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_COLOR_INDEX`, `GL_STENCIL_INDEX`, `GL_DEPTH_COMPONENT`, `GL_RED`, `GL_GREEN`, `GL_BLUE`, `GL_ALPHA`, `GL_RGB`, `GL_BGR`, `GL_RGBA`, `GL_BGRA`, `GL_LUMINANCE`, and `GL_LUMINANCE_ALPHA`.

`type`

  Specifies the data type of the pixel data. Must be one of `GL_UNSIGNED_BYTE`, `GL_BYTE`, `GL_BITMAP`, `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`, or `GL_UNSIGNED_INT_2_10_10_10_REV`.

`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`. Several parameters control the processing of the pixel data before it is placed into client memory. These parameters are set with three commands: glPixelStore, glPixelTransfer, and glPixelMap. 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.

When the `ARB_imaging` extension is supported, the pixel data may be processed by additional operations including color table lookup, color matrix transformations, convolutions, histograms, and minimum and maximum pixel value computations.

`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_COLOR_INDEX`

  Color indices are read from the color buffer selected by glReadBuffer. Each index is converted to fixed point, shifted left or right depending on the value and sign of `GL_INDEX_SHIFT`, and added to `GL_INDEX_OFFSET`. If `GL_MAP_COLOR` is `GL_TRUE`, indices are replaced by their mappings in the table `GL_PIXEL_MAP_I_TO_I`.

`GL_STENCIL_INDEX`

  Stencil values are read from the stencil buffer. Each index is converted to fixed point, shifted left or right depending on the value and sign of `GL_INDEX_SHIFT`, and added to `GL_INDEX_OFFSET`. If `GL_MAP_STENCIL` is `GL_TRUE`, indices are replaced by their mappings in the table `GL_PIXEL_MAP_S_TO_S`.

`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_RED`
`GL_GREEN`
`GL_BLUE`
`GL_ALPHA`
`GL_RGB`
`GL_BGR`
`GL_RGBA`
`GL_BGRA`
`GL_LUMINANCE`
`GL_LUMINANCE_ALPHA`

  Processing differs depending on whether color buffers store color indices or RGBA color components. If color indices are stored, they are read from the color buffer selected by glReadBuffer. Each index is converted to fixed point, shifted left or right depending on the value and sign of `GL_INDEX_SHIFT`, and added to `GL_INDEX_OFFSET`. Indices are then replaced by the red, green, blue, and alpha values obtained by indexing the tables `GL_PIXEL_MAP_I_TO_R`, `GL_PIXEL_MAP_I_TO_G`, `GL_PIXEL_MAP_I_TO_B`, and `GL_PIXEL_MAP_I_TO_A`. Each table must be of size 2 n , but n may be different for different tables. Before an index is used to look up a value in a table of size 2 n , it must be masked against 2 n - 1 .

  If RGBA color components are stored in the color buffers, they are read from the color buffer selected by glReadBuffer. Each color component is converted to floating point such that zero intensity maps to 0.0 and full intensity maps to 1.0. Each component is then multiplied by `GL_c_SCALE` and added to `GL_c_BIAS`, where c is RED, GREEN, BLUE, or ALPHA. Finally, if `GL_MAP_COLOR` is `GL_TRUE`, each component is clamped to the range 0 1 , scaled to the size of its corresponding table, and is then replaced by its mapping in the table `GL_PIXEL_MAP_c_TO_c`, where c is R, G, B, or A.

  Unneeded data is then discarded. For example, `GL_RED` discards the green, blue, and alpha 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 .

The shift, scale, bias, and lookup factors just described are all specified by glPixelTransfer. The lookup table contents themselves are specified by glPixelMap.

Finally, the indices or components are converted to the proper format, as specified by `type`. If `format` is `GL_COLOR_INDEX` or `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_ALPHA`, `GL_RGB`, `GL_BGR`, `GL_RGBA`, `GL_BGRA`, `GL_LUMINANCE`, or `GL_LUMINANCE_ALPHA` 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` | 2 8 - 1 | 2 8 - 1 &it; c
`GL_BYTE` | 2 7 - 1 | 2 8 - 1 &it; c - 1 2
`GL_BITMAP` | 1 | 1
`GL_UNSIGNED_SHORT` | 2 16 - 1 | 2 16 - 1 &it; c
`GL_SHORT` | 2 15 - 1 | 2 16 - 1 &it; c - 1 2
`GL_UNSIGNED_INT` | 2 32 - 1 | 2 32 - 1 &it; c
`GL_INT` | 2 31 - 1 | 2 32 - 1 &it; c - 1 2
`GL_FLOAT` | none | c

Return values are placed in memory as follows. If `format` is `GL_COLOR_INDEX`, `GL_STENCIL_INDEX`, `GL_DEPTH_COMPONENT`, `GL_RED`, `GL_GREEN`, `GL_BLUE`, `GL_ALPHA`, or `GL_LUMINANCE`, 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` and `GL_BGR` return three values, `GL_RGBA` and `GL_BGRA` return four values, and `GL_LUMINANCE_ALPHA` returns two 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_ENUM` is generated if `type` is `GL_BITMAP` and `format` is not `GL_COLOR_INDEX` or `GL_STENCIL_INDEX`.

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

`GL_INVALID_OPERATION` is generated if `format` is `GL_COLOR_INDEX` and the color buffers store RGBA color components.

`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 `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`.

The formats `GL_BGR`, and `GL_BGRA` and types `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` are available only if the GL version is 1.2 or greater.

`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 `glReadPixels` is executed between the execution of glBegin and the corresponding execution of glEnd.

## Associated Gets

glGet with argument `GL_INDEX_MODE`

glGet with argument `GL_PIXEL_PACK_BUFFER_BINDING`

## See Also

glCopyPixels, glDrawPixels, glPixelMap, glPixelStore, glPixelTransfer, glReadBuffer

## 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/.