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<STRONG>NAME</STRONG>
<STRONG>glReadPixels</STRONG> - read a block of pixels from the frame buffer
<STRONG>C</STRONG> <STRONG>SPECIFICATION</STRONG>
void <STRONG>glReadPixels</STRONG>( GLint <EM>x</EM>,
GLint <EM>y</EM>,
GLsizei <EM>width</EM>,
GLsizei <EM>height</EM>,
GLenum <EM>format</EM>,
GLenum <EM>type</EM>,
GLvoid *<EM>pixels</EM> )
<STRONG>PARAMETERS</STRONG>
<EM>x</EM>, <EM>y</EM> 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.
<EM>width</EM>, <EM>height</EM>
Specify the dimensions of the pixel rectangle. <EM>width</EM>
and <EM>height</EM> of one correspond to a single pixel.
<EM>format</EM>
Specifies the format of the pixel data. The following
symbolic values are accepted: <STRONG>GL_COLOR_INDEX</STRONG>,
<STRONG>GL_STENCIL_INDEX</STRONG>, <STRONG>GL_DEPTH_COMPONENT</STRONG>, <STRONG>GL_RED</STRONG>, <STRONG>GL_GREEN</STRONG>,
<STRONG>GL_BLUE</STRONG>, <STRONG>GL_ALPHA</STRONG>, <STRONG>GL_RGB</STRONG>, <STRONG>GL_RGBA</STRONG>, <STRONG>GL_LUMINANCE</STRONG>, and
<STRONG>GL_LUMINANCE_ALPHA</STRONG>.
<EM>type</EM> Specifies the data type of the pixel data. Must be one
of <STRONG>GL_UNSIGNED_BYTE</STRONG>, <STRONG>GL_BYTE</STRONG>, <STRONG>GL_BITMAP</STRONG>,
<STRONG>GL_UNSIGNED_SHORT</STRONG>, <STRONG>GL_SHORT</STRONG>, <STRONG>GL_UNSIGNED_INT</STRONG>, <STRONG>GL_INT</STRONG>,
or <STRONG>GL_FLOAT</STRONG>.
<EM>pixels</EM>
Returns the pixel data.
<STRONG>DESCRIPTION</STRONG>
<STRONG>glReadPixels</STRONG> returns pixel data from the frame buffer,
starting with the pixel whose lower left corner is at
location (<EM>x</EM>, <EM>y</EM>), into client memory starting at location
<EM>pixels</EM>. Several parameters control the processing of the
pixel data before it is placed into client memory. These
parameters are set with three commands: <STRONG>glPixelStore</STRONG>,
<STRONG>glPixelTransfer</STRONG>, and <STRONG>glPixelMap</STRONG>. This reference page
describes the effects on <STRONG>glReadPixels</STRONG> of most, but not all
of the parameters specified by these three commands.
<STRONG>glReadPixels</STRONG> returns values from each pixel with lower left
corner at (<EM>x</EM> + i, <EM>y</EM> + j) for 0<EM><</EM>i<<EM>width</EM> and 0<EM><</EM>j<<EM>height</EM>.
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.
<EM>format</EM> specifies the format for the returned pixel values;
accepted values are:
<STRONG>GL_COLOR_INDEX</STRONG>
Color indices are read from the color buffer
selected by <STRONG>glReadBuffer</STRONG>. Each index is converted
to fixed point, shifted left or right depending on
the value and sign of <STRONG>GL_INDEX_SHIFT</STRONG>, and added to
<STRONG>GL_INDEX_OFFSET</STRONG>. If <STRONG>GL_MAP_COLOR</STRONG> is <STRONG>GL_TRUE</STRONG>,
indices are replaced by their mappings in the
table <STRONG>GL_PIXEL_MAP_I_TO_I</STRONG>.
<STRONG>GL_STENCIL_INDEX</STRONG>
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
<STRONG>GL_INDEX_SHIFT</STRONG>, and added to <STRONG>GL_INDEX_OFFSET</STRONG>. If
<STRONG>GL_MAP_STENCIL</STRONG> is <STRONG>GL_TRUE</STRONG>, indices are replaced by
their mappings in the table <STRONG>GL_PIXEL_MAP_S_TO_S</STRONG>.
