glReadPixels

NAME

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 *pixels)

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	The format of the returned pixel data. The allowable values are GL_COLOR_INDEX, GL_STENCIL_INDEX, GL_DEPTH_COMPONENT, GL_RED, GL_GREEN, GL_BLUE, GL_ALPHA, GL_LUMINANCE, GL_LUMINANCE_ALPHA, GL_RGB, GL_RGBA, GL_BGR_EXT, GL_BGRA_EXT, GL_ARGB_I3D, GL_422_EXT, GL_422_REV_EXT, GL_422_AVERAGE_EXT, and GL_422_REV_AVERAGE_EXT.
type	Specifies the data type for pixel data. The allowable values are GL_BITMAP, GL_UNSIGNED_BYTE, GL_BYTE, GL_UNSIGNED_SHORT, GL_SHORT, GL_UNSIGNED_INT, GL_INT, GL_FLOAT, GL_UNSIGNED_BYTE_3_3_2_EXT, GL_UNSIGNED_BYTE_2_3_3_REV_EXT, GL_UNSIGNED_SHORT_5_6_5_EXT, GL_UNSIGNED_SHORT_5_6_5_REV_EXT, GL_UNSIGNED_SHORT_4_4_4_4_EXT, GL_UNSIGNED_SHORT_4_4_4_4_REV_EXT, GL_UNSIGNED_SHORT_5_5_5_1_EXT, GL_UNSIGNED_SHORT_1_5_5_5_REV_EXT, GL_UNSIGNED_INT_8_8_8_8_EXT, GL_UNSIGNED_INT_8_8_8_8_REV_EXT, GL_UNSIGNED_INT_10_10_10_2_EXT, and GL_UNSIGNED_INT_2_10_10_10_REV_EXT.
pixels	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 pixels. 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.

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 i^th pixel in the j^th row. Pixels are returned in row order from the lowest to the highest row, left to right in each row.

If GL_INTERLACE_READ_I3D is enabled, every other row is read. The j^th row is read from the window coordinate (x, y + j * 2).

The format parameter 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_LUMINANCE

GL_LUMINANCE_ALPHA

GL_RGB

GL_BGR_EXT

GL_RGBA

GL_BGRA_EXT

GL_ARGB_I3D

GL_422_EXT

GL_422_REV_EXT

GL_422_AVERAGE_EXT

GL_422_REV_AVERAGE_EXT

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 2ⁿ, but n may be different for different tables. Before an index is used to look up a value in a table of size 2ⁿ, it must be masked against 2ⁿ-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.

If GL_MAP_COLOR is true, each color component is clamped to the range [0,1] and scaled by the size of lookup table GL_PIXEL_MAP_c_TO_c, then replaced by the value that it references in that table. c is R, G, B, or A respectively.

If GL_COLOR_TABLE_EXT is true, each color component is clamped to the range [0,1] and scaled by the size of the GL_COLOR_TABLE_EXT, then replaced by the value that it references in that table according to the format of the color table (see glColorTableEXT).

If GL_CONVOLUTION_2D_EXT or GL_SEPARABLE_2D_EXT is true, two-dimensional convolution will be performed on each color component according to the format of the convolution filter (see glConvolutionFilter2DEXT and glSeparableFilter2DEXT).

If GL_POST_CONVOLUTION_COLOR_TABLE_EXT is true, each color component is clamped to the range [0,1] and scaled by the size of the GL_POST_CONVOLUTION_COLOR_TABLE_EXT, then replaced by the value that it references in that table according to the format of the color table (see glColorTableEXT).

Each color is then transformed by the color matrix (see glMatrixMode).

If GL_POST_COLOR_MATRIX_COLOR_TABLE_EXT is true, each color component is clamped to the range [0,1] and scaled by the size of the GL_POST_COLOR_MATRIX_COLOR_TABLE_EXT, then replaced by the value that it references in that table according to the format of the color table (see glColorTableEXT).

