GlPixelStore

_glPixelStore: set pixel storage modes

Syntax

 SUB _glPixelStoref (BYVAL pname AS _UNSIGNED LONG, BYVAL param AS SINGLE)
 void _glPixelStoref(GLenum pname, GLfloat param);

 SUB _glPixelStorei (BYVAL pname AS _UNSIGNED LONG, BYVAL param AS LONG)
 void _glPixelStorei(GLenum pname, GLint param);

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 _glReadPixels as well as the unpacking of texture patterns (see _glTexImage1D, _glTexImage2D, _glTexImage3D, _glTexSubImage1D, _glTexSubImage2D, _glTexSubImage3D), _glCompressedTexImage1D, _glCompressedTexImage2D, _glCompressedTexImage3D, _glCompressedTexSubImage1D, _glCompressedTexSubImage2D or _glCompressedTexSubImage1D.

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, or stencil indices is reversed. That is, if a four-byte component consists of bytes b0, b1, b2, b3, it is stored in memory as b3, b2, b1, b0 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 format 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.
_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; <math>k =

\begin{cases}

nl, & s \ge a\\
\tfrac{a}{s}\left \lceil \tfrac{snl}{a} \right \rceil, & s < a

\end{cases}</math>

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

<math>k = 8a\left \lceil \tfrac{nl}{8a} \right \rceil</math>

components or indices.

The word component in this description refers to the nonindex values red, green, blue, alpha, and depth. Storage format _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; <math>k =

\begin{cases}

nlh, & s \ge a\\
\tfrac{a}{s}\left \lceil \tfrac{snlh}{a} \right \rceil, & s < a

\end{cases}</math>

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).

The word component in this description refers to the nonindex values red, green, blue, alpha, and depth. Storage format _GL_RGB, for example, has three components per pixel: first red, then green, and finally blue.
_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 in 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 jm 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 kp, 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 _glTexImage1D, _glTexImage2D, _glTexImage3D, _glTexSubImage1D, _glTexSubImage2D, and _glTexSubImage3D

They are as follows:

_GL_UNPACK_SWAP_BYTES: If true, byte ordering for multibyte color components, depth components, or stencil indices is reversed. That is, if a four-byte component consists of bytes b0, b1, b2, b3, it is taken from memory as b3, b2, b1, b0 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 format 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.
_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; <math>k =

\begin{cases}

nl, & s \ge a\\
\tfrac{a}{s}\left \lceil \tfrac{snl}{a} \right \rceil, & s < a

\end{cases}</math>

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

<math>k = 8a\left \lceil \tfrac{nl}{8a} \right \rceil</math>

components or indices.

The word component in this description refers to the nonindex values red, green, blue, alpha, and depth. Storage format _GL_RGB, for example, has three components per pixel: first red, then green, and finally blue.
_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; <math>k =

\begin{cases}

nlh, & s \ge a\\
\tfrac{a}{s}\left \lceil \tfrac{snlh}{a} \right \rceil, & s < a

\end{cases}</math> 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). The word component in this description refers to the nonindex values red, green, blue, alpha, and depth. Storage format _GL_RGB, for example, has three components per pixel: first red, then green, and finally blue.

_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 glTexImage1D, glTexImage2D, glTexSubImage1D or glTexSubImage2D. Setting _GL_UNPACK_SKIP_PIXELS to i is equivalent to incrementing the pointer by in 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 jm components or indices, where m is the number of components or indices per row, as just computed in the _GL_UNPACK_ROW_LENGTH section.
_GL_UNPACK_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 following table gives the type, initial value, and range of valid values for each storage parameter that can be set with _glPixelStore.


pname	Type	Initial Value	Valid Range
_GL_PACK_SWAP_BYTES	boolean	false	true or false
_GL_PACK_LSB_FIRST	boolean	false	true or false
_GL_PACK_ROW_LENGTH	integer	0	[0, ∞)
_GL_PACK_IMAGE_HEIGHT	integer	0	[0, ∞)
_GL_PACK_SKIP_ROWS	integer	0	[0, ∞)
_GL_PACK_SKIP_PIXELS	integer	0	[0, ∞)
_GL_PACK_SKIP_IMAGES	integer	0	[0, ∞)
_GL_PACK_ALIGNMENT	integer	4	1, 2, 4, or 8
_GL_UNPACK_SWAP_BYTES	boolean	false	true or false
_GL_UNPACK_LSB_FIRST	boolean	false	true or false
_GL_UNPACK_ROW_LENGTH	integer	0	[0, ∞)
_GL_UNPACK_IMAGE_HEIGHT	integer	0	[0, ∞)
_GL_UNPACK_SKIP_ROWS	integer	0	[0, ∞)
_GL_UNPACK_SKIP_PIXELS	integer	0	[0, ∞)
_GL_UNPACK_SKIP_IMAGES	integer	0	[0, ∞)
_GL_UNPACK_ALIGNMENT	integer	4	1, 2, 4, or 8

_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.

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.

Use With:

_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