Magick::Image Class

Quick Contents

BLOBs
Constructors
Image Manipulation Methods
Image Attributes
Low-Level Image Pixel Access

Image is the primary object in Magick++ and represents a single image frame (see design ). The STL interface must be used to operate on image sequences or images (e.g. of format GIF, TIFF, MIFF, Postscript, & MNG) which are comprized of multiple image frames. Individual frames of a multi-frame image may be requested by adding array-style notation to the end of the file name (e.g. "animation.gif[3]" retrieves the fourth frame of a GIF animation. Various image manipulation operations may be applied to the image. Attributes may be set on the image to influence the operation of the manipulation operations. The Pixels class provides low-level access to image pixels. As a convenience, including <Magick++.h> is sufficient in order to use the complete Magick++ API. The Magick++ API is enclosed within the Magick namespace so you must either add the prefix " Magick:: " to each class/enumeration name or add the statement " using namespace Magick;" after including the Magick++.h header.

The preferred way to allocate Image objects is via automatic allocation (on the stack). There is no concern that allocating Image objects on the stack will excessively enlarge the stack since Magick++ allocates all large data objects (such as the actual image data) from the heap. Use of automatic allocation is preferred over explicit allocation (via new) since it is much less error prone and allows use of C++ scoping rules to avoid memory leaks. Use of automatic allocation allows Magick++ objects to be assigned and copied just like the C++ intrinsic data types (e.g. 'int '), leading to clear and easy to read code. Use of automatic allocation leads to naturally exception-safe code since if an exception is thrown, the object is automatically deallocated once the stack unwinds past the scope of the allocation (not the case for objects allocated via new ).

Image is very easy to use. For example, here is a the source to a program which reads an image, crops it, and writes it to a new file (the exception handling is optional but strongly recommended):

#include <Magick++.h>
#include <iostream>
using namespace std;
using namespace Magick;
int main(int argc,char **argv)
{
// Construct the image object. Seperating image construction from the
// the read operation ensures that a failure to read the image file
// doesn't render the image object useless.
Image image;
try {
    // Read a file into image object
    image.read( "girl.gif" );

    // Crop the image to specified size (width, height, xOffset, yOffset)
    image.crop( Geometry(100,100, 100, 100) );

    // Write the image to a file
    image.write( "x.gif" );
}
catch( Exception &error_ )
    {
      cout << "Caught exception: " << error_.what() << endl;
      return 1;
    }
return 0;
}

The following is the source to a program which illustrates the use of Magick++'s efficient reference-counted assignment and copy-constructor operations which minimize use of memory and eliminate unncessary copy operations (allowing Image objects to be efficiently assigned, and copied into containers). The program accomplishes the following:

Read master image.
Assign master image to second image.
Zoom second image to the size 640x480.
Assign master image to a third image.
Zoom third image to the size 800x600.
Write the second image to a file.
Write the third image to a file.

#include <Magick++.h>
#include <iostream>
using namespace std;
using namespace Magick;
int main(int argc,char **argv)
{
    Image master("horse.jpg");
    Image second = master;
    second.zoom("640x480");
    Image third = master;
    third.zoom("800x600");
    second.write("horse640x480.jpg");
    third.write("horse800x600.jpg");
    return 0;
}

During the entire operation, a maximum of three images exist in memory and the image data is never copied.

The following is the source for another simple program which creates a 100 by 100 pixel white image with a red pixel in the center and writes it to a file:

#include <Magick++.h>
using namespace std;
using namespace Magick;
int main(int argc,char **argv)
{
    Image image( "100x100", "white" );
    image.pixelColor( 49, 49, "red" );
    image.write( "red_pixel.png" );
    return 0;
}

If you wanted to change the color image to grayscale, you could add the lines:

    image.quantizeColorSpace( GRAYColorspace );
    image.quantizeColors( 256 );
    image.quantize( );

or, more simply:

image.type( GrayscaleType );

prior to writing the image.

BLOBs

While encoded images (e.g. JPEG) are most often written-to and read-from a disk file, encoded images may also reside in memory. Encoded images in memory are known as BLOBs (Binary Large OBjects) and may be represented using the Blob class. The encoded image may be initially placed in memory by reading it directly from a file, reading the image from a database, memory-mapped from a disk file, or could be written to memory by Magick++. Once the encoded image has been placed within a Blob, it may be read into a Magick++ Image via a constructor or read() . Likewise, a Magick++ image may be written to a Blob via write() .

An example of using Image to write to a Blob follows:

#include <Magick++.h>
using namespace std;
using namespace Magick;
int main(int argc,char **argv)
{
    // Read GIF file from disk
    Image image( "giraffe.gif" );
    // Write to BLOB in JPEG format
    Blob blob;
    image.magick( "JPEG" ) // Set JPEG output format
    image.write( &blob );

    [ Use BLOB data (in JPEG format) here ]

    return 0;
}

likewise, to read an image from a Blob, you could use one of the following examples:

[ Entry condition for the following examples is that data is pointer to encoded image data and length represents the size of the data ]

Blob blob( data, length );
Image image( blob );

Blob blob( data, length );
Image image;
image.read( blob);

some images do not contain their size or format so the size and format must be specified in advance:

Blob blob( data, length );
Image image;
image.size( "640x480")
image.magick( "RGBA" );
image.read( blob);

Constructors

Image may be constructed in a number of ways. It may be constructed from a file, a URL, or an encoded image (e.g. JPEG) contained in an in-memory BLOB . The available Image constructors are shown in the following table:

Image Constructors

Signature

Description

const std::string &imageSpec_

Construct Image by reading from file or URL specified by imageSpec_. Use array notation (e.g. filename[9]) to select a specific scene from a multi-frame image.

const Geometry &size_, const Color &color_

Construct a blank image canvas of specified size and color

const Blob &blob_

Construct Image by reading from encoded image data contained in an in-memory BLOB . Depending on the constructor arguments, the Blob size , depth , magick (format) may also be specified. Some image formats require that size be specified. The default ImageMagick uses for depth depends on the compiled-in Quantum size (8 or 16). If ImageMagick's Quantum size does not match that of the image, the depth may need to be specified. ImageMagick can usually automatically detect the image's format. When a format can't be automatically detected, the format (magick ) must be specified.

