glPixelStore

NAME

C SPECIFICATION

void glPixelStoref(GLenum pname,
                   GLfloat param)
void glPixelStorei(GLenum pname,
                   GLint param)

PARAMETERS

DESCRIPTION

pname is a symbolic constant indicating the parameter to be set, and param is the new value. The "pack" 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_0,, b₁, b₂, b₃, it is stored in memory as b_3,, 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. This parameter is significant for bitmap data only.

GL_PACK_ROW_LENGTH

If greater than zero, 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, the location of the first pixel of the next row is obtained by skipping

If (s >= a), k = nl
else, k = (a / s) * CEILING(snl / 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 = 8a * CEILING(nl / 8a)

components or indices.

The word component in this description refers to the non-index 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 and GL_PACK_SKIP_ROWS

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 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 jk components or indices, where k is the number of components or indices per row, as just computed in the GL_PACK_ROW_LENGTH 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).

GL_PACK_IMAGE_HEIGHT_EXT

If greater than zero, GL_PACK_IMAGE_HEIGHT_EXT defines the number of rows in each two-dimensional slice of a three-diminsional image.

GL_PACK_SKIP_IMAGES_EXT

This value is provided as a convenience to the programmer. Setting GL_PACK_SKIP_IMAGES_EXT to i is equivalent to incrementing the pointer past the first i two-dimensional slices of a three-dimensional image.

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_0,, b₁, b₂, b₃, it is taken from memory as b_3,, 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. This is relevant only for bitmap data.

GL_UNPACK_ROW_LENGTH

If greater than zero, 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, the location of the first pixel of the next row is obtained by skipping

If (s >= a), k = nl
else, k = (a / s) * CEILING(snl / 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 = 8a * CEILING(nl / 8a)

components or indices.

The word component in this description refers to the non-index 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 to the image. Setting GL_UNPACK_SKIP_PIXELS to iis 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 jk 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

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).

GL_UNPACK_IMAGE_HEIGHT_EXT

If greater than zero, GL_PACK_IMAGE_HEIGHT_EXT defines the number of rows in each two-dimensional slice of a three-diminsional image.

GL_UNPACK_SKIP_IMAGES_EXT

This value is provided as a convenience to the programmer. Setting GL_PACK_SKIP_IMAGES_EXT to i is equivalent to incrementing the pointer past the first i two-dimensional slices of a three-dimensional image.

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_SKIP_ROWS	integer	0	[0,∞)
GL_PACK_SKIP_PIXELS	integer	0	[0,∞)
GL_PACK_ALIGNMENT	integer	4	1, 2, 4, or 8
GL_PACK_IMAGE_HEIGHT	integer	0	[0,∞)
GL_PACK_SKIP_IMAGES	integer	0	[0,∞)
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_SKIP_ROWS	integer	0	[0,∞)
GL_UNPACK_SKIP_PIXELS	integer	0	[0,∞)
GL_UNPACK_ALIGNMENT	integer	4	1, 2, 4, or 8
GL_UNPACK_IMAGE_HEIGHT	integer	0	[0,∞)
GL_UNPACK_SKIP_IMAGES	integer	0	[0,∞)

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

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

EXTENSIONS

GL_EXT_texture3D

GL_PACK_IMAGE_HEIGHT_EXT, GL_PACK_SKIP_IMAGES_EXT, GL_UNPACK_IMAGE_HEIGHT_EXT, and GL_UNPACK_SKIP_IMAGES_EXT are part of the EXT_texture3D extension.

ERRORS

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

SEE ALSO