1991-2006
Silicon Graphics, Inc.
glPixelStore
3G

# glPixelStore

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 `ARB_imaging` extension is supported, pixel storage modes affect convolution 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 0 , b 1 , b 2 , b 3 , it is stored in memory as b 3 , b 2 , b 1 , b 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` 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 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 = n &it; l a s &it; s &it; n &it; l a &it; s >= a 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 &it; a &it; n &it; l 8 &it; a

  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

  k = n &it; l &it; h a s &it; s &it; n &it; l &it; h a &it; s >= a 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 ).

  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 i &it; 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 &it; 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 &it; 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 `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 0 , b 1 , b 2 , b 3 , it is taken from memory as b 3 , b 2 , b 1 , b 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` 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 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 = n &it; l a s &it; s &it; n &it; l a &it; s >= a 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 &it; a &it; n &it; l 8 &it; a

  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

  k = n &it; l &it; h a s &it; s &it; n &it; l &it; h a &it; s >= a 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 ).

  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 glDrawPixels, glTexImage1D, glTexImage2D, glTexSubImage1D, glTexSubImage2D, glBitmap, or glPolygonStipple. Setting `GL_UNPACK_SKIP_PIXELS` to i is equivalent to incrementing the pointer by i &it; 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 &it; 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`

  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.

## 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 `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`

## See Also

glBitmap, glColorTable, glColorSubTable, glConvolutionFilter1D, glConvolutionFilter2D, glSeparableFilter2D, glDrawPixels, glHistogram, glMinmax, glPixelMap, glPixelTransfer, glPixelZoom, glPolygonStipple, glPushClientAttrib, glReadPixels, glTexImage1D, glTexImage2D, glTexImage3D, glTexSubImage1D, glTexSubImage2D, glTexSubImage3D

## Copyright

Copyright 1991-2006 Silicon Graphics, Inc. This document is licensed under the SGI Free Software B License. For details, see http://oss.sgi.com/projects/FreeB/.