const Blob &blob_, const Geometry &size_

const Blob &blob_, const Geometry &size, unsigned int depth

const Blob &blob_, const Geometry &size, unsigned int depth_, const string &magick_

const Blob &blob_, const Geometry &size, const string &magick_

const unsigned int width_,
const unsigned int height_,
std::string map_,
const StorageType type_,
const unsigned char* pixels_

Construct a new Image based on an array of image pixels. The pixel data must be in scanline order top-to-bottom. The data can be character, short int, integer, float, or double. Float and double require the pixels to be normalized [0..1]. The other types are [0..QuantumRange]. For example, to create a 640x480 image from unsigned red-green-blue character data, use

Image image( 640, 480, "RGB", 0, pixels );

The parameters are as follows:

width_ Width in pixels of the image.

height_ Height in pixels of the image.

map_ This character string can be any combination or order of R = red, G = green, B = blue, A = alpha, C = cyan, Y = yellow M = magenta, and K = black. The ordering reflects the order of the pixels in the supplied pixel array.

type_ Pixel storage type (CharPixel, ShortPixel, IntegerPixel, FloatPixel, or DoublePixel)

pixels_ This array of values contain the pixel components as defined by the map_ and type_ parameters. The length of the arrays must equal the area specified by the width_ and height_ values and type_ parameters.

Image Manipulation Methods

Image supports access to all the single-image (versus image-list) manipulation operations provided by the ImageMagick library. If you must process a multi-image file (such as an animation), the STL interface , which provides a multi-image abstraction on top of Image, must be used.

Image manipulation methods are very easy to use. For example:

Image image;
image.read("myImage.tiff");
image.addNoise(GaussianNoise);
image.write("myImage.tiff");

adds gaussian noise to the image file "myImage.tiff".

The operations supported by Image are shown in the following table:

Image Manipulation Methods

Method Signature(s) Description

adaptiveThreshold

unsigned int width, unsigned int height, unsigned offset = 0
Apply adaptive thresholding to the image. Adaptive thresholding is useful if the ideal threshold level is not known in advance, or if the illumination gradient is not constant across the image. Adaptive thresholding works by evaulating the mean (average) of a pixel region (size specified by width and height) and using the mean as the thresholding value. In order to remove residual noise from the background, the threshold may be adjusted by subtracting a constant offset (default zero) from the mean to compute the threshold.

addNoise NoiseType noiseType_ Add noise to image with specified noise type.

affineTransform
const DrawableAffine &affine
Transform image by specified affine (or free transform) matrix.

annotate const std::string &text_, const Geometry &location_ Annotate using specified text, and placement location

string text_, const Geometry &boundingArea_, GravityType gravity_ Annotate using specified text, bounding area, and placement gravity. If boundingArea_ is invalid, then bounding area is entire image.

const std::string &text_, const Geometry &boundingArea_, GravityType gravity_, double degrees_, Annotate with text using specified text, bounding area, placement gravity, and rotation. If boundingArea_ is invalid, then bounding area is entire image.

const std::string &text_, GravityType gravity_ Annotate with text (bounding area is entire image) and placement gravity.

blur const double radius_ = 1, const double sigma_ = 0.5 Blur image. The radius_ parameter specifies the radius of the Gaussian, in pixels, not counting the center pixel. The sigma_ parameter specifies the standard deviation of the Laplacian, in pixels.

border const Geometry &geometry_ = "6x6+0+0" Border image (add border to image). The color of the border is specified by the borderColor attribute.

channel ChannelType layer_ Extract channel from image. Use this option to extract a particular channel from the image. MatteChannel for example, is useful for extracting the opacity values from an image.

charcoal const double radius_ = 1, const double sigma_ = 0.5 Charcoal effect image (looks like charcoal sketch). The radius_ parameter specifies the radius of the Gaussian, in pixels, not counting the center pixel. The sigma_ parameter specifies the standard deviation of the Laplacian, in pixels.

chop const Geometry &geometry_ Chop image (remove vertical or horizontal subregion of image)

colorize const unsigned int opacityRed_, const unsigned int opacityGreen_, const unsigned int opacityBlue_, const Color &penColor_ Colorize image with pen color, using specified percent opacity for red, green, and blue quantums.

const unsigned int opacity_, const Color &penColor_ Colorize image with pen color, using specified percent opacity.

comment const string &comment_ Comment image (add comment string to image). By default, each image is commented with its file name. Use this method to assign a specific comment to the image. Optionally you can include the image filename, type, width, height, or other image attributes by embedding special format characters.

compare
const Image &reference_
Compare current image with another image. Sets meanErrorPerPixel , normalizedMaxError , and normalizedMeanError in the current image. False is returned if the images are identical. An ErrorOption exception is thrown if the reference image columns, rows, colorspace, or matte differ from the current image.

composite const Image &compositeImage_, int xOffset_, int yOffset_, CompositeOperator compose_ = InCompositeOp Compose an image onto the current image at offset specified by xOffset_, yOffset_ using the composition algorithm specified by compose_.

const Image &compositeImage_, const Geometry &offset_, CompositeOperator compose_ = InCompositeOp Compose an image onto the current image at offset specified by offset_ using the composition algorithm specified by compose_ .

const Image &compositeImage_, GravityType gravity_, CompositeOperator compose_ = InCompositeOp Compose an image onto the current image with placement specified by gravity_ using the composition algorithm specified by compose_.

contrast unsigned int sharpen_ Contrast image (enhance intensity differences in image)

convolve unsigned int order_, const double *kernel_ Convolve image. Applies a user-specfied convolution to the image. The order_ parameter represents the number of columns and rows in the filter kernel, and kernel_ is a two-dimensional array of doubles representing the convolution kernel to apply.

crop const Geometry &geometry_ Crop image (subregion of original image)

cycleColormap int amount_ Cycle image colormap

despeckle void Despeckle image (reduce speckle noise)

display void Display image on screen.
Caution: if an image format is not compatible with the display visual (e.g. JPEG on a colormapped display) then the original image will be altered. Use a copy of the original if this is a problem.

draw const Drawable &drawable_ Draw shape or text on image.

const std::list<Drawable > &drawable_ Draw shapes or text on image using a set of Drawable objects contained in an STL list. Use of this method improves drawing performance and allows batching draw objects together in a list for repeated use.

edge unsigned int radius_ = 0.0 Edge image (hilight edges in image). The radius is the radius of the pixel neighborhood.. Specify a radius of zero for automatic radius selection.

