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glReadPixels(3G)
NAME
glReadPixels - read a block of pixels from the frame buffer
SYNOPSIS
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 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_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 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_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 2 sup 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 sup 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 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 , 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 .
If format is GL_RED, GL_GREEN, GL_BLUE, GL_ALPHA, GL_RGB, GL_BGR, GL_RGBA,
GL_BGRA, 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 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 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 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.
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 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
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(3), glPixelMap(3), glDrawPixels(3), glPixelStore(3),
glPixelTransfer(3), glReadBuffer(3)
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