glReadPixels(3G)
glReadPixels - read a block of pixels from the frame buffer
As shipped in IRIX 6.5. First release of IRIX 6.5.
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 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_RGBA, GL_ABGR_EXT, 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_EXT, GL_UNSIGNED_SHORT_4_4_4_4_EXT, GL_UNSIGNED_SHORT_5_5_5_1_EXT, GL_UNSIGNED_INT_8_8_8_8_EXT, or GL_UNSIGNED_INT_10_10_10_2_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 the commands glPixelStore, glPixelTransfer, and glPixelMap. The state for the EXT_convolution, SGI_color_matrix, EXT_histogram, and SGI_color_table extensions (see the reference pages for glPixelTransfer, glConvolutionFilter2DEXT, glColorTableSGI, glHistogramEXT, and glMinmaxEXT) also affect the results of glReadPixels. This reference page describes the effects on glReadPixels of some, but not all of the parameters specified by these commands. glReadPixels returns values from each pixel with 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. format specifies the format for the returned pixel values. Accepted values for format are as follows: 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 0.0 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_RED GL_GREEN GL_BLUE GL_ALPHA GL_RGB GL_RGBA GL_ABGR_EXT 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 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. Each of n these tables must be of size 2 , but n may be different for different tables. Before an index is used to look up a value in n n a table of size 2 it must be masked against 2 -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 described above are all specified by glPixelTransfer. The lookup table contents themselves are specified by glPixelMap. The final step involves converting the indices or components 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. type index mask ______________________________ 8 GL_UNSIGNED_BYTE 2 -1 7 GL_BYTE 2 -1 GL_BITMAP 1 16 GL_UNSIGNED_SHORT 2 -1 15 GL_SHORT 2 -1 32 GL_UNSIGNED_INT 2 -1 31 GL_INT 2 -1 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_RGBA, GL_ABGR_EXT, GL_LUMINANCE, or GL_LUMINANCE_ALPHA each component is first clamped to [0,1]. If type is not GL_FLOAT, each component is then multiplied by the multiplier shown in the following table. 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. type component conversion _____________________________________________________ 8 GL_UNSIGNED_BYTE (2 -1)*c 8 GL_BYTE [(2 -1)*c-1]/2 GL_BITMAP 1 16 GL_UNSIGNED_SHORT (2 -1)*c 16 GL_SHORT [(2 -1)*c-1]/2 32 GL_UNSIGNED_INT (2 -1)*c 32 GL_INT [(2 -1)*c-1]/2 N GL_UNSIGNED_BYTE_3_3_2_EXT (2 -1)*c N GL_UNSIGNED_SHORT_4_4_4_4_EXT (2 -1)*c N GL_UNSIGNED_SHORT_5_5_5_1_EXT (2 -1)*c N GL_UNSIGNED_INT_8_8_8_8_EXT (2 -1)*c N GL_UNSIGNED_INT_10_10_10_2_EXT (2 -1)*c GL_FLOAT c 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). 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*width + i. GL_RGB returns three values, GL_RGBA 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. (If type is a packed data format then all the elements of each group are stored in a a single unsigned byte, unsigned short, or unsigned int.) 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 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. If type is set to GL_UNSIGNED_BYTE_3_3_2_EXT, GL_UNSIGNED_SHORT_4_4_4_4_EXT, GL_UNSIGNED_SHORT_5_5_5_1_EXT, GL_UNSIGNED_INT_8_8_8_8_EXT, or GL_UNSIGNED_INT_10_10_10_2_EXT and the EXT_packed_pixels extension is not supported then a GL_INVALID_ENUM error is generated. 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 MACHINE DEPENDENCIES RealityEngine, RealityEngine2, and VTX systems do not support convolving images as they are read from the framebuffer. On RealityEngine, RealityEngine2, and VTX systems, histogramming and minmax never sink pixel data that is being read from the framebuffer. The EXT_packed_pixels extension is not supported on RealityEngine, RealityEngine2, and VTX systems. The following problems are known to exist on RealityEngine, RealityEngine2, and VTX systems: 1. It is not possible to read the depth or stencil buffers of multisample-capable Visuals. 2. Reading the depth-buffer is much slower than reading a color- buffer. 3. Reading RGBA format data from a color-index window does not work correctly. SEE ALSO glCopyPixels, glDrawPixels, glPixelMap, glPixelStore, glPixelTransfer, glReadBuffer