Magick::Pixels
The
Pixels
class provides efficient access to raw image
pixels. Image pixels (of type
PixelPacket.html
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
Image.html#modifyImage
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 the image type to TrueColor DirectClass
representation.
??
??image.type(TrueColorType
);
????
// 			Ensure that there is only one reference to
underlying image
???? 			// If this is not done, then image pixels will
not be modified.
????
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 ( int row = 0; row < rows ; ++row )
???????? 			for ( int column = 0; column < columns ;
++column )
???????????? 			*pixels++=green;
??
??
??// Save changes to
image.
???? 			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 ( int row = 0; row < rows ; ++row )
?????????? 			for ( 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");
??
??
??// Save changes to
image.
???? 			view.sync();
Pixels
supports the following
methods:
Pixel Cache Methods
Method
Returns
Signature
Description
get
PixelPacket.html
PixelPacket
*
const int x_, const int y_, const unsigned
const 			int columns_, const unsigned int rows_
Transfers read-write 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
sync
. The value returned is intended for pixel access
only. It should never be deallocated.
getConst
const
PixelPacket.html
PixelPacket
*
const int x_, const int y_, const unsigned int
columns_, const unsigned int rows_
Transfers read-only pixels from the image to
the 			pixel cache as defined by the specified rectangular region.
set
PixelPacket.html
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
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.
indexes
IndexPacket*
void
Returns the PsuedoColor pixel indexes
corresponding to the pixel region defined by the last
#get
get
,
#getConst
getConst
, or
#set
set
call. Only valid for PseudoColor and CMYKA images. The pixel
indexes (an array of type
IndexPacket
, which is typedef
Quantum
,
which is itself typedef
unsigned char
, or
unsigned short
,
depending on the value of the
QuantumDepth
define) provide
the
colormap
index 			(see
Image.html#colorMap
colorMap
) for each pixel in 			the
image. For CMYKA images, the indexes represent the matte 			(alpha)
channel. The value returned is intended for pixel access 			only. It
should never be deallocated.
x
int
void
Left ordinate of view
y
int
void
Top ordinate of view
columns
unsigned int
void
Width of view
rows
unsigned int
void
Height of view
