glReadPixels

glReadPixels: man2/glReadPixels.xml

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 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. 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. When the 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 maximum 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 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. format specifies the format for the returned pixel values; accepted values are: 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_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 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_BITMAP 1 1 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_FLOAT none 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 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 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.

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.