GLint y, GLsizei width, GLsizei height, GLenum format, GLenum type, GLvoid *pixels )
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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 $i$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 for the returned pixel values; accepted values are:
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).
| 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 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.
If an error is generated, no change is made to the contents of pixels.
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.