emboss const double radius_ = 1, const double sigma_ = 0.5 Emboss image (hilight edges with 3D effect). The radius_ parameter specifies the radius of the Gaussian, in pixels, not counting the center pixel. The sigma_ parameter specifies the standard deviation of the Laplacian, in pixels.

enhance void Enhance image (minimize noise)

equalize void Equalize image (histogram equalization)

erase void Set all image pixels to the current background color.

flip void Flip image (reflect each scanline in the vertical direction)

floodFill-
Color unsigned int x_, unsigned int y_, const Color &fillColor_ Flood-fill color across pixels that match the color of the target pixel and are neighbors of the target pixel. Uses current fuzz setting when determining color match.

const Geometry &point_, const Color &fillColor_

unsigned int x_, unsigned int y_, const Color &fillColor_, const Color &borderColor_ Flood-fill color across pixels starting at target-pixel and stopping at pixels matching specified border color. Uses current fuzz setting when determining color match.

const Geometry &point_, const Color &fillColor_, const Color &borderColor_

floodFillOpacity const long x_, const long y_, const unsigned int opacity_, const PaintMethod method_ Floodfill pixels matching color (within fuzz factor) of target pixel(x,y) with replacement opacity value using method.

floodFill-
Texture unsigned int x_, unsigned int y_, const Image &texture_ Flood-fill texture across pixels that match the color of the target pixel and are neighbors of the target pixel. Uses current fuzz setting when determining color match.

const Geometry &point_, const Image &texture_

unsigned int x_, unsigned int y_, const Image &texture_, const Color &borderColor_ Flood-fill texture across pixels starting at target-pixel and stopping at pixels matching specified border color. Uses current fuzz setting when determining color match.

const Geometry &point_, const Image &texture_, const Color &borderColor_

flop void Flop image (reflect each scanline in the horizontal direction)

frame const Geometry &geometry_ = "25x25+6+6" Add decorative frame around image

unsigned int width_, unsigned int height_, int x_, int y_, int innerBevel_ = 0, int outerBevel_ = 0

gamma double gamma_ Gamma correct image (uniform red, green, and blue correction).

double gammaRed_, double gammaGreen_, double gammaBlue_ Gamma correct red, green, and blue channels of image.

gaussianBlur const double width_, const double sigma_ Gaussian blur image. The number of neighbor pixels to be included in the convolution mask is specified by 'width_'. For example, a width of one gives a (standard) 3x3 convolution mask. The standard deviation of the gaussian bell curve is specified by 'sigma_'.

implode const double factor_ Implode image (special effect)

label const string &label_ Assign a label to an image. Use this option to assign a specific label to the image. Optionally you can include the image filename, type, width, height, or scene number in the label by embedding special format characters. If the first character of string is @, the image label is read from a file titled by the remaining characters in the string. When converting to Postscript, use this option to specify a header string to print above the image.

level
const double black_point, const double white_point, const double mid_point=1.0
Level image. Adjust the levels of the image by scaling the colors falling between specified white and black points to the full available quantum range. The parameters provided represent the black, mid (gamma), and white points. The black point specifies the darkest color in the image. Colors darker than the black point are set to zero. Mid point (gamma) specifies a gamma correction to apply to the image. White point specifies the lightest color in the image. Colors brighter than the white point are set to the maximum quantum value. The black and white point have the valid range 0 to QuantumRange while mid (gamma) has a useful range of 0 to ten.

levelChannel
const ChannelType channel, const double black_point, const double white_point, const double mid_point=1.0
Level image channel. Adjust the levels of the image channel by scaling the values falling between specified white and black points to the full available quantum range. The parameters provided represent the black, mid (gamma), and white points. The black point specifies the darkest color in the image. Colors darker than the black point are set to zero. Mid point (gamma) specifies a gamma correction to apply to the image. White point specifies the lightest color in the image. Colors brighter than the white point are set to the maximum quantum value. The black and white point have the valid range 0 to QuantumRange while mid (gamma) has a useful range of 0 to ten.

magnify void Magnify image by integral size

map const Image &mapImage_ , bool dither_ = false Remap image colors with closest color from reference image. Set dither_ to true in to apply Floyd/Steinberg error diffusion to the image. By default, color reduction chooses an optimal set of colors that best represent the original image. Alternatively, you can choose a particular set of colors from an image file with this option.

matteFloodfill const Color &target_, const unsigned int opacity_, const int x_, const int y_, PaintMethod method_ Floodfill designated area with a replacement opacity value.

medianFilter const double radius_ = 0.0 Filter image by replacing each pixel component with the median color in a circular neighborhood

minify void Reduce image by integral size

modifyImage void Prepare to update image. Ensures that there is only one reference to the underlying image so that the underlying image may be safely modified without effecting previous generations of the image. Copies the underlying image to a new image if necessary.

modulate double brightness_, double saturation_, double hue_ Modulate percent hue, saturation, and brightness of an image. Modulation of saturation and brightness is as a ratio of the current value (1.0 for no change). Modulation of hue is an absolute rotation of -180 degrees to +180 degrees from the current position corresponding to an argument range of 0 to 2.0 (1.0 for no change).

negate bool grayscale_ = false Negate colors in image. Replace every pixel with its complementary color (white becomes black, yellow becomes blue, etc.). Set grayscale to only negate grayscale values in image.

normalize void Normalize image (increase contrast by normalizing the pixel values to span the full range of color values).

oilPaint unsigned int radius_ = 3 Oilpaint image (image looks like oil painting)

opacity unsigned int opacity_ Set or attenuate the opacity channel in the image. If the image pixels are opaque then they are set to the specified opacity value, otherwise they are blended with the supplied opacity value. The value of opacity_ ranges from 0 (completely opaque) to QuantumRange . The defines OpaqueOpacity and TransparentOpacity are available to specify completely opaque or completely transparent, respectively.

opaque const Color &opaqueColor_, const Color &penColor_ Change color of pixels matching opaqueColor_ to specified penColor_.

ping const std::string &imageSpec_ Ping is similar to read except only enough of the image is read to determine the image columns, rows, and filesize. The columns , rows , and fileSize attributes are valid after invoking ping. The image data is not valid after calling ping.

const Blob &blob_

process
std::string name_, const int argc_, char **argv_
Execute the named process module, passing any arguments via an argument vector, with argc_ specifying the number of arguments in the vector, and argv_ passing the address of an array of null-terminated C strings which constitute the argument vector. An exception is thrown if the requested process module does not exist, fails to load, or fails during execution.

