glReadPixels Subroutine

OpenGL 1.2 for AIX: Reference Manual

glReadPixels Subroutine

Purpose

Reads a block of pixels from the frame buffer.

Library

OpenGL C bindings library: libGL.a

C Syntax


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 1 (one) correspond to a single pixel.
Format	Specifies the format of the pixel data. Symbolic constants GL_COLOR_INDEX, GL_STENCIL_INDEX, GL_DEPTH_COMPONENT, GL_RED, GL_GREEN, GL_BLUE, GL_ALPHA, GL_RGB, GL_RGBA, GL_BGR, GL_BGRA, GL_ABGR_EXT, GL_LUMINANCE, GL_LUMINANCE_ALPHA are accepted.
Type	Specifies the data type for Pixels. Sybolic constants 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, GL_UNSIGNED_INT_2_10_10_10_REV are accepted.
Pixels	Returns the pixel data.

Description

The glReadPixels subroutine returns pixel data from the frame buffer, starting with the pixel whose lower left corner is at location (X, Y), and puts it into client memory starting at the location specified by the Pixels parameter. Several parameters control the processing of the pixel data before it is placed into client memory. These parameters are set with three subroutines: glPixelStore, glPixelTransfer, and glPixelMap. The effects on glReadPixels of most, but not all, of the parameters specified by these three subroutines are described here.

The glReadPixels subroutine returns values from each pixel with the lower left-hand 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.

The Format parameter specifies the format for the returned pixel values. Accepted values for Format are as follows:

