glcopypixels
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glCopyPixels(3G) OpenGL Reference glCopyPixels(3G)
NAME
glCopyPixels - copy pixels in the frame buffer
C SPECIFICATION
void glCopyPixels( GLint x,
GLint y,
GLsizei width,
GLsizei height,
GLenum type )
PARAMETERS
x, y Specify the window coordinates of the lower left corner of the
rectangular region of pixels to be copied.
width, height
Specify the dimensions of the rectangular region of pixels to be
copied. Both must be nonnegative.
type Specifies whether color values, depth values, or stencil values are
to be copied. Symbolic constants GL_COLOR, GL_DEPTH, and GL_STENCIL
are accepted.
DESCRIPTION
glCopyPixels copies a screen-aligned rectangle of pixels from the
specified frame buffer location to a region relative to the current
raster position. Its operation is well defined only if the entire pixel
source region is within the exposed portion of the read window associated
with the current context. Results of copies from outside the window, or
from regions of the window that are not exposed, are hardware dependent
and undefined.
x and y specify the window coordinates of the lower left corner of the
rectangular region to be copied. width and height specify the dimensions
of the rectangular region to be copied. width and height must be non-
negative.
Several parameters control the processing of the pixel data while it is
being copied. These parameters are set with the commands
glPixelTransfer, glPixelMap, and glPixelZoom. The state of
GL_INTERLACE_SGIX, controlled by glEnable and glDisable, also affects the
result of glCopyPixels. This reference page describes the effects on
glCopyPixels of most, but not all, of these parameters. In addition the
convolution, histogram, and minmax operations may affect the result of
glCopyPixels. See the reference pages for glConvolutionFilter2DEXT,
glHistogramEXT, and glMinmaxEXT for more information.
glCopyPixels copies values from each pixel with lower left-hand corner at
(x + i, y + j) for 0<iglCopyPixels(3G) OpenGL Reference glCopyPixels(3G)
lowest to the highest row, left to right in each row.
type specifies whether color, depth, or stencil data is to be copied.
The pixels read from the source buffer are operated upon in a manner that
is dependent on type to generate various fragment values as detailed
below. The number of fragments generated for each source pixel and the
assignment of xdst and ydst window coordinates to these fragments are
common to all types and are as follows.
If the current raster position is invalid, no fragments are generated for
any of the source pixels. Otherwise, let (xr, yr) be the current raster
position. For each source pixel, fragments with values computed
according to that pixel are generated for destination pixels whose
centers are in the rectangle with corners at
(xr + i x zoomx, yr + j x interlace x zoomy) and
(xr + (i + 1) x zoomx,yr + (j x interlace + 1) x zoomy). zoomx and zoomy
are the values of GL_ZOOM_X and GL_ZOOM_Y, respectively, and interlace is
1 or 2 depending on whether GL_INTERLACE_SGIX is GL_FALSE or GL_TRUE,
respectively. GL_ZOOM_X and GL_ZOOM_Y, initially set to 1.0, are set
using glPixelZoom; GL_INTERLACE_SGIX, initially disabled, is controlled
by glEnable and glDisable.
GL_INTERLACE_SGIX is particularly useful when the pixels are copied from
fields of an interlaced video source. For additional discussion of
GL_INTERLACE_SGIX see the NOTES section of the reference page for
glDrawPixels.
Except for when type is GL_STENCIL the generated fragments are treated
just like the fragments generated by rasterizing points, lines, or
polygons. Texture mapping, fog, and all the fragment operations are
applied before the fragments are written to the frame buffer. When type
is GL_STENCIL then only the pixel ownership test, the scissor test, and
the stencil writemask affect the writes to the stencil buffer.
GL_COLOR Indices or RGBA colors are read from the buffer currently
specified as the read source buffer (see glReadBuffer).
If the GL is in color index mode, each index that is read
from this buffer is converted to a fixed-point format with
an unspecified number of bits to the right of the binary
point. Each index is then shifted left by GL_INDEX_SHIFT
bits, and added to GL_INDEX_OFFSET. If GL_INDEX_SHIFT is
negative, the shift is to the right. In either case, zero
bits fill otherwise unspecified bit locations in the
result. If GL_MAP_COLOR is true, the index is replaced
with the value that it references in lookup table
GL_PIXEL_MAP_I_TO_I. Whether the lookup replacement of
the index is done or not, the integer part of the index is
then ANDed with 2b-1, where b is the number of bits in a
color index buffer.
