glReadPixels(3G)
glReadPixels - read a block of pixels from the frame buffer
As shipped in IRIX 6.5.19. Last changed in IRIX 6.5.15.
NAME glReadPixels - read a block of pixels from the frame buffer C SPECIFICATION void glReadPixels( GLint x, GLint y, GLsizei width, GLsizei height, GLenum format, GLenum type, GLvoid *pixels ) PARAMETERS x, 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 format 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_ABGR_EXT, 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. DESCRIPTION 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 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 tranformations, 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 format 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_ABGR_EXT 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 n be of size 2 , but n may be different for different tables. Before an index is used to look up a value in a table of size n n 2 , it must be masked against 2 -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 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_ABGR_EXT, 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 ith pixel in the jth row is placed in location (j) width + i. GL_RGB and GL_BGR return three values, GL_RGBA, GL_BGRA and GL_ABGR_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_LSB_FIRST and GL_PACK_SWAP_BYTES, affect the way that data is written into memory. See glPixelStore for a description. Normally glReadPixels is synchronous: OpenGL executes a glReadPixels command in the order it is issued in the OpenGL command stream. Calling glEnable with parameter GL_ASYNC_READ_PIXELS_SGIX causes subsequent glReadPixels commands to be asynchronous as defined by the SGIX_async extension. An implementation of the asynchronous glReadPixels command may be non-blocking: It may return control to the program immediately rather than waiting for the command's results to be written into the destination rectangle in host memory. The glReadPixels command samples the OpenGL state vector at the time it is issued, but it may be executed at some undefined point in the future. In particular, if subsequent commands change the contents of the source rectangle in the framebuffer, then the result of the glReadPixels is undefined. Calling glDisable with parameter GL_ASYNC_READ_PIXELS_SGIX restores the default synchronous behavior for subsequent glReadPixels commands. It does not affect any pending asynchronous glReadPixels commands, or wait for their completion. When an asynchronous glReadPixels command is issued, it is associated with the current value of GL_ASYNC_MARKER_SGIX as defined by the SGIX_async extension. A program can determine if an asynchronous glReadPixels command has completed by using the glFinishAsyncSGIX or glPollAsyncSGIX commands. There is a maximum number of asynchronous glReadPixels commands that can be outstanding at any one time, defined by the implementation. This value can be queried with glGetIntegerv with argument GL_MAX_ASYNC_READ_PIXELS_SGIX. When GL_INTERLACE_READ_INGR is enabled, every other row of the source pixel rectangle is read. The height of the pixel rectangle is equivalent to 2xheight. Only rows (y+0,y+2,...) are read and affected by the pixel transfer and store operations. NOTES Values for pixels that lie outside the window connected to the current GL context are undefined. GL_ABGR_EXT is valid only if the EXT_abgr extension is supported. 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_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 not GL_RGBA, GL_BGRA or GL_ABGR_EXT. 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. GL_INVALID_OPERATION is generated when the SGIX_subsample extension is supported, and the pixel storage mode GL_PACK_SUBSAMPLE_RATE_SGIX is not GL_PIXEL_SUBSAMPLE_4444_SGIX, and width is not a multiple of 2, or format is not a 3 or 4 component format, or type is a packed pixels type. GL_INVALID_OPERATION is generated if GL_ASYNC_READ_PIXELS_SGIX is enabled and the number of asynchronous glReadPixels commands that have been issued but not queried (using glFinishAsyncSGIX or glPollAsyncSGIX) exceeds GL_MAX_ASYNC_READ_PIXELS_SGIX. ASSOCIATED GETS glGet with argument GL_INDEX_MODE MACHINE DEPENDENCIES The SGIX_async and SGIX_async_pixel extensions are implemented only on Octane2 VPro systems. RealityEngine, RealityEngine2, and VTX systems do not support convolving images as they are read from the framebuffer. The SGIX_ycrcb extension is supported only on O2 systems. When using GL_YCRCB_422_SGIX with glReadPixels on O2 systems, an odd integer value for GL_PACK_SKIP_PIXELS will be set to the next highest even integer value to preserve color alignment. On Octane2 VPro systems the format GL_DEPTH_COMPONENT24_SGIX can be used to transfer depth pixel values to and from the depth buffer in their internal eye-space range. There are performance advantages over transfers that convert to screen-space values, particularly for GL_UNSIGNED_INT type pixels. On RealityEngine, RealityEngine2, and VTX systems, histogramming and minmax never sink pixel data that is being read from the framebuffer. The INGR_interlace_read extension is supported only on Octane2 VPro systems, and on O2 systems. The EXT_packed_pixels extension is not supported on RealityEngine, RealityEngine2, and VTX systems. The following problems are known to exist on RealityEngine, RealityEngine2, and VTX systems: 1. It is not possible to read the depth or stencil buffers of multisample-capable Visuals. 2. Reading the depth-buffer is much slower than reading a color- buffer. 3. Reading RGBA format data from a color-index window does not work correctly. The SGIX_subsample and SGIX_resample extensions are supported only on Octane2 VPro systems. Applying the GL_PIXEL_SUBSAMPLE_2424_SGIX subsample rate is accelerated for direct immmediate mode transfers when the format is GL_RGB or GL_RGBA, and the type is GL_UNSIGNED_BYTE or GL_UNSIGNED_SHORT. SEE ALSO glCopyPixels, glDrawPixels, glPixelMap, glPixelStore, glPixelTransfer, glReadBuffer, glAsyncMarkerSGIX, glDeleteAsyncMarkersSGIX, glFinishAsyncSGIX, glGenAsyncMarkersSGIX, glIsAsyncMarkerSGIX, glPollAsyncSGIX