Each color is then clamped to the color clamp values (see glPixelTransfer).

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_LUMINANCE, GL_LUMINANCE_ALPHA, GL_RGB, GL_RGBA, GL_BGR_EXT, GL_BGRA_EXT, GL_ARGB_I3D, GL_422_EXT, GL_422_REV_EXT, GL_422_AVERAGE_EXT, or GL_422_REV_AVERAGE_EXT, 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). For the packed pixel formats, N is the number of bits in each component.

type Index Mask Component Conversion
GL_UNSIGNED_BYTE 2⁸ - 1 (2⁸ - 1) c
GL_BYTE 2⁷ - 1 [(2⁸ - 1) c - 1] / 2
GL_BITMAP 1 1
GL_UNSIGNED_SHORT 2¹⁶ - 1 (2¹⁶ - 1) c
GL_SHORT 2¹⁵ - 1 [(2¹⁶ - 1) c - 1] / 2
GL_UNSIGNED_INT 2³² - 1 (2³² - 1) c
GL_INT 2³¹ - 1 [(2³² - 1) c - 1] / 2
GL_FLOAT none c
GL_UNSIGNED_BYTE_3_3_2_EXT 2^N - 1 (2^N - 1) c
GL_UNSIGNED_BYTE_2_3_3_REV_EXT 2^N - 1 (2^N - 1) c
GL_UNSIGNED_SHORT_5_6_5_EXT 2^N - 1 (2^N - 1) c
GL_UNSIGNED_SHORT_5_6_5_REV_EXT 2^N - 1 (2^N - 1) c
GL_UNSIGNED_SHORT_4_4_4_4_EXT 2^N - 1 (2^N - 1) c
GL_UNSIGNED_SHORT_4_4_4_4_REV_EXT 2^N - 1 (2^N - 1) c
GL_UNSIGNED_SHORT_5_5_5_1_EXT 2^N - 1 (2^N - 1) c
GL_UNSIGNED_SHORT_1_5_5_5_REV_EXT 2^N - 1 (2^N - 1) c
GL_UNSIGNED_INT_8_8_8_8_EXT 2^N - 1 (2^N - 1) c
GL_UNSIGNED_INT_8_8_8_8_REV_EXT 2^N - 1 (2^N - 1) c
GL_UNSIGNED_INT_10_10_10_2_EXT 2^N - 1 (2^N - 1) c
GL_UNSIGNED_INT_2_10_10_10_REV_EXT 2^N - 1 (2^N - 1) c

type	Index Mask	Component Conversion
GL_UNSIGNED_BYTE	2⁸ - 1	(2⁸ - 1) c
GL_BYTE	2⁷ - 1	[(2⁸ - 1) c - 1] / 2
GL_BITMAP	1	1
GL_UNSIGNED_SHORT	2¹⁶ - 1	(2¹⁶ - 1) c
GL_SHORT	2¹⁵ - 1	[(2¹⁶ - 1) c - 1] / 2
GL_UNSIGNED_INT	2³² - 1	(2³² - 1) c
GL_INT	2³¹ - 1	[(2³² - 1) c - 1] / 2
GL_FLOAT	none	c
GL_UNSIGNED_BYTE_3_3_2_EXT	2^N - 1	(2^N - 1) c
GL_UNSIGNED_BYTE_2_3_3_REV_EXT	2^N - 1	(2^N - 1) c
GL_UNSIGNED_SHORT_5_6_5_EXT	2^N - 1	(2^N - 1) c
GL_UNSIGNED_SHORT_5_6_5_REV_EXT	2^N - 1	(2^N - 1) c
GL_UNSIGNED_SHORT_4_4_4_4_EXT	2^N - 1	(2^N - 1) c
GL_UNSIGNED_SHORT_4_4_4_4_REV_EXT	2^N - 1	(2^N - 1) c
GL_UNSIGNED_SHORT_5_5_5_1_EXT	2^N - 1	(2^N - 1) c
GL_UNSIGNED_SHORT_1_5_5_5_REV_EXT	2^N - 1	(2^N - 1) c
GL_UNSIGNED_INT_8_8_8_8_EXT	2^N - 1	(2^N - 1) c
GL_UNSIGNED_INT_8_8_8_8_REV_EXT	2^N - 1	(2^N - 1) c
GL_UNSIGNED_INT_10_10_10_2_EXT	2^N - 1	(2^N - 1) c
GL_UNSIGNED_INT_2_10_10_10_REV_EXT	2^N - 1	(2^N - 1) 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 i^th pixel in the j^th row is placed in location (j) width + i. GL_RGB and GL_BGR returns three values, GL_RGBA, GL_BGRA_EXT, and GL_ARGB_I3D returns four values, and GL_LUMINANCE_ALPHA returns two values for each pixel.

