GLAPI/glReadPixels

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glReadPixels
Core in version 4.4
Core since version 1.0

glReadPixels: read a block of pixels from the frame buffer

Function Definition

 void glReadPixels(GLint x​, GLint y​, GLsizei width​, GLsizei height​, GLenum format​, GLenum type​, GLvoid * data​);
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. For transfers of depth, stencil, or depth/stencil data, you must use GL_DEPTH_COMPONENT​, GL_STENCIL_INDEX​, or GL_DEPTH_STENCIL​, where appropriate. For transfers of normalized integer or floating-point color image data, you must use one of the following: GL_RED​, GL_GREEN​, GL_BLUE​, GL_RG​, GL_RGB​, GL_BGR​, GL_RGBA​, and GL_BGRA​. For transfers of non-normalized integer data, you must use one of the following: GL_RED_INTEGER​, GL_GREEN_INTEGER​, GL_BLUE_INTEGER​, GL_RG_INTEGER​, GL_RGB_INTEGER​, GL_BGR_INTEGER​, GL_RGBA_INTEGER​, and GL_BGRA_INTEGER​. Even if no actual pixel transfer is made (data​ is NULL and no buffer is bound to GL_PIXEL_UNPACK_BUFFER​), you must set this parameter correctly for the internal format of the destination image.
type
Specifies the data type of the pixel data. The following symbolic values are accepted: GL_UNSIGNED_BYTE​, GL_BYTE​, GL_UNSIGNED_SHORT​, GL_SHORT​, GL_UNSIGNED_INT​, GL_INT​, GL_FLOAT​, GL_UNSIGNED_BYTE_3_3_2​, GL_UNSIGNED_BYTE_2_3_3_REV​, GL_UNSIGNED_SHORT_5_6_5​, GL_UNSIGNED_SHORT_5_6_5_REV​, GL_UNSIGNED_SHORT_4_4_4_4​, GL_UNSIGNED_SHORT_4_4_4_4_REV​, GL_UNSIGNED_SHORT_5_5_5_1​, GL_UNSIGNED_SHORT_1_5_5_5_REV​, GL_UNSIGNED_INT_8_8_8_8​, GL_UNSIGNED_INT_8_8_8_8_REV​, GL_UNSIGNED_INT_10_10_10_2​, and GL_UNSIGNED_INT_2_10_10_10_REV​.
data
Specifies a pointer to the image data in memory, or if a buffer is bound to GL_PIXEL_PACK_BUFFER​, this provides an integer offset into the bound buffer object. If a buffer is not bound to GL_PIXEL_PACK_BUFFER​, this parameter may not be NULL.


Description

glReadPixels returns pixel data from the read framebuffer (the buffer bound to GL_READ_FRAMEBUFFER​), starting with the pixel whose lower left corner is at location (x​, y​), into client memory starting at location data​. Several parameters control the processing of the pixel data before it is placed into client memory. These parameters are set with glPixelStore. This reference page describes the effects on glReadPixels of most, but not all of the parameters specified by these three commands.

If a non-zero named buffer object is bound to the GL_PIXEL_PACK_BUFFER​ target (see glBindBuffer) while a block of pixels is requested, data​ is treated as a byte offset into the buffer object's data store rather than a pointer to 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_STENCIL_INDEX​
Stencil values are read from the stencil buffer.
GL_DEPTH_COMPONENT​
Depth values are read from the depth buffer. Each component is converted to floating point such that the minimum depth value maps to 0 and the maximum value maps to 1. Each component is then multiplied by GL_DEPTH_SCALE​, added to GL_DEPTH_BIAS​, and finally clamped to the range [0, 1].
GL_DEPTH_STENCIL​
Values are taken from both the depth and stencil buffers. The type​ parameter must be GL_UNSIGNED_INT_24_8​ or GL_FLOAT_32_UNSIGNED_INT_24_8_REV​.
GL_RED​
GL_GREEN​
GL_BLUE​
GL_RGB​
GL_BGR​
GL_RGBA​
GL_BGRA​
Finally, the indices or components are converted to the proper format, as specified by type​. If format​ is GL_STENCIL_INDEX​ and type​ is not GL_FLOAT​, each index is masked with the mask value given in the following table. If type​ is GL_FLOAT​, then each integer index is converted to single-precision floating-point format.

