Name

glReadPixels — read a block of pixels from the color buffer

C Specification

void glReadPixels(GLint x,
 GLint y,
 GLsizei width,
 GLsizei height,
 GLenum format,
 GLenum type,
 GLvoid * pixels);
 

Parameters

x, y

Specify the window coordinates of the first pixel that is read from the color 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. Must be either GL_RGBA or the value of GL_IMPLEMENTATION_COLOR_READ_FORMAT_OES.

type

Specifies the data type of the pixel data. Must be either GL_UNSIGNED_BYTE or the value of GL_IMPLEMENTATION_COLOR_READ_TYPE_OES.

pixels

Returns the pixel data.

Description

glReadPixels returns pixel data from the color buffer, starting with the pixel whose lower left corner is at location (x, y), into client memory starting at location pixels. The processing of the pixel data before it is placed into client memory can be controlled with glPixelStorei.

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 of 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. GL_LUMINANCE computes a single-component value as the sum of the red, green, and blue components, and GL_LUMINANCE_ALPHA does the same, while keeping alpha as a second value. The final values are clamped to the range [0, 1].

Finally, the components are converted to the proper, as specified by type where each component is multiplied by 2n-1 , where n is the number of bits per component.

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*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 pixels. Storage parameter GL_PACK_ALIGNMENT set by glPixelStorei, affects the way that data is written into memory. See glPixelStorei for a description.

Notes

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_OES and GL_IMPLEMENTATION_COLOR_READ_TYPE_OES.

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 pixels.

Errors

GL_INVALID_ENUM is generated if format is not GL_RGBA or the value of GL_IMPLEMENTATION_COLOR_READ_FORMAT_OES.

GL_INVALID_ENUM is generated if type is not GL_UNSIGNED_BYTE or the value of GL_IMPLEMENTATION_COLOR_READ_TYPE_OES.

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_OES and GL_IMPLEMENTATION_COLOR_READ_TYPE_OES.

Associated Gets

glGet with argument GL_IMPLEMENTATION_COLOR_READ_FORMAT_OES

glGet with argument GL_IMPLEMENTATION_COLOR_READ_TYPE_OES

glGet with argument GL_PACK_ALIGNMENT

See Also

glPixelStorei

Copyright

Copyright © 2003-2004 Silicon Graphics, Inc. This document is licensed under the SGI Free Software B License. For details, see http://oss.sgi.com/projects/FreeB/.