glCopyPixels(3G)glCopyPixels(3G)NAMEglCopyPixels - copy pixels in the frame buffer
SYNOPSIS
void glCopyPixels(
GLint x,
GLint y,
GLsizei width,
GLsizei height,
GLenum type );
PARAMETERS
Specify the window coordinates of the lower left corner of the rectan‐
gular region of pixels to be copied. Specify the dimensions of the
rectangular region of pixels to be copied. Both must be nonnegative.
Specifies whether color values, depth values, or stencil values are to
be copied. Symbolic constants GL_COLOR, GL_DEPTH, and GL_STENCIL are
accepted.
DESCRIPTIONglCopyPixels() 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 window. 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 dimen‐
sions 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: glPixel‐
Transfer, glPixelMap, and glPixelZoom. This reference page describes
the effects on glCopyPixels() of most, but not all, of the parameters
specified 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.
type specifies whether color, depth, or stencil data is to be copied.
The details of the transfer for each data type are as follows: 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 loca‐
tions 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 2^b-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 com‐
ponents 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 components.
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 refer‐
ences in that table. c is R, G, B, or A.
If the GL_ARB_imaging extension is supported, the color values
may be additionally processed 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 texture coordinates to each pixel, then assigning window
coordinates (x[r] + i , y[r] + j), where (x[r], y[r]) 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.
Depth values are read from the depth buffer and converted
directly to an internal floating-point with unspecified preci‐
sion. The resulting floating-point depth value is then multi‐
plied 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 components to fragments
by attaching the current raster position color or color index
and texture coordinates to each pixel, then assigning window
coordinates (x[r] + i , y[r] + j), where (x[r], y[r]) 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.
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_OFF‐
SET. If GL_INDEX_SHIFT is negative, the shift is to the right.
In either case, zero bits fill otherwise unspecified bit loca‐
tions 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
2 sup b -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 (x[r] + i , y[r] + j), where (x[r],
y[r]) is the current raster position. Only the pixel ownership
test, the scissor test, and the stencil writemask affect these
write operations.
The rasterization described thus far assumes pixel zoom factors of 1.0.
If glPixelZoom is used to change the x and y pixel zoom factors, pixels
are converted to fragments as follows. If (x[r], y[r]) is the current
raster position, and a given pixel is in the ith location in the jth
row of the source pixel rectangle, then fragments are generated for
pixels whose centers are in the rectangle with corners at (x[r] +
zoom[x]^ i, y[r] + zoom[y] j)
and (x[r] + zoom[x] (i + 1), y[r] + zoom[y] ( j + 1 ))
where zoom[x] is the value of GL_ZOOM_X and zoom[y] is the value of
GL_ZOOM_Y.
EXAMPLES
To copy the color pixel in the lower left corner of the window to the
current raster position, use 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 GETSglGet() with argument GL_CURRENT_RASTER_POSITION
glGet() with argument GL_CURRENT_RASTER_POSITION_VALID
SEE ALSOglColorTable(3), glConvolutionFilter1D(3), glConvolutionFilter2D(3),
glDepthFunc(3), glDrawBuffer(3), glDrawPixels(3), glMatrixMode(3),
glPixelMap(3), glPixelTransfer(3), glPixelZoom(3), glRasterPos(3),
glReadBuffer(3), glReadPixels(3), glSeparableFilter2D(3), glStencil‐
Func(3)glCopyPixels(3G)