quantize bool measureError_ = false Quantize image (reduce number of colors). Set measureError_ to true in order to calculate error attributes.

raise const Geometry &geometry_ = "6x6+0+0", bool raisedFlag_ = false Raise image (lighten or darken the edges of an image to give a 3-D raised or lowered effect)

read const string &imageSpec_ Read image into current object

const Geometry &size_, const std::string &imageSpec_ Read image of specified size into current object. This form is useful for images that do not specifiy their size or to specify a size hint for decoding an image. For example, when reading a Photo CD, JBIG, or JPEG image, a size request causes the library to return an image which is the next resolution greater or equal to the specified size. This may result in memory and time savings.

const Blob &blob_ Read encoded image of specified size from an in-memory BLOB into current object. Depending on the method arguments, the Blob size, depth, and format may also be specified. Some image formats require that size be specified. The default ImageMagick uses for depth depends on its Quantum size (8 or 16). If ImageMagick's Quantum size does not match that of the image, the depth may need to be specified. ImageMagick can usually automatically detect the image's format. When a format can't be automatically detected, the format must be specified.

const Blob &blob_, const Geometry &size_

const Blob &blob_, const Geometry &size_, unsigned int depth_

const Blob &blob_, const Geometry &size_, unsigned short depth_, const string &magick_

const Blob &blob_, const Geometry &size_, const string &magick_

const unsigned int width_, const unsigned int height_, std::string map_, const StorageType type_, const unsigned char* pixels_ Read image based on an array of image pixels. The pixel data must be in scanline order top-to-bottom. The data can be character, short int, integer, float, or double. Float and double require the pixels to be normalized [0..1]. The other types are [0..QuantumRange]. For example, to create a 640x480 image from unsigned red-green-blue character data, use
image.read( 640, 480, "RGB", 0, pixels );

The parameters are as follows:

width_ Width in pixels of the image.

height_ Height in pixels of the image.

map_ This character string can be any combination or order of R = red, G = green, B = blue, A = alpha, C = cyan, Y = yellow M = magenta, and K = black. The ordering reflects the order of the pixels in the supplied pixel array.

type_ Pixel storage type (CharPixel, ShortPixel, IntegerPixel, FloatPixel, or DoublePixel)

pixels_ This array of values contain the pixel components as defined by the map_ and type_ parameters. The length of the arrays must equal the area specified by the width_ and height_ values and type_ parameters.

reduceNoise void Reduce noise in image using a noise peak elimination filter.

unsigned int order_

roll int columns_, int rows_ Roll image (rolls image vertically and horizontally) by specified number of columnms and rows)

rotate double degrees_ Rotate image counter-clockwise by specified number of degrees.

sample const Geometry &geometry_ Resize image by using pixel sampling algorithm

scale const Geometry &geometry_ Resize image by using simple ratio algorithm

segment double clusterThreshold_ = 1.0,
double smoothingThreshold_ = 1.5 Segment (coalesce similar image components) by analyzing the histograms of the color components and identifying units that are homogeneous with the fuzzy c-means technique. Also uses quantizeColorSpace and verbose image attributes. Specify clusterThreshold_ , as the number of pixels each cluster must exceed the cluster threshold to be considered valid. SmoothingThreshold_ eliminates noise in the second derivative of the histogram. As the value is increased, you can expect a smoother second derivative. The default is 1.5.

shade double azimuth_ = 30, double elevation_ = 30,
bool colorShading_ = false Shade image using distant light source. Specify azimuth_ and elevation_ as the position of the light source. By default, the shading results as a grayscale image.. Set colorShading_ to true to shade the red, green, and blue components of the image.

sharpen const double radius_ = 1, const double sigma_ = 0.5 Sharpen pixels in image. The radius_ parameter specifies the radius of the Gaussian, in pixels, not counting the center pixel. The sigma_ parameter specifies the standard deviation of the Laplacian, in pixels.

shave const Geometry &geometry_ Shave pixels from image edges.

shear double xShearAngle_, double yShearAngle_ Shear image (create parallelogram by sliding image by X or Y axis). Shearing slides one edge of an image along the X or Y axis, creating a parallelogram. An X direction shear slides an edge along the X axis, while a Y direction shear slides an edge along the Y axis. The amount of the shear is controlled by a shear angle. For X direction shears, x degrees is measured relative to the Y axis, and similarly, for Y direction shears y degrees is measured relative to the X axis. Empty triangles left over from shearing the image are filled with the color defined as borderColor.

solarize double factor_ = 50.0 Solarize image (similar to effect seen when exposing a photographic film to light during the development process)

spread unsigned int amount_ = 3 Spread pixels randomly within image by specified amount

stegano const Image &watermark_ Add a digital watermark to the image (based on second image)

stereo const Image &rightImage_ Create an image which appears in stereo when viewed with red-blue glasses (Red image on left, blue on right)

swirl double degrees_ Swirl image (image pixels are rotated by degrees)

texture const Image &texture_ Layer a texture on pixels matching image background color.

threshold double threshold_ Threshold image

transform const Geometry &imageGeometry_ Transform image based on image and crop geometries. Crop geometry is optional.

const Geometry &imageGeometry_, const Geometry &cropGeometry_

transparent const Color &color_ Add matte image to image, setting pixels matching color to transparent.

trim void Trim edges that are the background color from the image.

unsharpmask double radius_, double sigma_, double amount_, double threshold_ Replace image with a sharpened version of the original image using the unsharp mask algorithm. The radius_ parameter specifies the radius of the Gaussian, in pixels, not counting the center pixel. The sigma_ parameter specifies the standard deviation of the Gaussian, in pixels. The amount_ parameter specifies the percentage of the difference between the original and the blur image that is added back into the original. The threshold_ parameter specifies the threshold in pixels needed to apply the diffence amount.

wave double amplitude_ = 25.0, double wavelength_ = 150.0 Alter an image along a sine wave.

write const string &imageSpec_ Write image to a file using filename imageSpec_ .
Caution: if an image format is selected which is capable of supporting fewer colors than the original image or quantization has been requested, the original image will be quantized to fewer colors. Use a copy of the original if this is a problem.

Blob *blob_ Write image to a in-memory BLOB stored in blob_. The magick_ parameter specifies the image format to write (defaults to magick ). The depth_ parameter species the image depth (defaults to depth ).
Caution: if an image format is selected which is capable of supporting fewer colors than the original image or quantization has been requested, the original image will be quantized to fewer colors. Use a copy of the original if this is a problem.

