File: Pixels.tex

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\section{Magick::Pixels}
\scriptsize{
\begin{verbatim}

The Pixels class provides efficient access to raw image pixels. Image pixels
(of type PixelPacket) may be accessed directly via the Image Pixel Cache.
The image pixel cache is a rectangular window (a view) into the actual image
pixels (which may be in memory, memory-mapped from a disk file, or entirely
on disk). Obtain existing image pixels via get(). Create a new pixel region
using set().

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 sync() insures that the base image is updated with the contents of
the modified pixel cache. The method decode() supports copying foreign pixel
data formats into the pixel cache according to the QuantumTypes. The method
encode() supports copying the pixels in the cache to a foreign pixel
representation according to the format specified by QuantumTypes.

Setting a view using the Pixels class does not cause the number of
references to the underlying image to be reduced to one. Therefore, 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.

The PixelPacket* returned by the set and get methods, and the IndexPacket*
returned by the indexes method point to pixel data managed by the Pixels
class. The Pixels class is responsible for releasing resources associated
with the pixel view. This means that the pointer should never be passed to
delete() or free().

The pixel view is a small image in which the pixels may be accessed,
addressed, and updated, as shown in the following example, which produces an
image similar to the one on the right (minus lines and text):

  // Create base image
   Image image(Geometry(254,218), "white");

  // Set image pixels to DirectClass representation
   image.classType( DirectClass );

   // Ensure that there is only one reference to underlying
image
   image.modifyImage();

  // Allocate pixel view
   Pixels view(image);

  // Set all pixels in region anchored at 38x36, with size
160x230 to green.
   unsigned int columns = 196; unsigned int rows = 162;
   Color green("green");
   PixelPacket *pixels = view.get(38,36,columns,rows);
   for ( unsigned int row = 0; row < rows ; ++row )
     for ( unsigned int column = 0; column < columns ; ++column  [Cache.png]
)
       *pixels++=green;
   view.sync();

  // Set all pixels in region anchored at 86x72, with size
108x67 to yellow.
   columns = 108; rows = 67;
   Color yellow("yellow");
   pixels = view.get(86,72,columns,rows);
   for ( unsigned int row = 0; row < rows ; ++row )
      for ( unsigned int column = 0; column < columns ;
++column )
         *pixels++=yellow;
    view.sync();

  // Set pixel at position 108,94 to red
   *(view.get(108,94,1,1)) = Color("red");
   view.sync();


Pixels supports the following methods:

                             Pixel Cache Methods

 Method  Returns      Signature              Description

                                             Transfers pixels from the
                                             image to the pixel cache as
                                             defined by the specified
                      int x_, int y_,        rectangular region. Modified
 get     PixelPacket* unsigned int columns_, pixels may be subsequently
                      unsigned int rows_     transferred back to the image
                                             via sync. The value returned
                                             is intended for pixel access
                                             only. It should never be
                                             deallocated.

                                             Allocates a pixel cache region
                                             to store image pixels as
                                             defined by the region
                      int x_, int y_,        rectangle.  This area is
 set     PixelPacket* unsigned int columns_, subsequently transferred from
                      unsigned int rows_     the pixel cache to the image
                                             via sync. The value returned
                                             is intended for pixel access
                                             only. It should never be
                                             deallocated.


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

                                             Returns the PsuedoColor pixel
                                             indexes corresponding to the
                                             pixel region defined by the
                                             last get or set call. Only
                                             valid for PseudoColor and
                                             CMYKA images. The pixel
                                             indexes (an array of type
                                             IndexPacket, which is typedef
                                             Quantum, which is itself
 indexes IndexPacket* void                   typedef unsigned char, or
                                             unsigned short, depending on
                                             the value of the QuantumDepth
                                             define) provide the colormap
                                             index (see colorMap) for each
                                             pixel in the image. For CMYKA
                                             images, the indexes represent
                                             the matte channel. The value
                                             returned is intended for pixel
                                             access only. It should never
                                             be deallocated.

 x       unsigned int void                   Left ordinate of view

 y       unsigned int void                   Top ordinate of view

 columns unsigned int void                   Width of view

 rows    unsigned int void                   Height of view




\end{verbatim}
}