GLCOPYPIXELS(3G) UNIX Programmer's Manual GLCOPYPIXELS(3G)
glCopyPixels - copy pixels in the frame buffer
void glCopyPixels( GLint x,
GLint y,
GLsizei width,
GLsizei height,
GLenum type )
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 sten-
cil values are to be copied. Symbolic constants
GL_COLOR, GL_DEPTH, and GL_STENCIL are accepted.
glCopyPixels copies a screen-aligned rectangle of pixels
from the specified frame buffer location to a region rela-
tive to the current raster position. Its operation is well
defined only if the entire pixel source region is within the
exposed portion of the window. Results of copies from out-
side 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. Both width and height must not be negative.
Several parameters control the processing of the pixel data
while it is being copied. These parameters are set with
three commands: glPixelTransfer, glPixelMap, and
glPixelZoom. This reference page describes the effects on
glCopyPixels of most, but not all, of the parameters speci-
fied by these three commands.
glCopyPixels copies values from each pixel with the lower
left-hand 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 copied in row order from the lowest to
the highest row, left to right in each row.
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type specifies whether color, depth, or stencil data is to
be copied. The details of the transfer for each data type
are as follows:
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 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.
If the GL is in RGBA mode, the red, green,
blue, and alpha components of each pixel that
is read are converted to an internal
floating-point with unspecified precision.
The conversion maps the largest representable
component value to 1.0, and component value 0
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, and ALPHA for the respective color com-
ponents. The results are clamped to the range
[0,1]. If GL_MAP_COLOR is true, each color
component is scaled 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.
If the GL_ARB_imaging extension is supported,
the color values may be additionally pro-
cessed by color-table lookups, color-matrix
transformations, and convolution filters.
The GL then converts the resulting indices or
RGBA colors to fragments by attaching the
current raster position z coordinate and tex-
ture coordinates to each pixel, then assign-
ing window coordinates (xr + i,yr + j), where
(xr,yr) is the current raster position, and
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the pixel was the ith pixel in the jth row.
These pixel fragments are then 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.
GL_DEPTH Depth values are read from the depth buffer
and converted directly to an internal
floating-point 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 GL then converts the resulting depth com-
ponents to fragments by attaching the current
raster position color or color index and tex-
ture coordinates to each pixel, then assign-
ing window coordinates (xr + i,yr + j), where
(xr,yr) is the current raster position, and
the pixel was the ith pixel in the jth row.
These pixel fragments are then 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.
GL_STENCIL Stencil indices are read from the stencil
buffer and converted to an internal fixed-
point 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 nega-
tive, 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 indices
are then written to the stencil buffer such
that the index read from the ith location of
the jth row is written to location
(xr + i,yr + j), where (xr,yr) is the current
raster position. Only the pixel ownership
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test, the scissor test, and the stencil wri-
temask affect these write operations.
The rasterization described thus far assumes pixel zoom fac-
tors of 1.0. If
glPixelZoom is used to change the x and y pixel zoom fac-
tors, pixels are converted to fragments as follows. If (xr,
yr) is the current raster position, and a given pixel is in
the ith location in the jth row of the source pixel rectan-
gle, then fragments are generated for pixels whose centers
are in the rectangle with corners at
(xr + zoomxi, yr + zoomyj)
and
(xr + zoomx(i + 1), yr + zoomy(j + 1))
where zoomx is the value of GL_ZOOM_X and zoomy is the value
of GL_ZOOM_Y.
To copy the color pixel in the lower left corner of the win-
dow to the current raster position, use glCopyPixels(0, 0,
1, 1, GL_COLOR);
Modes specified by glPixelStore have no effect on the opera-
tion of glCopyPixels.
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 exe-
cuted between the execution of glBegin and the corresponding
execution of glEnd.
glGet with argument GL_CURRENT_RASTER_POSITION
glGet with argument GL_CURRENT_RASTER_POSITION_VALID
glColorTable(3G), glConvolutionFilter1D(3G),
glConvolutionFilter2D(3G), glDepthFunc(3G),
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glDrawBuffer(3G), glDrawPixels(3G), glMatrixMode(3G),
glPixelMap(3G), glPixelTransfer(3G), glPixelZoom(3G),
glRasterPos(3G), glReadBuffer(3G), glReadPixels(3G),
glSeparableFilter2D(3G), glStencilFunc(3G)
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