HP Workstation zx2000 OpenGL 1.1 Reference for HP-UX 11.x - Page 382

R glReadPixels 382 If format is GL_RED, GL_GREEN, GL_BLUE, GL_ALPHA, GL_RGB, GL_RGBA, 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 GL_UNSIGNED_BYTE 28 - 1 GL_BYTE 27 - 1 GL_BITMAP 1 GL_UNSIGNED_SHORT 216 - 1 GL_SHORT 215 - 1 GL_UNSIGNED_INT 232 - 1 GL_INT 231 - 1 GL_FLOAT none Component Conversion (28 - 1)c [(28 - 1)c - 1] / 2 1 (216- 1)c [(216 - 1)c - 1] / 2 (232 - 1)c [(232 - 1)c - 1] / 2 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 ith pixel in the jth row is placed in location j · width + i. GL_RGB returns three values, GL_RGBA returns 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. 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. Errors • GL_INVALID_ENUM is generated if format or type is not an accepted value. • 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. Chapter 16