If format is GL_422_EXT, GL_422_REV_EXT, GL_422_AVERAGE_EXT, or GL_422_REV_AVERAGE_EXT, the RGBA components are converted to a 422 format and stored as two values for each pixel. For GL_422_EXT and GL_422_REV_EXT, the red component is assigned to luminance for each pixel, and the green and blue components alternate as chrominance. For GL_422_AVERAGE_EXT and GL_422_AVERAGE_REV_EXT, the red component is assigned to luminance for the even and odd pixels, the chrominance for even pixel is the average of the even pixel's green component and the odd pixel's green component, and the chrominance for the odd pixel is the average of the even pixel's blue component and the odd pixel's blue component.

All values corresponding to a single pixel occupy contiguous space in pixels. 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 pixels.

EXTENSIONS

Some routines and constants for this function are part of an extension, not part of the core GL command set. The following extension names must be present in the string returned by glGetString when called with argument GL_EXTENSIONS to use these routines and constants.

GL_EXT_422_pixels: GL_422_EXT, GL_422_REV_EXT, GL_422_AVERAGE_EXT, and GL_422_REV_AVERAGE_EXT are part of the EXT_422_pixels extension.
GL_I3D_argb: GL_ARGB_I3D is part of the I3D_argb extension.
GL_EXT_bgra: GL_BGR_EXT and GL_BGRA_EXT are part of the EXT_bgra extension.
GL_I3D_color_clamp: The color clamp is part of the I3D_color_clamp extension.
GL_EXT_color_matrix: The color matrix is part of the EXT_color_matrix extension.
GL_EXT_color_table: GL_COLOR_TABLE_EXT, GL_POST_CONVOLUTION_COLOR_TABLE_EXT, and GL_POST_COLOR_MATRIX_COLOR_TABLE_EXT are part of the EXT_color_table extension.
GL_EXT_convolution: GL_CONVOLUTION_2D_EXT and GL_SEPARABLE_2D_EXT are part of the EXT_convolution extension.
EXT_interlace_read: GL_INTERLACE_READ_I3D is part of the EXT_interlace_read extension.
GL_EXT_packed_pixels: GL_UNSIGNED_BYTE_3_3_2_EXT, GL_UNSIGNED_BYTE_2_3_3_REV_EXT, GL_UNSIGNED_SHORT_5_6_5_EXT, GL_UNSIGNED_SHORT_5_6_5_REV_EXT, GL_UNSIGNED_SHORT_4_4_4_4_EXT, GL_UNSIGNED_SHORT_4_4_4_4_REV_EXT, GL_UNSIGNED_SHORT_5_5_5_1_EXT, GL_UNSIGNED_SHORT_1_5_5_5_REV_EXT, GL_UNSIGNED_INT_8_8_8_8_EXT, GL_UNSIGNED_INT_8_8_8_8_REV_EXT, GL_UNSIGNED_INT_10_10_10_2_EXT, and GL_UNSIGNED_INT_2_10_10_10_REV_EXT are part of the EXT_packed_pixels extension.

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

ASSOCIATED GETS

glGet with argument GL_INDEX_MODE