glReadPixels(3G) OpenGL Reference 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 format 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_ABGR_EXT, 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 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.
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glReadPixels(3G) OpenGL Reference glReadPixels(3G)
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 format for the returned pixel values; accepted
values are:
GL_COLOR_INDEX [Toc] [Back]
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 [Toc] [Back]
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 [Toc] [Back]
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 [Toc] [Back]
GL_GREEN [Toc] [Back]
GL_BLUE [Toc] [Back]
GL_ALPHA [Toc] [Back]
GL_RGB [Toc] [Back]
GL_BGR [Toc] [Back]
GL_RGBA [Toc] [Back]
GL_BGRA [Toc] [Back]
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glReadPixels(3G) OpenGL Reference glReadPixels(3G)
GL_ABGR_EXT [Toc] [Back]
GL_LUMINANCE [Toc] [Back]
GL_LUMINANCE_ALPHA [Toc] [Back]
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
n
be of size 2 , but n may be different for different tables.
Before an index is used to look up a value in a table of size
n n
2 , it must be masked against 2 -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 singlecomponent
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 format, 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 format.
If format is GL_RED, GL_GREEN, GL_BLUE, GL_ALPHA, GL_RGB, GL_BGR,
GL_RGBA, GL_BGRA, GL_ABGR_EXT, 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
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glReadPixels(3G) OpenGL Reference glReadPixels(3G)
is passed as is (or converted to the client's single-precision floatingpoint
format 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, GL_BGRA
and GL_ABGR_EXT 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.
Normally glReadPixels is synchronous: OpenGL executes a glReadPixels
command in the order it is issued in the OpenGL command stream. Calling
glEnable with parameter GL_ASYNC_READ_PIXELS_SGIX causes subsequent
glReadPixels commands to be asynchronous as defined by the SGIX_async
extension. An implementation of the asynchronous glReadPixels command
may be non-blocking: It may return control to the program immediately
rather than waiting for the command's results to be written into the
destination rectangle in host memory. The glReadPixels command samples
the OpenGL state vector at the time it is issued, but it may be executed
at some undefined point in the future. In particular, if subsequent
commands change the contents of the source rectangle in the framebuffer,
then the result of the glReadPixels is undefined.
Calling glDisable with parameter GL_ASYNC_READ_PIXELS_SGIX restores the
default synchronous behavior for subsequent glReadPixels commands. It
does not affect any pending asynchronous glReadPixels commands, or wait
for their completion.
When an asynchronous glReadPixels command is issued, it is associated
with the current value of GL_ASYNC_MARKER_SGIX as defined by the
SGIX_async extension. A program can determine if an asynchronous
glReadPixels command has completed by using the glFinishAsyncSGIX or
glPollAsyncSGIX commands.
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glReadPixels(3G) OpenGL Reference glReadPixels(3G)
There is a maximum number of asynchronous glReadPixels commands that can
be outstanding at any one time, defined by the implementation. This
value can be queried with glGetIntegerv with argument
GL_MAX_ASYNC_READ_PIXELS_SGIX.
When GL_INTERLACE_READ_INGR is enabled, every other row of the source
pixel rectangle is read. The height of the pixel rectangle is equivalent
to 2xheight. Only rows (y+0,y+2,...) are read and affected by the pixel
transfer and store operations.
Values for pixels that lie outside the window connected to the current GL
context are undefined.
GL_ABGR_EXT is valid only if the EXT_abgr extension is supported.
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 not GL_RGBA, GL_BGRA or GL_ABGR_EXT.
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glReadPixels(3G) OpenGL Reference glReadPixels(3G)
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.
GL_INVALID_OPERATION is generated when the SGIX_subsample extension is
supported, and the pixel storage mode GL_PACK_SUBSAMPLE_RATE_SGIX is not
GL_PIXEL_SUBSAMPLE_4444_SGIX, and width is not a multiple of 2, or format
is not a 3 or 4 component format, or type is a packed pixels type.
GL_INVALID_OPERATION is generated if GL_ASYNC_READ_PIXELS_SGIX is enabled
and the number of asynchronous glReadPixels commands that have been
issued but not queried (using glFinishAsyncSGIX or glPollAsyncSGIX)
exceeds GL_MAX_ASYNC_READ_PIXELS_SGIX.
glGet with argument GL_INDEX_MODE
MACHINE DEPENDENCIES
The SGIX_async and SGIX_async_pixel extensions are implemented only on
Octane2 VPro systems.
RealityEngine, RealityEngine2, and VTX systems do not support convolving
images as they are read from the framebuffer.
The SGIX_ycrcb extension is supported only on O2 systems. When using
GL_YCRCB_422_SGIX with glReadPixels on O2 systems, an odd integer value
for GL_PACK_SKIP_PIXELS will be set to the next highest even integer
value to preserve color alignment.
On Octane2 VPro systems the format GL_DEPTH_COMPONENT24_SGIX can be used
to transfer depth pixel values to and from the depth buffer in their
internal eye-space range. There are performance advantages over
transfers that convert to screen-space values, particularly for
GL_UNSIGNED_INT type pixels.
On RealityEngine, RealityEngine2, and VTX systems, histogramming and
minmax never sink pixel data that is being read from the framebuffer.
The INGR_interlace_read extension is supported only on Octane2 VPro
systems, and on O2 systems.
The EXT_packed_pixels extension is not supported on RealityEngine,
RealityEngine2, and VTX systems.
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glReadPixels(3G) OpenGL Reference glReadPixels(3G)
The following problems are known to exist on RealityEngine,
RealityEngine2, and VTX systems:
1. It is not possible to read the depth or stencil buffers of
multisample-capable Visuals.
2. Reading the depth-buffer is much slower than reading a colorbuffer.
3. Reading RGBA format data from a color-index window does not work
correctly.
The SGIX_subsample and SGIX_resample extensions are supported only on
Octane2 VPro systems. Applying the GL_PIXEL_SUBSAMPLE_2424_SGIX
subsample rate is accelerated for direct immmediate mode transfers when
the format is GL_RGB or GL_RGBA, and the type is GL_UNSIGNED_BYTE or
GL_UNSIGNED_SHORT.
glCopyPixels, glDrawPixels, glPixelMap, glPixelStore, glPixelTransfer,
glReadBuffer, glAsyncMarkerSGIX, glDeleteAsyncMarkersSGIX,
glFinishAsyncSGIX, glGenAsyncMarkersSGIX, glIsAsyncMarkerSGIX,
glPollAsyncSGIX
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