<STRONG>GL_DEPTH_COMPONENT</STRONG>
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 <STRONG>GL_DEPTH_SCALE</STRONG>, added to
<STRONG>GL_DEPTH_BIAS</STRONG>, and finally clamped to the range
[0,1].
<STRONG>GL_RED</STRONG>
<STRONG>GL_GREEN</STRONG>
<STRONG>GL_BLUE</STRONG>
<STRONG>GL_ALPHA</STRONG>
<STRONG>GL_RGB</STRONG>
<STRONG>GL_RGBA</STRONG>
<STRONG>GL_LUMINANCE</STRONG>
<STRONG>GL_LUMINANCE_ALPHA</STRONG>
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
<STRONG>glReadBuffer</STRONG>. Each index is converted to fixed
point, shifted left or right depending on the
value and sign of <STRONG>GL_INDEX_SHIFT</STRONG>, and added to
<STRONG>GL_INDEX_OFFSET</STRONG>. Indices are then replaced by the
red, green, blue, and alpha values obtained by
indexing the tables <STRONG>GL_PIXEL_MAP_I_TO_R</STRONG>,
<STRONG>GL_PIXEL_MAP_I_TO_G</STRONG>, <STRONG>GL_PIXEL_MAP_I_TO_B</STRONG>, and
<STRONG>GL_PIXEL_MAP_I_TO_A</STRONG>. Each table must be of size
2^n, but n may be different for different tables.
Before an index is used to look up a value in a
table of size 2^n, it must be masked against 2^n-
1.
If RGBA color components are stored in the color
buffers, they are read from the color buffer
selected by <STRONG>glReadBuffer</STRONG>. 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
<STRONG>GL_c_SCALE</STRONG> and added to <STRONG>GL_c_BIAS</STRONG>, where <EM>c</EM> is RED,
GREEN, BLUE, or ALPHA. Finally, if <STRONG>GL_MAP_COLOR</STRONG> is
<STRONG>GL_TRUE</STRONG>, 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 <STRONG>GL_PIXEL_MAP_c_TO_c</STRONG>, where <EM>c</EM> is R, G, B, or
A.
Unneeded data is then discarded. For example,
<STRONG>GL_RED</STRONG> discards the green, blue, and alpha
components, while <STRONG>GL_RGB</STRONG> discards only the alpha
component. <STRONG>GL_LUMINANCE</STRONG> computes a single-
component value as the sum of the red, green, and
blue components, and <STRONG>GL_LUMINANCE_ALPHA</STRONG> 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
<STRONG>glPixelTransfer</STRONG>. The lookup table contents themselves are
specified by <STRONG>glPixelMap</STRONG>.
Finally, the indices or components are converted to the
proper format, as specified by <EM>type</EM>. If <EM>format</EM> is
<STRONG>GL_COLOR_INDEX</STRONG> or <STRONG>GL_STENCIL_INDEX</STRONG> and <EM>type</EM> is not <STRONG>GL_FLOAT</STRONG>,
each index is masked with the mask value given in the
following table. If <EM>type</EM> is <STRONG>GL_FLOAT</STRONG>, then each integer
index is converted to single-precision floating-point
format.
If <EM>format</EM> is <STRONG>GL_RED</STRONG>, <STRONG>GL_GREEN</STRONG>, <STRONG>GL_BLUE</STRONG>, <STRONG>GL_ALPHA</STRONG>, <STRONG>GL_RGB</STRONG>,
<STRONG>GL_RGBA</STRONG>, <STRONG>GL_LUMINANCE</STRONG>, or <STRONG>GL_LUMINANCE_ALPHA</STRONG> and <EM>type</EM> is not
<STRONG>GL_FLOAT</STRONG>, each component is multiplied by the multiplier
shown in the following table. If type is <STRONG>GL_FLOAT</STRONG>, 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).
