The glReadPixels function reads a block of pixels from the frame buffer.
void glReadPixels(
GLint x,
GLint y,
GLsizei width,
GLsizei height,
GLenum format,
GLenum type,
GLvoid *pixels
);
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 GL_RED, GL_GREEN, GL_BLUE, and GL_ALPHA. Each component is clamped to the range [0,1]. Finally, if GL_MAP_COLOR is GL_TRUE, each color component c is replaced by its mapping in the table GL_PIXEL_MAP_c_TO_c, where c again is GL_RED, GL_GREEN, GL_BLUE, and GL_ALPHA. Each component is scaled to the size of its corresponding table before the lookup is performed.
Finally, unneeded data is 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.
| Type | Index Mask | Component Conversion |
|---|---|---|
| GL_UNSIGNED_BYTE | 2^8–1 | (2^8–1)c |
| GL_BYTE | 2^7–1 | [2^7–1]c-1]/2 |
| GL_BITMAP | 1 | 1 |
| GL_UNSIGNED_SHORT | 2^16–1 | (2^16–1) c |
| GL_SHORT | 2^15–1 | [(2^15–1) c–1] / 2 |
| GL_UNSIGNED_INT | 2^32–1 | (2^32–1) c |
| GL_INT | 2^31–1 | [(2^31–1) c–1] / 2 |
| GL_FLOAT | none | c |
The glReadPixels function 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 topic describes the effects on glReadPixels of most, but not all of the parameters specified by these three commands.
The glReadPixels function 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.
The shift, scale, bias, and lookup factors described above are all specified by glPixelTransfer. The lookup table contents are specified by glPixelMap.
The final step involves converting the indexes 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. 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_BGR_EXT, GL_BGRA_EXT, GL_LUMINANCE, or GL_LUMINANCE_ALPHA and type is not GL_FLOAT, each component is multiplied by the multiplier shown in the preceding 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 OpenGL).
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_EXT return three values, GL_RGBA and GL_BGRA_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. 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.
Values for pixels that lie outside the window connected to the current OpenGL context are undefined.
If an error is generated, no change is made to the contents of pixels.
The following function retrieves information related to glReadPixels:
glGet with argument GL_INDEX_MODE
The following are the error codes generated and their conditions.
| Error Code | Condition |
|---|---|
| GL_INVALID_ENUM | format or type was not an accepted value. |
| GL_INVALID_VALUE | either width or height was negative. |
| GL_INVALID_OPERATION | format was GL_COLOR_INDEX and the color buffers stored RGBA or BGRA color components. |
| GL_INVALID_OPERATION | format was GL_STENCIL_INDEX and there was no stencil buffer. |
| GL_INVALID_OPERATION | format was GL_DEPTH_COMPONENT and there was no depth buffer. |
| GL_INVALID_OPERATION | glReadPixels was called between a call to glBegin and the corresponding call to glEnd. |
Windows NT: Use version 3.5 and later.
Windows: Use Windows 95 and later.
Windows CE: Unsupported.
Header: Declared in gl.h.
Import Library: Link with opengl32.lib.
glBegin, glCopyPixels, glDrawPixels, glEnd, glPixelMap, glPixelStore, glPixelTransfer, glReadBuffer