If format​ is GL_RED​, GL_GREEN​, GL_BLUE​, GL_RGB​, GL_BGR​, GL_RGBA​, or GL_BGRA​ and type​ is not GL_FLOAT​, each component is multiplied by the multiplier shown in the following table. If type is GL_FLOAT​, then each component is passed as is (or converted to the client's single-precision floating-point format if it is different from the one used by the GL).

type​ Index Mask Component Conversion
GL_UNSIGNED_BYTE​ 28 - 1
GL_BYTE​ 27 - 1
GL_UNSIGNED_SHORT​ 216 - 1
GL_SHORT​ 215 - 1
GL_UNSIGNED_INT​ 232 - 1
GL_INT​ 231 - 1
GL_HALF_FLOAT​ none c
GL_FLOAT​ none c
GL_UNSIGNED_BYTE_3_3_2​ 2N - 1
GL_UNSIGNED_BYTE_2_3_3_REV​ 2N - 1
GL_UNSIGNED_SHORT_5_6_5​ 2N - 1
GL_UNSIGNED_SHORT_5_6_5_REV​ 2N - 1
GL_UNSIGNED_SHORT_4_4_4_4​ 2N - 1
GL_UNSIGNED_SHORT_4_4_4_4_REV​ 2N - 1
GL_UNSIGNED_SHORT_5_5_5_1​ 2N - 1
GL_UNSIGNED_SHORT_1_5_5_5_REV​ 2N - 1
GL_UNSIGNED_INT_8_8_8_8​ 2N - 1
GL_UNSIGNED_INT_8_8_8_8_REV​ 2N - 1
GL_UNSIGNED_INT_10_10_10_2​ 2N - 1
GL_UNSIGNED_INT_2_10_10_10_REV​ 2N - 1
GL_UNSIGNED_INT_24_8​ 2N - 1
GL_UNSIGNED_INT_10F_11F_11F_REV​ -- Special
GL_UNSIGNED_INT_5_9_9_9_REV​ -- Special
GL_FLOAT_32_UNSIGNED_INT_24_8_REV​ none c (Depth Only)

Return values are placed in memory as follows. If format​ is GL_STENCIL_INDEX​, GL_DEPTH_COMPONENT​, GL_RED​, GL_GREEN​, or GL_BLUE​, 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​ and GL_BGR​ return three values, GL_RGBA​ and GL_BGRA​ return four values for each pixel, with all values corresponding to a single pixel occupying contiguous space in data​. Storage parameters set by glPixelStore, such as GL_PACK_LSB_FIRST​ and GL_PACK_SWAP_BYTES​, affect the way that data is written into memory. See glPixelStore for a description.

Notes

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 format​ is GL_STENCIL_INDEX​ and there is no stencil buffer.

GL_INVALID_OPERATION​ is generated if format​ is GL_DEPTH_COMPONENT​ and there is no depth buffer.

GL_INVALID_OPERATION​ is generated if format​ is GL_DEPTH_STENCIL​ and there is no depth buffer or if there is no stencil buffer.

GL_INVALID_ENUM​ is generated if format​ is GL_DEPTH_STENCIL​ and type​ is not GL_UNSIGNED_INT_24_8​ or GL_FLOAT_32_UNSIGNED_INT_24_8_REV​.

GL_INVALID_OPERATION​ is generated if type​ is one of GL_UNSIGNED_BYTE_3_3_2​, GL_UNSIGNED_BYTE_2_3_3_REV​, GL_UNSIGNED_SHORT_5_6_5​, or GL_UNSIGNED_SHORT_5_6_5_REV​ and format​ is not GL_RGB​.

GL_INVALID_OPERATION​ is generated if type​ is one of GL_UNSIGNED_SHORT_4_4_4_4​, GL_UNSIGNED_SHORT_4_4_4_4_REV​, GL_UNSIGNED_SHORT_5_5_5_1​, GL_UNSIGNED_SHORT_1_5_5_5_REV​, GL_UNSIGNED_INT_8_8_8_8​, GL_UNSIGNED_INT_8_8_8_8_REV​, GL_UNSIGNED_INT_10_10_10_2​, or GL_UNSIGNED_INT_2_10_10_10_REV​ and format​ is neither GL_RGBA​ nor GL_BGRA​.

GL_INVALID_OPERATION​ is generated if a non-zero buffer object name is bound to the GL_PIXEL_PACK_BUFFER​ target and the buffer object's data store is currently mapped.

GL_INVALID_OPERATION​ is generated if a non-zero buffer object name is bound to the GL_PIXEL_PACK_BUFFER​ target and the data would be packed to the buffer object such that the memory writes required would exceed the data store size.

GL_INVALID_OPERATION​ is generated if a non-zero buffer object name is bound to the GL_PIXEL_PACK_BUFFER​ target and data​ is not evenly divisible into the number of bytes needed to store in memory a datum indicated by type​.

GL_INVALID_OPERATION​ is generated if GL_READ_FRAMEBUFFER_BINDING​ is non-zero, the read framebuffer is complete, and the value of GL_SAMPLE_BUFFERS​ for the read framebuffer is greater than zero.

Associated Gets

glGet with argument GL_PIXEL_PACK_BUFFER_BINDING​

See Also

glBindFramebuffer, glClampColor, glDrawBuffer, glDrawBuffers, glPixelStore, glReadBuffer

Pixel Transfer, Pixel Buffer Object

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/.