1991-2006
Silicon Graphics, Inc.
glReadPixels
3G

# glReadPixels

read a block of pixels from the frame buffer

## C Specification

                    void glReadPixels
                    GLint x
                    GLint y
                    GLsizei width
                    GLsizei height
                    GLenum format
                    GLenum type
                    GLvoid * data

## Parameters

`x`
`y`

  Specify the window coordinates of the first pixel that is read from the frame buffer. This location is the lower left corner of a rectangular block of pixels.

`width`
`height`

  Specify the dimensions of the pixel rectangle. `width` and `height` of one correspond to a single pixel.

`format`

  Specifies the format of the pixel data. The following symbolic values are accepted: `GL_ALPHA`, `GL_RGB`, and `GL_RGBA`.

`type`

  Specifies the data type of the pixel data. Must be one of `GL_UNSIGNED_BYTE`, `GL_UNSIGNED_SHORT_5_6_5`, `GL_UNSIGNED_SHORT_4_4_4_4`, or `GL_UNSIGNED_SHORT_5_5_5_1`.

`data`

  Returns the pixel data.

## Description

`glReadPixels` returns pixel data from the frame buffer, starting with the pixel whose lower left corner is at location (`x`, `y`), into client memory starting at location `data`. The `GL_PACK_ALIGNMENT` parameter, set with the glPixelStorei command, affects the processing of the pixel data before it is placed into client memory.

`glReadPixels` returns values from each pixel with lower left corner at x + i y + j for 0 <= i < width and 0 <= j < height . This pixel is said to be the ith pixel in the jth row. Pixels are returned in row order from the lowest to the highest row, left to right in each row.

`format` specifies the format for the returned pixel values; accepted values are:

`GL_ALPHA`
`GL_RGB`
`GL_RGBA`

RGBA color components are read from the color buffer. Each color component is converted to floating point such that zero intensity maps to 0.0 and full intensity maps to 1.0.

Unneeded data is then discarded. For example, `GL_ALPHA` discards the red, green, and blue components, while `GL_RGB` discards only the alpha component. The final values are clamped to the range 0 1 .

Finally, the components are converted to the proper format, as specified by `type`. When `type` is `GL_UNSIGNED_BYTE`, each component is multiplied by 2 8 - 1 . When `type` is `GL_UNSIGNED_SHORT_5_6_5`, `GL_UNSIGNED_SHORT_4_4_4_4`, or `GL_UNSIGNED_SHORT_5_5_5_1`, each component is multiplied by 2 N - 1 , where N is the number of bits in the bitfield.

Return values are placed in memory as follows. If `format` is `GL_ALPHA`, a single value is returned and the data for the ith pixel in the jth row is placed in location j &it; width + i . `GL_RGB` returns three values and `GL_RGBA` returns four values for each pixel, with all values corresponding to a single pixel occupying contiguous space in `data`. Storage parameter `GL_PACK_ALIGNMENT`, set by glPixelStorei, affects the way that data is written into memory. See glPixelStorei for a description.

## Notes

If the currently bound framebuffer is not the default framebuffer object, color components are read from the color image attached to the `GL_COLOR_ATTACHMENT0` attachment point.

Only two `format`/`type` parameter pairs are accepted. `GL_RGBA`/`GL_UNSIGNED_BYTE` is always accepted, and the other acceptable pair can be discovered by querying `GL_IMPLEMENTATION_COLOR_READ_FORMAT` and `GL_IMPLEMENTATION_COLOR_READ_TYPE`.

Values for pixels that lie outside the window connected to the current GL context are undefined.

If an error is generated, no change is made to the contents of `data`.

## Errors

`GL_INVALID_ENUM` is generated if `format` or `type` is not an accepted value.

`GL_INVALID_VALUE` is generated if either `width` or `height` is negative.

`GL_INVALID_OPERATION` is generated if `type` is `GL_UNSIGNED_SHORT_5_6_5` and `format` is not `GL_RGB`.

`GL_INVALID_OPERATION` is generated if `type` is `GL_UNSIGNED_SHORT_4_4_4_4` or `GL_UNSIGNED_SHORT_5_5_5_1` and `format` is not `GL_RGBA`.

`GL_INVALID_OPERATION` is generated if `format` and `type` are neither `GL_RGBA` and `GL_UNSIGNED_BYTE`, respectively, nor the format/type pair returned by querying `GL_IMPLEMENTATION_COLOR_READ_FORMAT` and `GL_IMPLEMENTATION_COLOR_READ_TYPE`.

`GL_INVALID_FRAMEBUFFER_OPERATION` is generated if the currently bound framebuffer is not framebuffer complete (i.e. the return value from glCheckFramebufferStatus is not `GL_FRAMEBUFFER_COMPLETE`).

## Associated Gets

glGet with argument `GL_IMPLEMENTATION_COLOR_READ_FORMAT` or `GL_IMPLEMENTATION_COLOR_READ_TYPE`

glGet with argument `GL_PACK_ALIGNMENT`

## See Also

glCheckFramebufferStatus, glPixelStorei

## 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/.