GLCOPYPIXELS() MachTen Programmer’s Manual GLCOPYPIXELS()

NAME
glCopyPixels - copy pixels in the frame buffer

C SPECIFICATION
void glCopyPixels( GLint x,
GLint y,
GLsizei width,
GLsizei height,
GLenum type )

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PARAMETERS
x, y Specify the window coordinates of the lower left cor-
ner 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 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 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. 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 spec-
ified 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 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 num-
ber 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 neg-
ative, 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 converted to an internal
floating-point format with unspecified pre-
cision. 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 multi-
plied 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 references in that table. c
is R, G, B, or A.

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 ($x sub r + i
, y sub r + j$), 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 map-
ping, fog, and all the fragment operations
are applied before the fragments are writ-
ten to the frame buffer.

GL_DEPTH Depth values are read from the depth buffer
and converted directly to an internal
floating-point format with unspecified pre-
cision. 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 neg-
ative, 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 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, pixels 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 location 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 exe-
cuted between the execution of glBegin and the correspond-
ing 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

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