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GLPIXELSTORE

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GLPIXELSTORE

NAME
C SPECIFICATION
PARAMETERS
DESCRIPTION
NOTES
ERRORS
ASSOCIATED GETS
SEE ALSO

NAME

glPixelStoref, glPixelStorei − set pixel storage modes

C SPECIFICATION

void glPixelStoref( GLenum pname,

GLfloat param )

void glPixelStorei( GLenum pname,

GLint param )

PARAMETERS

	pname		Specifies the symbolic name of the parameter to be set. Six values affect the packing of pixel data into memory: GL_PACK_SWAP_BYTES, GL_PACK_LSB_FIRST, GL_PACK_ROW_LENGTH, GL_PACK_IMAGE_HEIGHT, GL_PACK_SKIP_PIXELS, GL_PACK_SKIP_ROWS, GL_PACK_SKIP_IMAGES, and GL_PACK_ALIGNMENT. Six more affect the unpacking of pixel data from memory: GL_UNPACK_SWAP_BYTES, GL_UNPACK_LSB_FIRST, GL_UNPACK_ROW_LENGTH, GL_UNPACK_IMAGE_HEIGHT, GL_UNPACK_SKIP_PIXELS, GL_UNPACK_SKIP_ROWS, GL_UNPACK_SKIP_IMAGES, and GL_UNPACK_ALIGNMENT.
	param		Specifies the value that pname is set to.

DESCRIPTION

glPixelStore sets pixel storage modes that affect the operation of subsequent glDrawPixels and glReadPixels as well as the unpacking of polygon stipple patterns (see glPolygonStipple), bitmaps (see glBitmap), texture patterns (see glTexImage1D, glTexImage2D, glTexImage3D, glTexSubImage1D, glTexSubImage2D, glTexSubImage3D). Additionally, if the GL_ARB_imaging extension is supported, pixle storage modes affect convlution filters (see glConvolutionFilter1D, glConvolutionFilter2D, and glSeparableFilter2D, color table (see glColorTable, and glColorSubTable, and unpacking histogram (See glHistogram), and minmax (See glMinmax) data.

pname is a symbolic constant indicating the parameter to be set, and param is the new value. Six of the twelve storage parameters affect how pixel data is returned to client memory. They are as follows:
GL_PACK_SWAP_BYTES

If true, byte ordering for multibyte color components, depth components, color indices, or stencil indices is reversed. That is, if a four-byte component consists of bytes $b sub 0$, $b sub 1$, $b sub 2$, $b sub 3$, it is stored in memory as $b sub 3$, $b sub 2$, $b sub 1$, $b sub 0$ if GL_PACK_SWAP_BYTES is true. GL_PACK_SWAP_BYTES has no effect on the memory order of components within a pixel, only on the order of bytes within components or indices. For example, the three components of a GL_RGB pixel are always stored with red first, green second, and blue third, regardless of the value of GL_PACK_SWAP_BYTES.

GL_PACK_LSB_FIRST

If true, bits are ordered within a byte from least significant to most significant; otherwise, the first bit in each byte is the most significant one. This parameter is significant for bitmap data only.

GL_PACK_ROW_LENGTH

If greater than 0, GL_PACK_ROW_LENGTH defines the number of pixels in a row. If the first pixel of a row is placed at location $p$ in memory, then the location of the first pixel of the next row is obtained by skipping

$k ~=~~ left { ^ lpile { n l above {a over s left ceiling { s n l } over a right ceiling}} ~~ lpile {s ~>=~ a above s ~<~ a }$

components or indices, where $n$ is the number of components or indices in a pixel, $l$ is the number of pixels in a row (GL_PACK_ROW_LENGTH if it is greater than 0, the $width$ argument to the pixel routine otherwise), $a$ is the value of GL_PACK_ALIGNMENT, and $s$ is the size, in bytes, of a single component (if $ a < s$, then it is as if $a ~=~ s$). In the case of 1-bit values, the location of the next row is obtained by skipping

$k ~=~ 8 a left ceiling { n l } over { 8 a } ^ right ceiling$

components or indices.

The word component in this description refers to the nonindex values red, green, blue, alpha, and depth. Storage GL_RGB, for example, has three components per pixel: first red, then green, and finally blue.