GL_COLOR_INDEX	Color indexes are read from the color buffer selected by the glReadBuffer subroutine. Each index is converted to fixed-point format, 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, indexes 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 format, 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, indexes 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 format such that the minimum depth value maps to 0.0 and the maximum value maps to 1.0. Each component is then multiplied by GL_DEPTH_SCALE, added to GL_DEPTH_BIAS, and finally clamped to the range [0,1].
GL_ABGR_EXT	Each pixel is a four-component group: for GL_RGBA, the red component is first, followed by green, followed by blue, followed by alpha; for GL_BGRA, the blue component is first, followed by green, followed by red, followed by alpha; for GL_ABGR_EXT the order is alpha, blue, green, and then red. Floating-point values are converted directly to an internal floatingpoint format with unspecified precision. Signed integer values are mapped linearly to the internal floating-point format such that the most positive representable integer value maps to 1.0, and the most negative representable value maps to -1.0. Unsigned integer data is mapped similarly: the largest integer value maps to 1.0, and zero maps to 0.0. The resulting floating-point color values are then multiplied by GL_c_SCALE and added to GL_c_BIAS, where c is RED, GREEN, BLUE, and ALPHA for the respective color components. The results are clamped to the range [0,1].
GL_RED	Each pixel is a single red component. This component is converted to the internal floating-point format in the same way as the red component of an RGBA pixel is, then it is converted to an RGBA pixel with green and blue set to 0.0, and alpha set to 1.0. After this conversion, the pixel is treated just as if it had been read as an RGBA pixel.
GL_GREEN	Each pixel is a single green component. This component is converted to the internal floating-point format in the same way as the green component of an RGBA pixel is, then it is converted to an RGBA pixel with red and blue set to 0.0, and alpha set to 1.0. After this conversion, the pixel is treated just as if it had been read as an RGBA pixel.
GL_BLUE	Each pixel is a single blue component. This component is converted to the internal floating-point format in the same way as the blue component of an RGBA pixel is, then it is converted to an RGBA pixel with red and green set to 0.0, and alpha set to 1.0. After this conversion, the pixel is treated just as if it had been read as an RGBA pixel.
GL_ALPHA	Each pixel is a single alpha component. This component is converted to the internal floating-point format in the same way as the alpha component of an RGBA pixel is, then it is converted to an RGBA pixel with red, green, and blue set to 0.0. After this conversion, the pixel is treated just as if it had been read as an RGBA pixel.
GL_RGB	Each pixel is a three-component group, red first, followed by green, followed by blue. Each component is converted to the internal floating-point format in the same way as the red, green, and blue components of an RGBA pixel are. The color triple is converted to an RGBA pixel with alpha set to 1.0. After this conversion, the pixel is treated just as if it had been read as an RGBA pixel.
GL_RGBA	Each pixel is a four-component group, red first, followed by green, followed by blue, followed by alpha. Floating-point values are converted directly to an internal floating-point format with unspecified precision. Signed integer values are mapped linearly to the internal floating-point format such that the most positive representable integer value maps to 1.0, and the most negative representable value maps to -1.0. Unsigned integer data are mapped similarly: the largest integer value maps to 1.0, and 0 maps to 0.0. The resulting floating-point color values are then multiplied by GL_c_SCALE and added to GL_c_BIAS, where c is RED, GREEN, BLUE, and ALPHA for the respective color components. The results are clamped to the range [0,1]. If GL_MAP_COLOR is True, each color component is scaled by the size of the 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. The resulting RGBA colors are then converted to fragments by attaching the current raster position z coordinate and texture coordinates to each pixel, then assigning x and y window coordinates to the nth fragment such that xn = xr + n mod Width and yn = yr + [n/Width], where (xr, yr) is the current raster position. These pixel fragments are then treated just like the fragments generated by rasterizing points, lines, or polygons. Texture mapping, fog, and all the fragment operations are applied before the fragments are written to the frame buffer.
GL_BGR	Each pixel is a three-component group, blue first, followed by green, followed by red. Each component is converted to the internal floating-point format in the same way as the blue, green, and red components of an BGRA pixel are. The color triple is converted to an BGRA pixel with alpha set to 1.0. After this conversion, the pixel is treated just as if it had been read as an BGRA pixel.
GL_BGRA	Each pixel is a four-component group, blue first, followed by green, followed by red, followed by alpha. Floating-point values are converted directly to an internal floating-point format with unspecified precision. Signed integer values are mapped linearly to the internal floating-point format such that the most positive representable integer value maps to 1.0, and the most negative representable value maps to -1.0. Unsigned integer data are mapped similarly: the largest integer value maps to 1.0, and 0 maps to 0.0. The resulting floating-point color values are then multiplied by GL_c_SCALE and added to GL_c_BIAS, where c is BLUE, GREEN, RED, and ALPHA for the respective color components. The results are clamped to the range [0,1]. If GL_MAP_COLOR is True, each color component is scaled by the size of the lookup table GL_PIXEL_MAP_c_TO_c, then replaced by the value that it references in that table. c is B, G, R, or A, respectively. The resulting BGRA colors are then converted to fragments by attaching the current raster position z coordinate and texture coordinates to each pixel, then assigning x and y window coordinates to the nth fragment such that xn = xr + n mod Width and yn = yr + [n/Width], where (xr, yr) is the current raster position. These pixel fragments are then treated just like the fragments generated by rasterizing points, lines, or polygons. Texture mapping, fog, and all the fragment operations are applied before the fragments are written to the frame buffer.
GL_LUMINANCE	Each pixel is a single luminance component. This component is converted to the internal floating-point format in the same way as the red component of an RGBA pixel is, then it is converted to an RGBA pixel with red, green, and blue set to the converted luminance value, and alpha set to 1.0. After this conversion, the pixel is treated just as if it had been read as an RGBA pixel.
GL_LUMINANCE_ALPHA	Processing differs depending on whether color buffers store color indexes or red, green, blue, alpha (RGBA) color components. If color indexes are stored, they are read from the color buffer selected by glReadBuffer. Each index is converted to fixed-point format, shifted left or right depending on the value and sign of GL_INDEX_SHIFT, and added to GL_INDEX_OFFSET. Indexes are then replaced by the RGBA values obtained by indexing the 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 tables. 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 format 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 GL_RED, GL_GREEN, GL_BLUE, and GL_ALPHA. Each component is clamped to the range [0,1]. Finally, if GL_MAP_COLOR is GL_TRUE, each color component c is replaced by its mapping in the table GL_PIXEL_MAP_c_TO_c, where c again is GL_RED, GL_GREEN, GL_BLUE, and GL_ALPHA. Each component is scaled to the size its corresponding table before the lookup is performed. Finally, unneeded data is 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 shift, scale, bias, and lookup factors described in the preceding section are all specified by glPixelTransfer. The lookup table contents themselves are specified by the glPixelMap subroutine.

