GLREADPIXELS - Online Linux Manual PageSection : 3G
NAME"glReadPixels − read a block of pixels from the frame buffer
C SPECIFICATIONvoid glReadPixels( GLint x, GLint y, GLsizei width, GLsizei height, GLenum format, GLenum type, GLvoid *pixels )
PARAMETERSx, 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 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. pixels  Returns the pixel data.
DESCRIPTION­glReadPixels 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. When the ­GL_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 maximun 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 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 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. 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 , 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 . 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 if it is different from the one used by the GL). typeindex maskcomponent conversion ­GL_UNSIGNED_BYTE28-1(28-1)c ­GL_BYTE27-1[(28-1)c-1]/2 ­GL_BITMAP11 ­GL_UNSIGNED_SHORT216-1(216-1)c ­GL_SHORT215-1[(216-1)c-1]/2 ­GL_UNSIGNED_INT232-1(232-1)c ­GL_INT231-1[(232-1)c-1]/2 ­GL_FLOATnonec 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 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 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.
NOTESValues 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.
ERRORS­GL_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. ­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.
ASSOCIATED GETS­glGet with argument ­GL_INDEX_MODE
SEE ALSO­glCopyPixels(3G), ­glDrawPixels(3G), ­glPixelMap(3G), ­glPixelStore(3G), ­glPixelTransfer(3G),
­glReadBuffer(3G) 0
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