GL_PACK_IMAGE_HEIGHT

If greater than 0, GL_PACK_IMAGE_HEIGHT defines the number of pixels in an image three-dimensional texture volume. Where ‘‘image’’ is defined by all pixels sharing the same third dimension index. If the first pixel of a row is placed at location $p$ in memory, then the location of the first pixel of the next row is obtained by skipping

$k ~=~~ left { ~ lpile { n l h above {a over s left ceiling { s n l h } over a ^ right ceiling}} ~~ lpile {s ~>=~ a above s ~<~ a }$

components or indices, where $n$ is the number of components or indices in a pixel, $l$ is the number of pixels in a row (GL_PACK_ROW_LENGTH if it is greater than 0, the $width$ argument to glTexImage3d otherwise), $h$ is the number of rows in a pixel image (GL_PACK_IMAGE_HEIGHT if it is greater than 0, the $height$ argument to the glTexImage3D routine otherwise), $a$ is the value of GL_PACK_ALIGNMENT, and $s$ is the size, in bytes, of a single component (if $ a < s$, then it is as if $a = s$).

GL_PACK_SKIP_PIXELS, GL_PACK_SKIP_ROWS, and GL_PACK_SKIP_IMAGES

These values are provided as a convenience to the programmer; they provide no functionality that cannot be duplicated simply by incrementing the pointer passed to glReadPixels. Setting GL_PACK_SKIP_PIXELS to $i$ is equivalent to incrementing the pointer by $i n$ components or indices, where $n$ is the number of components or indices in each pixel. Setting GL_PACK_SKIP_ROWS to $j$ is equivalent to incrementing the pointer by $j m$ components or indices, where $m$ is the number of components or indices per row, as just computed in the GL_PACK_ROW_LENGTH section. Setting GL_PACK_SKIP_IMAGES to $k$ is equivalent to incrementing the pointer by $k p$, where $p$ is the number of components or indices per image, as computed in the GL_PACK_IMAGE_HEIGHT section.

GL_PACK_ALIGNMENT

Specifies the alignment requirements for the start of each pixel row in memory. The allowable values are 1 (byte-alignment), 2 (rows aligned to even-numbered bytes), 4 (word-alignment), and 8 (rows start on double-word boundaries).

The other six of the twelve storage parameters affect how pixel data is read from client memory. These values are significant for glDrawPixels, glTexImage1D, glTexImage2D, glTexImage3D, glTexSubImage1D, glTexSubImage2D, glTexSubImage3D, glBitmap, and glPolygonStipple.

Additionally, if the GL_ARB_imaging extension is supported, glColorTable, glColorSubTable, glConvolutionFilter1D, glConvolutionFilter2D, and glSeparableFilter2D. They are as follows:
GL_UNPACK_SWAP_BYTES

If true, byte ordering for multibyte color components, depth components, color indices, or stencil indices is reversed. That is, if a four-byte component consists of bytes $b sub 0$, $b sub 1$, $b sub 2$, $b sub 3$, it is taken from memory as $b sub 3$, $b sub 2$, $b sub 1$, $b sub 0$ if GL_UNPACK_SWAP_BYTES is true. GL_UNPACK_SWAP_BYTES has no effect on the memory order of components within a pixel, only on the order of bytes within components or indices. For example, the three components of a GL_RGB pixel are always stored with red first, green second, and blue third, regardless of the value of GL_UNPACK_SWAP_BYTES.

GL_UNPACK_LSB_FIRST

If true, bits are ordered within a byte from least significant to most significant; otherwise, the first bit in each byte is the most significant one. This is relevant only for bitmap data.

GL_UNPACK_ROW_LENGTH

If greater than 0, GL_UNPACK_ROW_LENGTH defines the number of pixels in a row. If the first pixel of a row is placed at location $p$ in memory, then the location of the first pixel of the next row is obtained by skipping

$k ~=~~ left { ~ lpile { n l above {a over s left ceiling { s n l } over a ^ right ceiling}} ~~ lpile {s ~>=~ a above s ~<~ a }$

components or indices, where $n$ is the number of components or indices in a pixel, $l$ is the number of pixels in a row (- GL_UNPACK_ROW_LENGTH if it is greater than 0, the $width$ argument to the pixel routine otherwise), $a$ is the value of GL_UNPACK_ALIGNMENT, and $s$ is the size, in bytes, of a single component (if $ a < s$, then it is as if $a = s$). In the case of 1-bit values, the location of the next row is obtained by skipping

$k ~=~ 8 a left ceiling { n l } over { 8 a } right ceiling$

components or indices.

