GlReadPixels

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_glReadPixels: read a block of pixels from the frame buffer


Syntax

 SUB _glReadPixels (BYVAL x AS LONG, BYVAL y AS LONG, BYVAL width AS LONG, BYVAL height AS LONG, BYVAL format AS _UNSIGNED LONG, BYVAL type AS _UNSIGNED LONG, pixels AS _OFFSET)
 void _glReadPixels(GLint x, GLint y, GLsizei width, GLsizei height, GLenum format, GLenum type, GLvoid * data);


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.

Template:Glapi pixeltransferupparams


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 data. Several parameters control the processing of the pixel data before it is placed into client memory. These parameters are set with _glPixelStore. 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.

_glReadPixels returns values from each pixel with lower left corner at (x + i, y + j) for 0 &le; i < width and 0 &le; 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_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_DEPTH_STENCIL
Values are taken from both the depth and stencil buffers. The type parameter must be _GL_UNSIGNED_INT_24_8 or _GL_FLOAT_32_UNSIGNED_INT_24_8_REV.
_GL_RED
_GL_GREEN
_GL_BLUE
_GL_RGB
_GL_BGR
_GL_RGBA
_GL_BGRA
Finally, the indices or components are converted to the proper format, as specified by type. If format is _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_RGB, _GL_BGR, _GL_RGBA, or _GL_BGRA 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<sup>8</sup> - 1 <math>(2^8 - 1)c</math>
_GL_BYTE 2<sup>7</sup> - 1 <math>\tfrac{(2^8 - 1)c - 1}{2}</math>
_GL_UNSIGNED_SHORT 2<sup>16</sup> - 1 <math>(2^{16} - 1)c</math>
_GL_SHORT 2<sup>15</sup> - 1 <math>\tfrac{(2^{16} - 1)c - 1}{2}</math>
_GL_UNSIGNED_INT 2<sup>32</sup> - 1 <math>(2^{32} - 1)c</math>
_GL_INT 2<sup>31</sup> - 1 <math>\tfrac{(2^{32} - 1)c - 1}{2}</math>
_GL_HALF_FLOAT none c
_GL_FLOAT none c
_GL_UNSIGNED_BYTE_3_3_2 2<sup>N</sup> - 1 <math>(2^N - 1)c</math>
_GL_UNSIGNED_BYTE_2_3_3_REV 2<sup>N</sup> - 1 <math>(2^N - 1)c</math>
_GL_UNSIGNED_SHORT_5_6_5 2<sup>N</sup> - 1 <math>(2^N - 1)c</math>
_GL_UNSIGNED_SHORT_5_6_5_REV 2<sup>N</sup> - 1 <math>(2^N - 1)c</math>
_GL_UNSIGNED_SHORT_4_4_4_4 2<sup>N</sup> - 1 <math>(2^N - 1)c</math>
_GL_UNSIGNED_SHORT_4_4_4_4_REV 2<sup>N</sup> - 1 <math>(2^N - 1)c</math>
_GL_UNSIGNED_SHORT_5_5_5_1 2<sup>N</sup> - 1 <math>(2^N - 1)c</math>
_GL_UNSIGNED_SHORT_1_5_5_5_REV 2<sup>N</sup> - 1 <math>(2^N - 1)c</math>
_GL_UNSIGNED_INT_8_8_8_8 2<sup>N</sup> - 1 <math>(2^N - 1)c</math>
_GL_UNSIGNED_INT_8_8_8_8_REV 2<sup>N</sup> - 1 <math>(2^N - 1)c</math>
_GL_UNSIGNED_INT_10_10_10_2 2<sup>N</sup> - 1 <math>(2^N - 1)c</math>
_GL_UNSIGNED_INT_2_10_10_10_REV 2<sup>N</sup> - 1 <math>(2^N - 1)c</math>
_GL_UNSIGNED_INT_24_8 2<sup>N</sup> - 1 <math>(2^N - 1)c</math>
_GL_UNSIGNED_INT_10F_11F_11F_REV -- Special
_GL_UNSIGNED_INT_5_9_9_9_REV -- Special
_GL_FLOAT_32_UNSIGNED_INT_24_8_REV none c (Depth Only)

Return values are placed in memory as follows. If format is _GL_STENCIL_INDEX, _GL_DEPTH_COMPONENT, _GL_RED, _GL_GREEN, or _GL_BLUE, 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 and _GL_BGRA return four 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.


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 data.


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_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 format is _GL_DEPTH_STENCIL and there is no depth buffer or if there is no stencil buffer.

_GL_INVALID_ENUM is generated if format is _GL_DEPTH_STENCIL and type is not _GL_UNSIGNED_INT_24_8 or _GL_FLOAT_32_UNSIGNED_INT_24_8_REV.

_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.

_GL_INVALID_OPERATION is generated if a non-zero buffer object name is bound to the _GL_PIXEL_PACK_BUFFER target and the buffer object's data store is currently mapped.

_GL_INVALID_OPERATION is generated if a non-zero buffer object name is bound to the _GL_PIXEL_PACK_BUFFER target and the data would be packed to the buffer object such that the memory writes required would exceed the data store size.

_GL_INVALID_OPERATION is generated if a non-zero buffer object name is bound to the _GL_PIXEL_PACK_BUFFER target and data is not evenly divisible into the number of bytes needed to store in memory a datum indicated by type.

_GL_INVALID_OPERATION is generated if _GL_READ_FRAMEBUFFER_BINDING is non-zero, the read framebuffer is complete, and the value of _GL_SAMPLE_BUFFERS for the read framebuffer is greater than zero.


Use With:

_glGet with argument _GL_INDEX_MODE

_glGet with argument _GL_PIXEL_PACK_BUFFER_BINDING


See also

_GL _glBindFramebuffer, _glClampColor, _glDrawBuffers, _glDrawBuffers, _glPixelStore

Pixel Transfer, Pixel Buffer Object


Copyright: 1991-2006 Silicon Graphics, Inc. This document is licensed under the SGI Free Software B License.
For details, see http://oss.sgi.com/projects/FreeB/.



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Copyright 1991-2006 Silicon Graphics, Inc. This document is licensed under the SGI Free Software B License. For details, see http://oss.sgi.com/projects/FreeB/.