The final step involves converting the indexes or components to the proper format, as specified by the Type parameter. If the Format parameter 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 the Type parameter is GL_FLOAT, each integer index is converted to single-precision floating-point format.

If the Format parameter is GL_RED, GL_GREEN, GL_BLUE, GL_ALPHA, GL_RGB, GL_RGBA, GL_BGR, GL_BGRA, GL_ABGR_EXT, GL_LUMINANCE, or GL_LUMINANCE_ALPHA and the Type parameter is not GL_FLOAT, each component is multiplied by the multiplier shown in the following table. If Type is GL_FLOAT, 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	(2⁸ -1)c
GL_BYTE	2⁷ -1	[(2⁷ -1)c-1]/2
GL_BITMAP	1	1
GL_UNSIGNED_SHORT	2¹⁶ -1	(2¹⁶ -1)c
GL_SHORT	2¹⁵ -1	[(2¹⁵ -1)c-1]/2
GL_UNSIGNED_INT	2³² -1	(2³² -1)c
GL_INT	2³¹ -1	[(2³¹ -1)c-1]/2
GL_FLOAT	none	c
GL_UNSIGNED_BYTE_3_3_2	2⁸ -1	(2^N -1)c
GL_UNSIGNED_BYTE_2_3_3_REV	2⁸ -1	(2^N -1)c
GL_UNSIGNED_SHORT_5_6_5	2¹⁶ -1	(2^N -1)c
GL_UNSIGNED_SHORT_5_6_5_REV	2¹⁶ -1	(2^N -1)c
GL_UNSIGNED_SHORT_4_4_4_4	2¹⁶ -1	(2^N -1)c
GL_UNSIGNED_SHORT_4_4_4_4_REV	2¹⁶ -1	(2^N -1)c
GL_UNSIGNED_SHORT_5_5_5_1	2¹⁶ -1	(2^N -1)c
GL_UNSIGNED_SHORT_1_5_5_5_REV	2¹⁶ -1	(2^N -1)c
GL_UNSIGNED_INT_8_8_8_8	2³² -1	(2^N -1)c
GL_UNSIGNED_INT_8_8_8_8_REV	2³² -1	(2^N -1)c
GL_UNSIGNED_INT_10_10_10_2	2³² -1	(2^N -1)c
GL_UNSIGNED_INT_2_10_10_10_REV	2³² -1	(2^N -1)c

Equations with N as the exponent are performed for each bitfield of the packed data type, with N set to the number of bits in the bitfield.

Return values are placed in memory as follows. If the Format parameter 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) Width + i. GL_RGB and GL_BGR return three values, GL_RGBA, GL_BGRA, and GL_ABGR_EXT 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 Pixels. Storage parameters set by glPixelStore, such as GL_PACK_SWAP_BYTES and GL_PACK_LSB_FIRST, affect the way that data is written into memory. See the glPixelStore subroutine 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.

Format of GL_ABGR_EXT is part of the _extname (EXT_abgr) extension, not part of the core GL command set.

Packed pixel types and BGR/BGRA formats are only supported in OpenGL 1.2 and later.

Error Codes

GL_INVALID_ENUM	Format or Type is not an accepted value.
GL_INVALID_VALUE	Width or Height is negative.
GL_INVALID_OPERATION	Format is GL_COLOR_INDEX and the color buffers store RGBA color components.
GL_INVALID_OPERATION	Format is GL_STENCIL_INDEX and there is no stencil buffer.
GL_INVALID_OPERATION	Format is GL_DEPTH_COMPONENT and there is no depth buffer.
GL_INVALID_OPERATION	The glReadPixels subroutine is called between a call to glBegin and the corresponding call to glEnd.

Associated Gets

Associated gets for the glReadPixels subroutine are as follows. (See the glGet subroutine for more information.)

glGet with argument GL_INDEX_MODE.

Files

/usr/include/GL/gl.h

Contains C language constants, variable type definitions, and ANSI function prototypes for OpenGL.

Related Information

The glBegin or glEnd subroutine, glCopyPixels subroutine, glDrawPixels subroutine, glPixelMap subroutine, glPixelStore subroutine, glPixelTransfer subroutine, glReadBuffer subroutine.

OpenGL Overview.