GL_UNPACK_IMAGE_HEIGHT

If greater than 0, GL_UNPACK_IMAGE_HEIGHT defines the number of pixels in an image of a three-dimensional texture volume. Where ‘‘image’’ is defined by all pixel sharing the same third dimension index. If the first pixel of a row is placed at location $p$ in memory, then the location of the first pixel of the next row is obtained by skipping

$k ~=~~ left {~ lpile { n l h above {a over s left ceiling { s n l h } over a ^ right ceiling}} ~~ lpile {s ~ >=~ a above s ~<~ a }$

components or indices, where $n$ is the number of components or indices in a pixel, $l$ is the number of pixels in a row (- GL_UNPACK_ROW_LENGTH if it is greater than 0, the $width$ argument to glTexImage3D otherwise), $h$ is the number of rows in an image (GL_UNPACK_IMAGE_HEIGHT if it is greater than 0, the $height$ argument to glTexImage3D otherwise), $a$ is the value of GL_UNPACK_ALIGNMENT, and $s$ is the size, in bytes, of a single component (if $ a < s$, then it is as if $a ~=~ s$).

GL_UNPACK_SKIP_PIXELS and GL_UNPACK_SKIP_ROWS

These values are provided as a convenience to the programmer; they provide no functionality that cannot be duplicated by incrementing the pointer passed to glDrawPixels, glTexImage1D, glTexImage2D, glTexSubImage1D, glTexSubImage2D, glBitmap, or glPolygonStipple. Setting GL_UNPACK_SKIP_PIXELS to $i$ is equivalent to incrementing the pointer by $i n$ components or indices, where $n$ is the number of components or indices in each pixel. Setting GL_UNPACK_SKIP_ROWS to $j$ is equivalent to incrementing the pointer by $j k$ components or indices, where $k$ is the number of components or indices per row, as just computed in the GL_UNPACK_ROW_LENGTH section.

GL_UNPACK_ALIGNMENT

The following table gives the type, initial value, and range of valid values for each storage parameter that can be set with glPixelStore.

glPixelStoref can be used to set any pixel store parameter. If the parameter type is boolean, then if param is 0, the parameter is false; otherwise it is set to true. If pname is a integer type parameter, param is rounded to the nearest integer.

Likewise, glPixelStorei can also be used to set any of the pixel store parameters. Boolean parameters are set to false if param is 0 and true otherwise.

NOTES

The pixel storage modes in effect when glDrawPixels, glReadPixels, glTexImage1D, glTexImage2D, glTexImage3D, glTexSubImage1D, glTexSubImage2D, glTexSubImage3D, glBitmap, or glPolygonStipple is placed in a display list control the interpretation of memory data. Likewise, if the GL_ARB_imaging extension is supported, the pixel storage modes in effect when glColorTable, glColorSubTable, glConvolutionFilter1D, glConvolutionFilter2D, of glSeparableFilter2D is placed in a display list control the interpretation of memory data. The pixel storage modes in effect when a display list is executed are not significant.

Pixel storage modes are client state and must be pushed and restored using
glPushClientAttrib and glPopClientAttrib.

ERRORS

GL_INVALID_ENUM is generated if pname is not an accepted value.

GL_INVALID_VALUE is generated if a negative row length, pixel skip, or row skip value is specified, or if alignment is specified as other than 1, 2, 4, or 8.

GL_INVALID_OPERATION is generated if glPixelStore is executed between the execution of glBegin and the corresponding execution of glEnd.

ASSOCIATED GETS

glGet with argument GL_PACK_SWAP_BYTES
glGet with argument GL_PACK_LSB_FIRST
glGet with argument GL_PACK_ROW_LENGTH
glGet with argument GL_PACK_IMAGE_HEIGHT
glGet with argument GL_PACK_SKIP_ROWS
glGet with argument GL_PACK_SKIP_PIXELS
glGet with argument GL_PACK_SKIP_IMAGES
glGet with argument GL_PACK_ALIGNMENT
glGet with argument GL_UNPACK_SWAP_BYTES
glGet with argument GL_UNPACK_LSB_FIRST
glGet with argument GL_UNPACK_ROW_LENGTH
glGet with argument GL_UNPACK_IMAGE_HEIGHT
glGet with argument GL_UNPACK_SKIP_ROWS
glGet with argument GL_UNPACK_SKIP_PIXELS
glGet with argument GL_UNPACK_SKIP_IMAGES
glGet with argument GL_UNPACK_ALIGNMENT

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