Blob *blob_, std::string &magick_

Blob *blob_, std::string &magick_, unsigned int depth_

const int x_, const int y_, const unsigned int columns_, const unsigned int rows_, const std::string &map_, const StorageType type_, unsigned char* pixels_ Write pixel data into a buffer you supply. The data is saved either as char, short int, integer, float or double format in the order specified by the type_ parameter. For example, we want to extract scanline 1 of a 640x480 image as character data in red-green-blue order:
image.write(0,0,640,1,"RGB",0,pixels);

The parameters are as follows:

x_ Horizontal ordinate of left-most coordinate of region to extract.

y_ Vertical ordinate of top-most coordinate of region to extract.

columns_ Width in pixels of the region to extract.

rows_ Height in pixels of the region to extract.

map_ This character string can be any combination or order of R = red, G = green, B = blue, A = alpha, C = cyan, Y = yellow, M = magenta, and K = black. The ordering reflects the order of the pixels in the supplied pixel array.

type_ Pixel storage type (CharPixel, ShortPixel, IntegerPixel, FloatPixel, or DoublePixel)

pixels_ This array of values contain the pixel components as defined by the map_ and type_ parameters. The length of the arrays must equal the area specified by the width_ and height_ values and type_ parameters.

zoom const Geometry &geometry_ Zoom image to specified size.

Image Attributes

Image attributes are set and obtained via methods in Image. Except for methods which accept pointer arguments (e.g. chromaBluePrimary) all methods return attributes by value.

Image attributes are easily used. For example, to set the resolution of the TIFF file "file.tiff" to 150 dots-per-inch (DPI) in both the horizontal and vertical directions, you can use the following example code:

string filename("file.tiff");
Image image;
image.read(filename);
image.resolutionUnits(PixelsPerInchResolution);
image.density(Geometry(150,150)); // could also use image.density("150x150")
image.write(filename)

The supported image attributes and the method arguments required to obtain them are shown in the following table:

