man glReadPixels (3): 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_STENCIL_INDEX, GL_DEPTH_COMPONENT, GL_DEPTH_STENCIL, GL_RED, GL_GREEN, GL_BLUE, GL_RGB, GL_BGR, GL_RGBA, and GL_BGRA.

type

: Specifies the data type of the pixel data. Must be one of GL_UNSIGNED_BYTE, GL_BYTE, GL_UNSIGNED_SHORT, GL_SHORT, GL_UNSIGNED_INT, GL_INT, GL_HALF_FLOAT, 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, GL_UNSIGNED_INT_2_10_10_10_REV, GL_UNSIGNED_INT_24_8, GL_UNSIGNED_INT_10F_11F_11F_REV, GL_UNSIGNED_INT_5_9_9_9_REV, or GL_FLOAT_32_UNSIGNED_INT_24_8_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 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. 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_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	2 8 - 1	2 8 - 1 ⁢ c
GL_BYTE	2 7 - 1	2 8 - 1 ⁢ c - 1 2
GL_UNSIGNED_SHORT	2 16 - 1	2 16 - 1 ⁢ c
GL_SHORT	2 15 - 1	2 16 - 1 ⁢ c - 1 2
GL_UNSIGNED_INT	2 32 - 1	2 32 - 1 ⁢ c
GL_INT	2 31 - 1	2 32 - 1 ⁢ c - 1 2
GL_HALF_FLOAT	none	c
GL_FLOAT	none	c
GL_UNSIGNED_BYTE_3_3_2	2 N - 1	2 N - 1 ⁢ c
GL_UNSIGNED_BYTE_2_3_3_REV	2 N - 1	2 N - 1 ⁢ c
GL_UNSIGNED_SHORT_5_6_5	2 N - 1	2 N - 1 ⁢ c
GL_UNSIGNED_SHORT_5_6_5_REV	2 N - 1	2 N - 1 ⁢ c
GL_UNSIGNED_SHORT_4_4_4_4	2 N - 1	2 N - 1 ⁢ c
GL_UNSIGNED_SHORT_4_4_4_4_REV	2 N - 1	2 N - 1 ⁢ c
GL_UNSIGNED_SHORT_5_5_5_1	2 N - 1	2 N - 1 ⁢ c
GL_UNSIGNED_SHORT_1_5_5_5_REV	2 N - 1	2 N - 1 ⁢ c
GL_UNSIGNED_INT_8_8_8_8	2 N - 1	2 N - 1 ⁢ c
GL_UNSIGNED_INT_8_8_8_8_REV	2 N - 1	2 N - 1 ⁢ c
GL_UNSIGNED_INT_10_10_10_2	2 N - 1	2 N - 1 ⁢ c
GL_UNSIGNED_INT_2_10_10_10_REV	2 N - 1	2 N - 1 ⁢ c
GL_UNSIGNED_INT_24_8	2 N - 1	2 N - 1 ⁢ c
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_INDEX_MODE

glGet() with argument GL_PIXEL_PACK_BUFFER_BINDING