glReadPixels
GLREADPIXELS(3G) GLREADPIXELS(3G)
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 )
delim $$
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 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 transformations, convolutions, histograms and minimum and max-
imun 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 $i$th pixel in the $j$th 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 map-
pings 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 com-
ponent 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 $2 sup n - 1$.
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 inten-
sity 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 map-
ping 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 dis-
cards 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 $i$th pixel in the $j$th 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 for-
mat 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(3G), glDrawPixels(3G), glPixelMap(3G), glPixelStore(3G),
glPixelTransfer(3G),
glReadBuffer(3G)
GLREADPIXELS(3G)
