GLCOPYPIXELS()GLCOPYPIXELS()NAMEglCopyPixels - 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.
DESCRIPTIONglCopyPixels 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: 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 $i$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 glRead‐
Buffer). 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_OFF‐
SET. If GL_INDEX_SHIFT is negative, the shift is to the
right. In either case, zero bits fill otherwise unspec‐
ified bit locations in the result. If GL_MAP_COLOR is
true, the index is replaced with the value that it ref‐
erences 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 format with unspec‐
ified 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.
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 current
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 fragments 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 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 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 current
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 fragments are
written to the frame buffer.
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 $2 sup b -1$, where
$b$ is the number of bits in the stencil buffer. The
resulting stencil indices are then written to the sten‐
cil buffer such that the index read from the $i$th loca‐
tion 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
glDepthFunc, glDrawBuffer, glDrawPixels, glPixelMap, glPixelTransfer,
glPixelZoom, glRasterPos, glReadBuffer, glReadPixels, glStencilFunc
GLCOPYPIXELS()