Image Attributes
Function	Type	Get Signature	Set Signature	Description
adjoin	bool	void	bool flag_	Join images into a single multi-image file.
antiAlias	bool	void	bool flag_	Control antialiasing of rendered Postscript and Postscript or TrueType fonts. Enabled by default.
animation- Delay	unsigned int (0 to 65535)	void	unsigned int delay_	Time in 1/100ths of a second (0 to 65535) which must expire before displaying the next image in an animated sequence. This option is useful for regulating the animation of a sequence of GIF images within Netscape.
animation- Iterations	unsigned int	void	unsigned int iterations_	Number of iterations to loop an animation (e.g. Netscape loop extension) for.
attribute	string	const std::string name_	const std::string name_, const std::string value_	An arbitrary named image attribute. Any number of named attributes may be attached to the image. For example, the image comment is a named image attribute with the name "comment". EXIF tags are attached to the image as named attributes. Use the syntax "[EXIF:<tag>]" to request an EXIF tag similar to "[EXIF:DateTime]".
background- Color	Color	void	const Color &color_	Image background color
background- Texture	string	void	const string &texture_	Image file name to use as the background texture. Does not modify image pixels.
baseColumns	unsigned int	void		Base image width (before transformations)
baseFilename	string	void		Base image filename (before transformations)
baseRows	unsigned int	void		Base image height (before transformations)
borderColor	Color	void	const Color &color_	Image border color
boundingBox	Geometry	void		Return smallest bounding box enclosing non-border pixels. The current fuzz value is used when discriminating between pixels. This is the crop bounding box used by crop(Geometry(0,0)).
boxColor	Color	void	const Color &boxColor_	Base color that annotation text is rendered on.
cacheThreshold	unsigned int		const int	Pixel cache threshold in megabytes. Once this threshold is exceeded, all subsequent pixels cache operations are to/from disk. This is a static method and the attribute it sets is shared by all Image objects.
channelDepth	unsigned int	const ChannelType channel_	const ChannelType channel_, const unsigned int depth_	Channel modulus depth. The channel modulus depth represents the minimum number of bits required to support the channel without loss. Setting the channel's modulus depth modifies the channel (i.e. discards resolution) if the requested modulus depth is less than the current modulus depth, otherwise the channel is not altered. There is no attribute associated with the modulus depth so the current modulus depth is obtained by inspecting the pixels. As a result, the depth returned may be less than the most recently set channel depth. Subsequent image processing may result in increasing the channel depth.
chroma- BluePrimary	double x & y	double x_, double y_	double x_, double y_	Chromaticity blue primary point (e.g. x=0.15, y=0.06)
chroma- GreenPrimary	double x & y	double x_, double y_	double x_, double y_	Chromaticity green primary point (e.g. x=0.3, y=0.6)
chroma- RedPrimary	double x & y	double x_, double y_	double x_, double y_	Chromaticity red primary point (e.g. x=0.64, y=0.33)
chroma- WhitePoint	double x & y	doublex_, double y_	double x_, double y_	Chromaticity white point (e.g. x=0.3127, y=0.329)
classType	ClassType	void	ClassType class_	Image storage class. Note that conversion from a DirectClass image to a PseudoClass image may result in a loss of color due to the limited size of the palette (256 or 65535 colors).
clipMask	Image	void	const Image &clipMask_	Associate a clip mask image with the current image. The clip mask image must have the same dimensions as the current image or an exception is thrown. Clipping occurs wherever pixels are transparent in the clip mask image. Clipping Pass an invalid image to unset an existing clip mask.
colorFuzz	double	void	double fuzz_	Colors within this distance are considered equal. A number of algorithms search for a target color. By default the color must be exact. Use this option to match colors that are close to the target color in RGB space.
colorMap	Color	unsigned int index_	unsigned int index_, const Color &color_	Color at colormap index.
colorMapSize	unsigned int	void	unsigned int entries_	Number of entries in the colormap. Setting the colormap size may extend or truncate the colormap. The maximum number of supported entries is specified by the MaxColormapSizeconstant, and is dependent on the value of QuantumDepth when ImageMagick is compiled. An exception is thrown if more entries are requested than may be supported. Care should be taken when truncating the colormap to ensure that the image colormap indexes reference valid colormap entries.
colorSpace	ColorspaceType colorSpace_	void	ColorspaceType colorSpace_	The colorspace (e.g. CMYK) used to represent the image pixel colors. Image pixels are always stored as RGB(A) except for the case of CMY(K).
columns	unsigned int	void		Image width
comment	string	void		Image comment
compose	CompositeOperator	void	CompositeOperator compose_	Composition operator to be used when composition is implicitly used (such as for image flattening).
compress- Type	CompressionType	void	CompressionType compressType_	Image compresion type. The default is the compression type of the specified image file.
debug	bool	void	bool flag_	Enable printing of internal debug messages from ImageMagick as it executes.
defineValue	string	const std::string &magick_, const std::string &key_	const std::string &magick_, const std::string &key_, const std::string &value_	Set or obtain a definition string to applied when encoding or decoding the specified format. The meanings of the definitions are format specific. The format is designated by the magick_ argument, the format-specific key is designated by key_, and the associated value is specified by value_. See the defineSet() method if the key must be removed entirely.
defineSet	bool	const std::string &magick_, const std::string &key_	const std::string &magick_, const std::string &key_, bool flag_	Set or obtain a definition flag to applied when encoding or decoding the specified format.. Similar to the defineValue() method except that passing the flag_ value 'true' creates a value-less define with that format and key. Passing the flag_ value 'false' removes any existing matching definition. The method returns 'true' if a matching key exists, and 'false' if no matching key exists.
density	Geometry (default 72x72)	void	const Geometry &density_	Vertical and horizontal resolution in pixels of the image. This option specifies an image density when decoding a Postscript or Portable Document page. Often used with psPageSize.
depth	unsigned int (8, 16, or 32)	void	unsigned int depth_	Image depth. Used to specify the bit depth when reading or writing raw images or when the output format supports multiple depths. Defaults to the quantum depth that ImageMagick is compiled with.
endian	EndianType	void	EndianType endian_	Specify (or obtain) endian option for formats which support it.
directory	string	void		Tile names from within an image montage
fileName	string	void	const string &fileName_	Image file name.
fileSize	off_t	void		Number of bytes of the image on disk
fillColor	Color	void	const Color &fillColor_	Color to use when filling drawn objects
fillPattern	Image	void	const Image &fillPattern_	Pattern image to use when filling drawn objects.
fillRule	FillRule	void	const Magick::FillRule &fillRule_	Rule to use when filling drawn objects.
filterType	FilterTypes	void	FilterTypes filterType_	Filter to use when resizing image. The reduction filter employed has a sigificant effect on the time required to resize an image and the resulting quality. The default filter is Lanczos which has been shown to produce high quality results when reducing most images.
font	string	void	const string &font_	Text rendering font. If the font is a fully qualified X server font name, the font is obtained from an X server. To use a TrueType font, precede the TrueType filename with an @. Otherwise, specify a Postscript font name (e.g. "helvetica").
fontPointsize	unsigned int	void	unsigned int pointSize_	Text rendering font point size
fontTypeMetrics	TypeMetric	const std::string &text_, TypeMetric *metrics		Update metrics with font type metrics using specified text, and current font and fontPointSize settings.
format	string	void		Long form image format description.
gamma	double (typical range 0.8 to 2.3)	void		Gamma level of the image. The same color image displayed on two different workstations may look different due to differences in the display monitor. Use gamma correction to adjust for this color difference.
geometry	Geometry	void		Preferred size of the image when encoding.
gifDispose- Method	unsigned int { 0 = Disposal not specified, 1 = Do not dispose of graphic, 3 = Overwrite graphic with background color, 4 = Overwrite graphic with previous graphic. }	void	unsigned int disposeMethod_	GIF disposal method. This option is used to control how successive frames are rendered (how the preceding frame is disposed of) when creating a GIF animation.
iccColorProfile	Blob	void	const Blob &colorProfile_	ICC color profile. Supplied via a Blob since Magick++/ and ImageMagick do not currently support formating this data structure directly. Specifications are available from the International Color Consortium for the format of ICC color profiles.
interlace- Type	InterlaceType	void	InterlaceType interlace_	The type of interlacing scheme (default NoInterlace ). This option is used to specify the type of interlacing scheme for raw image formats such as RGB or YUV. NoInterlace means do not interlace, LineInterlace uses scanline interlacing, and PlaneInterlace uses plane interlacing. PartitionInterlace is like PlaneInterlace except the different planes are saved to individual files (e.g. image.R, image.G, and image.B). Use LineInterlace or PlaneInterlace to create an interlaced GIF or progressive JPEG image.
iptcProfile	Blob	void	const Blob & iptcProfile_	IPTC profile. Supplied via a Blob since Magick++ and ImageMagick do not currently support formating this data structure directly. Specifications are available from the International Press Telecommunications Council for IPTC profiles.
label	string	void	const string &label_	Image label
magick	string	void	const string &magick_	Get image format (e.g. "GIF")
matte	bool	void	bool matteFlag_	True if the image has transparency. If set True, store matte channel if the image has one otherwise create an opaque one.
matteColor	Color	void	const Color &matteColor_	Image matte (frame) color
meanError- PerPixel	double	void		The mean error per pixel computed when an image is color reduced. This parameter is only valid if verbose is set to true and the image has just been quantized.
modulusDepth	unsigned int	void	unsigned int depth_	Image modulus depth (minimum number of bits required to support red/green/blue components without loss of accuracy). The pixel modulus depth may be decreased by supplying a value which is less than the current value, updating the pixels (reducing accuracy) to the new depth. The pixel modulus depth can not be increased over the current value using this method.
monochrome	bool	void	bool flag_	Transform the image to black and white
montage- Geometry	Geometry	void		Tile size and offset within an image montage. Only valid for montage images.
normalized- MaxError	double	void		The normalized max error per pixel computed when an image is color reduced. This parameter is only valid if verbose is set to true and the image has just been quantized.
normalized- MeanError	double	void		The normalized mean error per pixel computed when an image is color reduced. This parameter is only valid if verbose is set to true and the image has just been quantized.
orientation	OrientationType	void	OrientationType orientation_	Image orientation. Supported by some file formats such as DPX and TIFF. Useful for turning the right way up.
packets	unsigned int	void		The number of runlength-encoded packets in the image
packetSize	unsigned int	void		The number of bytes in each pixel packet
page	Geometry	void	const Geometry &pageSize_	Preferred size and location of an image canvas. Use this option to specify the dimensions and position of the Postscript page in dots per inch or a TEXT page in pixels. This option is typically used in concert with density . Page may also be used to position a GIF image (such as for a scene in an animation)
pixelColor	Color	unsigned int x_, unsigned int y_	unsigned int x_, unsigned int y_, const Color &color_	Get/set pixel color at location x & y.
profile	Blob	const std::string name_	const std::string name_, const Blob &colorProfile_	Get/set/remove a named profile. Valid names include "*", "8bim", "icm", "iptc", or a user/format-defined profile name.
quality	unsigned int (0 to 100)	void	unsigned int quality_	JPEG/MIFF/PNG compression level (default 75).
quantize- Colors	unsigned int	void	unsigned int colors_	Preferred number of colors in the image. The actual number of colors in the image may be less than your request, but never more. Images with less unique colors than specified with this option will have any duplicate or unused colors removed.
quantize- ColorSpace	ColorspaceType	void	ColorspaceType colorSpace_	Colorspace to quantize colors in (default RGB). Empirical evidence suggests that distances in color spaces such as YUV or YIQ correspond to perceptual color differences more closely than do distances in RGB space. These color spaces may give better results when color reducing an image.
quantize- Dither	bool	void	bool flag_	Apply Floyd/Steinberg error diffusion to the image. The basic strategy of dithering is to trade intensity resolution for spatial resolution by averaging the intensities of several neighboring pixels. Images which suffer from severe contouring when reducing colors can be improved with this option. The quantizeColors or monochrome option must be set for this option to take effect.
quantize- TreeDepth	unsigned int	void	unsigned int treeDepth_	Depth of the quantization color classification tree. Values of 0 or 1 allow selection of the optimal tree depth for the color reduction algorithm. Values between 2 and 8 may be used to manually adjust the tree depth.
rendering- Intent	RenderingIntent	void	RenderingIntent render_	The type of rendering intent
resolution- Units	ResolutionType	void	ResolutionType units_	Units of image resolution
rows	unsigned int	void		The number of pixel rows in the image
samplingFactor	string (e.g. "2x1,1x1,1x1")	void	const string &samplingFactor_	JPEG sampling factors
scene	unsigned int	void	unsigned int scene_	Image scene number
signature	string	bool force_ = false		Image MD5 signature. Set force_ to 'true' to force re-computation of signature.
size	Geometry	void	const Geometry &geometry_	Width and height of a raw image (an image which does not support width and height information). Size may also be used to affect the image size read from a multi-resolution format (e.g. Photo CD, JBIG, or JPEG.
strokeAntiAlias	bool	void	bool flag_	Enable or disable anti-aliasing when drawing object outlines.
strokeColor	Color	void	const Color &strokeColor_	Color to use when drawing object outlines
strokeDashOffset	unsigned int	void	double strokeDashOffset_	While drawing using a dash pattern, specify distance into the dash pattern to start the dash (default 0).
strokeDashArray	const double*	void	const double* strokeDashArray_	Specify the pattern of dashes and gaps used to stroke paths. The strokeDashArray represents a zero-terminated array of numbers that specify the lengths (in pixels) of alternating dashes and gaps in user units. If an odd number of values is provided, then the list of values is repeated to yield an even number of values. A typical strokeDashArray_ array might contain the members 5 3 2 0, where the zero value indicates the end of the pattern array.
strokeLineCap	LineCap	void	LineCap lineCap_	Specify the shape to be used at the corners of paths (or other vector shapes) when they are stroked. Values of LineJoin are UndefinedJoin, MiterJoin, RoundJoin, and BevelJoin.
strokeLineJoin	LineJoin	void	LineJoin lineJoin_	Specify the shape to be used at the corners of paths (or other vector shapes) when they are stroked. Values of LineJoin are UndefinedJoin, MiterJoin, RoundJoin, and BevelJoin.
strokeMiterLimit	unsigned int	void	unsigned int miterLimit_	Specify miter limit. When two line segments meet at a sharp angle and miter joins have been specified for 'lineJoin', it is possible for the miter to extend far beyond the thickness of the line stroking the path. The miterLimit' imposes a limit on the ratio of the miter length to the 'lineWidth'. The default value of this parameter is 4.
strokeWidth	double	void	double strokeWidth_	Stroke width for use when drawing vector objects (default one)
strokePattern	Image	void	const Image &strokePattern_	Pattern image to use while drawing object stroke (outlines).
subImage	unsigned int	void	unsigned int subImage_	Subimage of an image sequence
subRange	unsigned int	void	unsigned int subRange_	Number of images relative to the base image
textEncoding	string	void	const std::string &encoding_	Specify the code set to use for text annotations. The only character encoding which may be specified at this time is "UTF-8" for representing Unicode as a sequence of bytes. Specify an empty string to use the default ASCII encoding. Successful text annotation using Unicode may require fonts designed to support Unicode.
tileName	string	void	const string &tileName_	Tile name
totalColors	unsigned long	void		Number of colors in the image
type	ImageType	void	ImageType	Image type.
verbose	bool	void	bool verboseFlag_	Print detailed information about the image
view	string	void	const string &view_	FlashPix viewing parameters.
x11Display	string (e.g. "hostname:0.0")	void	const string &display_	X11 display to display to, obtain fonts from, or to capture image from
xResolution	double	void		x resolution of the image
yResolution	double	void		y resolution of the image

