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glReadPixels(3G)
Contents
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glReadPixels - read a block of pixels from the frame
buffer
void glReadPixels(
GLint x,
GLint y,
GLsizei width,
GLsizei height,
GLenum format,
GLenum type,
GLvoid *pixels );
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. Specify the
dimensions of the pixel rectangle. width and height of
one correspond to a single pixel. 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.
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. Returns the pixel data.
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: 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. 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. 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].
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 floatingpoint
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.
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.
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.
glGet() with argument GL_INDEX_MODE
glCopyPixels(3), glPixelMap(3), glDrawPixels(3), glPixelStore(3), glPixelTransfer(3), glReadBuffer(3)
glReadPixels(3G)
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