C SPECIFICATION
void GLAPIENTRY glReadPixels( GLint x,
GLint GLint )
PARAMETERS
x, GLint 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.
_param3, _param4
Specify the dimensions of the pixel rectangle. _param3 and
_param4 of one correspond to a single pixel.
_param5 Specifies the format of the pixel data. The following sym-
bolic 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_LUMINANCE, and
GL_LUMINANCE_ALPHA.
_param6 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.
_param7 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, GLint), into
client memory starting at location _param7. Several parameters control
the processing of the pixel data before it is placed into client mem-
ory. 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 speci-
fied 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, GLint + j) for 0 <= i < _param3 and 0 <= j < _param4. 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.
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 compo-
nent 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_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 be of size 2n, but n
may be different for different tables. Before an index is
used to look up a value in a table of size 2n, it must be
masked against 2n-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
glPixelTransfer. The lookup table contents themselves are specified by
glPixelMap.
Finally, the indices or components are converted to the proper format,
as specified by _param6. If _param5 is GL_COLOR_INDEX or
GL_STENCIL_INDEX and _param6 is not GL_FLOAT, each index is masked with
the mask value given in the following table. If _param6 is GL_FLOAT,
then each integer index is converted to single-precision floating-point
format.
If _param5 is GL_RED, GL_GREEN, GL_BLUE, GL_ALPHA, GL_RGB, GL_BGR,
GL_RGBA, GL_BGRA, GL_LUMINANCE, or GL_LUMINANCE_ALPHA and _param6 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).
------------------------------------------------------
_param6 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 _param5 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)_eqn-
param3+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 con-
tiguous space in _param7. 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 _param7.
ERRORS
GL_INVALID_ENUM is generated if _param5 or _param6 is not an accepted
value.
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 _param6 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 _param5 is
not GL_RGB.
GL_INVALID_OPERATION is generated if _param6 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
_param5 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.
ASSOCIATED GETS
glGet with argument GL_INDEX_MODE
SEE ALSO
glCopyPixels, glDrawPixels, glPixelMap, glPixelStore, glPixelTransfer,
glReadBuffer
GLREADPIXELS(3G)
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