glCopyPixels(3)
NAME
glCopyPixels - copy pixels in the frame buffer
C SPECIFICATION
void glCopyPixels( GLint x,
GLint y,
GLsizei width,
GLsizei height,
GLenum type )
delim $$
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 spec-
ified 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:
glPixelTransfer, 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 $i$th pixel in the $j$th 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:
GL_COLOR Indices or RGBA colors are read from the buffer cur-
rently 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 $2 sup b -1$, 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 components of each pixel that is read are con-
verted to an internal floating-point with unspecified
precision. The conversion maps the largest repre-
sentable 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 respec-
tive color components. The results are clamped to the
range [0,1]. If GL_MAP_COLOR is true, each color compo-
nent 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 processed by color-table
lookups, color-matrix transformations, and convolution
filters.
The GL then converts the resulting indices or RGBA col-
ors to fragments by attaching the current raster posi-
tion z coordinate and texture coordinates to each pixel,
then assigning window coordinates ($x sub r ~+~ i , y
sub r ~+~ j$), where ($x sub r , y sub r$) is the cur-
rent raster position, and the pixel was the $i$th pixel
in the $j$th 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 frag-
ments are written to the frame buffer.
GL_DEPTH Depth values are read from the depth buffer and con-
verted 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 components to
fragments by attaching the current raster position color
or color index and texture coordinates to each pixel,
then assigning window coordinates ($x sub r ~+~ i , y
sub r ~+~ j$), where ($x sub r , y sub r$) is the cur-
rent raster position, and the pixel was the $i$th pixel
in the $j$th 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 frag-
ments 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 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 ta-
ble 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 $i$th location of the
$j$th row is written to location ($x sub r ~+~ i , y sub
r ~+~ j$), where ($x sub r , y sub r$) 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, pix-
els are converted to fragments as follows. If ($x sub r$, $y sub r$)
is the current raster position, and a given pixel is in the $i$th loca-
tion in the $j$th row of the source pixel rectangle, then fragments are
generated for pixels whose centers are in the rectangle with corners at
($x sub r ~+~ zoom sub x^ i$, $y sub r ~+~ zoom sub y^j$)
and
($x sub r ~+~ zoom sub x^ (i ~+~ 1)$, $y sub r ~+~ zoom sub y^ ( j ~+~
1 )$)
where $zoom sub x$ is the value of GL_ZOOM_X and $zoom sub y$ 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 GETS
glGet with argument GL_CURRENT_RASTER_POSITION
glGet with argument GL_CURRENT_RASTER_POSITION_VALID
SEE ALSO
glColorTable(3G), glConvolutionFilter1D(3G), glConvolutionFilter2D(3G),
glDepthFunc(3G), glDrawBuffer(3G), glDrawPixels(3G), glMatrixMode(3G),
glPixelMap(3G), glPixelTransfer(3G), glPixelZoom(3G), glRasterPos(3G),
glReadBuffer(3G), glReadPixels(3G), glSeparableFilter2D(3G),
glStencilFunc(3G)
GLCOPYPIXELS(3G)
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