GLREADPIXELS(3G) UNIX Programmer's Manual GLREADPIXELS(3G)
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 )
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 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.
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.
glReadPixels returns pixel data from the frame buffer,
starting with the pixel whose lower left corner is at loca-
tion (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
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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, convolu-
tions, histograms and minimum and maximun pixel value compu-
tations.
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
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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 com-
ponents. 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 inten-
sity 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 com-
ponents, while GL_RGB discards only the alpha com-
ponent. GL_LUMINANCE computes a single-component
value as the sum of the red, green, and blue com-
ponents, 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
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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).
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 sin-
gle 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 occu-
pying 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.
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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 com-
ponents.
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 exe-
cuted 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(3G), glDrawPixels(3G), glPixelMap(3G),
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glPixelStore(3G), glPixelTransfer(3G),
glReadBuffer(3G)
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