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glCopyPixels(3G) OpenGL Reference glCopyPixels(3G)
If the GL is in RGBA mode, each of the red, green, blue,
and alpha components of each pixel that is read is
converted to an internal floating-point format with
unspecified precision. The conversion maps the largest
representable component value to 1.0, and component value
zero to 0.0. The resulting floating-point color values
are then multiplied by GL_c_SCALE and added to GL_c_BIAS,
where c is RED, GREEN, BLUE, or ALPHA for the respective
color components. The results are clamped to the range
[0,1]. If GL_MAP_COLOR is true, each color component is
multiplied by the size of lookup table
GL_PIXEL_MAP_c_TO_c, then replaced by the value that it
references in that table. c is R, G, B, or A,
respectively.
The resulting index or RGBA color components, and the
current raster position z and texture coordinates are
assigned to each of the fragments generated for the source
pixel.
GL_DEPTH Depth values are read from the depth buffer and converted
directly to an internal floating-point format with
unspecified precision. The resulting floating-point depth
value is then multiplied by GL_DEPTH_SCALE and added to
GL_DEPTH_BIAS. The result is clamped to the range [0,1].
The resulting depth component, and the current raster
position texture coordinates and color or color index are
assigned to each of the fragments generated for the source
pixel.
GL_STENCIL Stencil indices are read from the stencil buffer and
converted to an internal fixed-point format with an
unspecified number of bits to the right of the binary
point. Each fixed-point index is then shifted left by
GL_INDEX_SHIFT bits, and added to GL_INDEX_OFFSET. If
GL_INDEX_SHIFT is negative, the shift is to the right. In
either case, zero bits fill otherwise unspecified bit
locations in the result. If GL_MAP_STENCIL is true, the
index is replaced with the value that it references in
lookup table GL_PIXEL_MAP_S_TO_S. Whether the lookup
replacement of the index is done or not, the integer part
of the index is then ANDed with 2b-1, where b is the
number of bits in the stencil buffer.
The resulting stencil value is assigned to each of the
fragments generated for the source pixel.
EXAMPLES
To copy the color pixel in the lower left corner of the window to the
current raster position, use
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glCopyPixels(3G) OpenGL Reference glCopyPixels(3G)
glCopyPixels(0, 0, 1, 1, GL_COLOR);
NOTES
Modes specified by glPixelStore have no effect on the operation of
glCopyPixels.
ERRORS
GL_INVALID_ENUM is generated if type is not an accepted value.
GL_INVALID_VALUE is generated if either width or height is negative.
GL_INVALID_OPERATION is generated if type is GL_DEPTH and there is no
depth buffer.
GL_INVALID_OPERATION is generated if type is GL_STENCIL and there is no
stencil buffer.
GL_INVALID_OPERATION is generated if glCopyPixels is executed between the
execution of glBegin and the corresponding execution of glEnd.
ASSOCIATED GETS
glGet with argument GL_CURRENT_RASTER_POSITION
glGet with argument GL_CURRENT_RASTER_POSITION_VALID
glGet with argument GL_INTERLACE_SGIX
MACHINE DEPENDENCIES
On RealityEngine, RealityEngine2, and VTX systems convolution may not be
used in the following circumstances:
1. When rendering to pixmaps.
2. When fragment processing (texturing, depth buffering, alpha
testing, multisampling, fog) is enabled.
3. When histogramming or minmax is enabled.
4. When either of the pixel zoom factors has a value other than 1.0
or -1.0.
In these cases, glDrawPixels and glCopyPixels report a
GL_INVALID_OPERATION error and do not transfer any pixels.
Loading a convolution filter with some combinations of pixel transfer
modes may be unreliable on InfiniteReality systems. Straightforward
transfers are known to work, though. The remaining problems will be
fixed in the next release.
RealityEngine, RealityEngine2, and VTX systems cannot copy pixels from a
video source (see glXCreateGLXVideoSourceSGIX) when RGBA scale and bias
are not 1 and 0, respectively, or when any of texturing, alpha or depth
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glCopyPixels(3G) OpenGL Reference glCopyPixels(3G)
testing, pixel mapping, multisampling, convolution, histogram, or minmax
is enabled. A GL_INVALID_OPERATION error will result if any of these
conditions is true when glCopyPixels is executed.
Performance of glCopyPixels on InfiniteReality systems is expected to
increase in a future release.
The SGIX_interlace extension is supported only on InfiniteReality systems
and on RealityEngine, RealityEngine2, and VTX systems.
SEE ALSO
glConvolutionFilter2DEXT, glDepthFunc, glDrawBuffer, glDrawPixels,
glEnable, glHistogramEXT, glMinmaxEXT, glPixelMap, glPixelTransfer,
glPixelZoom, glRasterPos, glReadBuffer, glReadPixels,
glSeparableFilter2DEXT, glStencilFunc.
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