Low-Level Image Pixel Access

Image pixels (of type PixelPacket ) may be accessed directly via the Image Pixel Cache . The image pixel cache is a rectangular window into the actual image pixels (which may be in memory, memory-mapped from a disk file, or entirely on disk). Two interfaces exist to access the Image Pixel Cache. The interface described here (part of the Image class) supports only one view at a time. See the Pixels class for a more abstract interface which supports simultaneous pixel views (up to the number of rows). As an analogy, the interface described here relates to the Pixels class as stdio's gets() relates to fgets(). The Pixels class provides the more general form of the interface.

Obtain existing image pixels via getPixels(). Create a new pixel region using setPixels().

In order to ensure that only the current generation of the image is modified, the Image's modifyImage() method should be invoked to reduce the reference count on the underlying image to one. If this is not done, then it is possible for a previous generation of the image to be modified due to the use of reference counting when copying or constructing an Image.

Depending on the capabilities of the operating system, and the relationship of the window to the image, the pixel cache may be a copy of the pixels in the selected window, or it may be the actual image pixels. In any case calling syncPixels() insures that the base image is updated with the contents of the modified pixel cache. The method readPixels() supports copying foreign pixel data formats into the pixel cache according to the QuantumTypes. The method writePixels() supports copying the pixels in the cache to a foreign pixel representation according to the format specified by QuantumTypes.