_______________________________________________________
| <EM>type</EM> | <EM>index</EM> <EM>mask</EM> | <EM>component</EM> <EM>conversion</EM> |
<EM>|</EM>__________________<EM>|</EM>____________<EM>|</EM>______________________|
| | | |
|<STRONG>GL_UNSIGNED_BYTE</STRONG> | 2^8-1 | (2^8-1)c |
| <STRONG>GL_BYTE</STRONG> | 2^7-1 | [(2^8-1)c-1]/2 |
| <STRONG>GL_BITMAP</STRONG> | 1 | 1 |
|<STRONG>GL_UNSIGNED_SHORT</STRONG> | 2^16-1 | (2^16-1)c |
| <STRONG>GL_SHORT</STRONG> | 2^15-1 | [(2^16-1)c-1]/2 |
| <STRONG>GL_UNSIGNED_INT</STRONG> | 2^32-1 | (2^32-1)c |
| <STRONG>GL_INT</STRONG> | 2^31-1 | [(2^32-1)c-1]/2 |
| <STRONG>GL_FLOAT</STRONG> | none | c |
<EM>|</EM>__________________<EM>|</EM>____________<EM>|</EM>______________________|
Return values are placed in memory as follows. If <EM>format</EM> is
<STRONG>GL_COLOR_INDEX</STRONG>, <STRONG>GL_STENCIL_INDEX</STRONG>, <STRONG>GL_DEPTH_COMPONENT</STRONG>,
<STRONG>GL_RED</STRONG>, <STRONG>GL_GREEN</STRONG>, <STRONG>GL_BLUE</STRONG>, <STRONG>GL_ALPHA</STRONG>, or <STRONG>GL_LUMINANCE</STRONG>, a
single value is returned and the data for the ith pixel in
the jth row is placed in location (j) width + i. <STRONG>GL_RGB</STRONG>
returns three values, <STRONG>GL_RGBA</STRONG> returns four values, and
<STRONG>GL_LUMINANCE_ALPHA</STRONG> returns two values for each pixel, with
all values corresponding to a single pixel occupying
contiguous space in <EM>pixels</EM>. Storage parameters set by
<STRONG>glPixelStore</STRONG>, such as <STRONG>GL_PACK_LSB_FIRST</STRONG> and
<STRONG>GL_PACK_SWAP_BYTES</STRONG>, affect the way that data is written into
memory. See <STRONG>glPixelStore</STRONG> for a description.
<STRONG>NOTES</STRONG>
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 <EM>pixels</EM>.
<STRONG>ERRORS</STRONG>
<STRONG>GL_INVALID_ENUM</STRONG> is generated if <EM>format</EM> or <EM>type</EM> is not an
accepted value.
<STRONG>GL_INVALID_ENUM</STRONG> is generated if <EM>type</EM> is <STRONG>GL_BITMAP</STRONG> and <EM>format</EM>
is not <STRONG>GL_COLOR_INDEX</STRONG> or <STRONG>GL_STENCIL_INDEX</STRONG>.
<STRONG>GL_INVALID_VALUE</STRONG> is generated if either <EM>width</EM> or <EM>height</EM> is
negative.
<STRONG>GL_INVALID_OPERATION</STRONG> is generated if <EM>format</EM> is
<STRONG>GL_COLOR_INDEX</STRONG> and the color buffers store RGBA color
components.
<STRONG>GL_INVALID_OPERATION</STRONG> is generated if <EM>format</EM> is
<STRONG>GL_STENCIL_INDEX</STRONG> and there is no stencil buffer.
<STRONG>GL_INVALID_OPERATION</STRONG> is generated if <EM>format</EM> is
<STRONG>GL_DEPTH_COMPONENT</STRONG> and there is no depth buffer.
<STRONG>GL_INVALID_OPERATION</STRONG> is generated if <STRONG>glReadPixels</STRONG> is
executed between the execution of <STRONG>glBegin</STRONG> and the
corresponding execution of <STRONG>glEnd</STRONG>.
<STRONG>ASSOCIATED</STRONG> <STRONG>GETS</STRONG>
<STRONG>glGet</STRONG> with argument <STRONG>GL_INDEX_MODE</STRONG>
<STRONG>SEE</STRONG> <STRONG>ALSO</STRONG>
<STRONG>glCopyPixels</STRONG>, <STRONG>glDrawPixels</STRONG>, <STRONG>glPixelMap</STRONG>, <STRONG>glPixelStore</STRONG>,
<STRONG>glPixelTransfer</STRONG>,
<STRONG>glReadBuffer</STRONG>
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