The pixel region is effectively a small image in which the pixels may be accessed, addressed, and updated, as shown in the following example:

Image image("cow.png");
// Ensure that there are no other references to this image.
image.modifyImage();
// Set the image type to TrueColor DirectClass representation.
image.type(TrueColorType);
// Request pixel region with size 60x40, and top origin at 20x30
int columns = 60;
PixelPacket *pixel_cache = image.getPixels(20,30,columns,40);
// Set pixel at column 5, and row 10 in the pixel cache to red.
int column = 5;
int row = 10;
PixelPacket *pixel = pixel_cache+row*columns+column;
*pixel = Color("red");
// Save changes to underlying image .
image.syncPixels();
// Save updated image to file.
image.write("horse.png");

The image cache supports the following methods:

Image Cache Methods

Method Returns Signature Description

getConstPixels const PixelPacket * const int x_, const int y_, const unsigned int columns_, const unsigned int rows_ Transfers pixels from the image to the pixel cache as defined by the specified rectangular region. The returned pointer remains valid until the next getPixel, getConstPixels, or setPixels call and should never be deallocated by the user.

getConstIndexes const IndexPacket* void Returns a pointer to the Image pixel indexes corresponding to a previous getPixel, getConstPixels, or setPixels call. The returned pointer remains valid until the next getPixel, getConstPixels, or setPixels call and should never be deallocated by the user. Only valid for PseudoClass images or CMYKA images. The pixel indexes represent an array of type IndexPacket, with each entry corresponding to an x,y pixel position. For PseudoClass images, the entry's value is the offset into the colormap (see colorMap ) for that pixel. For CMYKA images, the indexes are used to contain the alpha channel.

getIndexes IndexPacket* void Returns a pointer to the Image pixel indexes corresponding to the pixel region requested by the last getConstPixels , getPixels , or setPixels call. The returned pointer remains valid until the next getPixel, getConstPixels, or setPixels call and should never be deallocated by the user. Only valid for PseudoClass images or CMYKA images. The pixel indexes represent an array of type IndexPacket, with each entry corresponding to a pixel x,y position. For PseudoClass images, the entry's value is the offset into the colormap (see colorMap ) for that pixel. For CMYKA images, the indexes are used to contain the alpha channel.

getPixels PixelPacket * const int x_, const int y_, const unsigned int columns_, const unsigned int rows_ Transfers pixels from the image to the pixel cache as defined by the specified rectangular region. Modified pixels may be subsequently transferred back to the image via syncPixels. The returned pointer remains valid until the next getPixel, getConstPixels, or setPixels call and should never be deallocated by the user.

setPixels PixelPacket * const int x_, const int y_, const unsigned int columns_, const unsigned int rows_ Allocates a pixel cache region to store image pixels as defined by the region rectangle. This area is subsequently transferred from the pixel cache to the image via syncPixels. The returned pointer remains valid until the next getPixel, getConstPixels, or setPixels call and should never be deallocated by the user.

syncPixels void void Transfers the image cache pixels to the image.

readPixels void QuantumTypes quantum_, unsigned char *source_, Transfers one or more pixel components from a buffer or file into the image pixel cache of an image. ReadPixels is typically used to support image decoders. The region transferred corresponds to the region set by a preceding setPixels call.

writePixels void QuantumTypes quantum_, unsigned char *destination_ Transfers one or more pixel components from the image pixel cache to a buffer or file. WritePixels is typically used to support image encoders. The region transferred corresponds to the region set by a preceding getPixels or getConstPixels call.

Method	Returns	Signature	Description
getConstPixels	const PixelPacket *	const int x_, const int y_, const unsigned int columns_, const unsigned int rows_	Transfers pixels from the image to the pixel cache as defined by the specified rectangular region. The returned pointer remains valid until the next getPixel, getConstPixels, or setPixels call and should never be deallocated by the user.
getConstIndexes	const IndexPacket*	void	Returns a pointer to the Image pixel indexes corresponding to a previous getPixel, getConstPixels, or setPixels call. The returned pointer remains valid until the next getPixel, getConstPixels, or setPixels call and should never be deallocated by the user. Only valid for PseudoClass images or CMYKA images. The pixel indexes represent an array of type IndexPacket, with each entry corresponding to an x,y pixel position. For PseudoClass images, the entry's value is the offset into the colormap (see colorMap ) for that pixel. For CMYKA images, the indexes are used to contain the alpha channel.
getIndexes	IndexPacket*	void	Returns a pointer to the Image pixel indexes corresponding to the pixel region requested by the last getConstPixels , getPixels , or setPixels call. The returned pointer remains valid until the next getPixel, getConstPixels, or setPixels call and should never be deallocated by the user. Only valid for PseudoClass images or CMYKA images. The pixel indexes represent an array of type IndexPacket, with each entry corresponding to a pixel x,y position. For PseudoClass images, the entry's value is the offset into the colormap (see colorMap ) for that pixel. For CMYKA images, the indexes are used to contain the alpha channel.
getPixels	PixelPacket *	const int x_, const int y_, const unsigned int columns_, const unsigned int rows_	Transfers pixels from the image to the pixel cache as defined by the specified rectangular region. Modified pixels may be subsequently transferred back to the image via syncPixels. The returned pointer remains valid until the next getPixel, getConstPixels, or setPixels call and should never be deallocated by the user.
setPixels	PixelPacket *	const int x_, const int y_, const unsigned int columns_, const unsigned int rows_	Allocates a pixel cache region to store image pixels as defined by the region rectangle. This area is subsequently transferred from the pixel cache to the image via syncPixels. The returned pointer remains valid until the next getPixel, getConstPixels, or setPixels call and should never be deallocated by the user.
syncPixels	void	void	Transfers the image cache pixels to the image.
readPixels	void	QuantumTypes quantum_, unsigned char *source_,	Transfers one or more pixel components from a buffer or file into the image pixel cache of an image. ReadPixels is typically used to support image decoders. The region transferred corresponds to the region set by a preceding setPixels call.
writePixels	void	QuantumTypes quantum_, unsigned char *destination_	Transfers one or more pixel components from the image pixel cache to a buffer or file. WritePixels is typically used to support image encoders. The region transferred corresponds to the region set by a preceding getPixels or getConstPixels call.