 |
Index for
Section 3G |
|
 |
Alphabetical
listing for G |
|
 |
Bottom of
page |
|
glCopyPixels(3G)
NAME
glCopyPixels - copy pixels in the frame buffer
SYNOPSIS
void glCopyPixels(
GLint x,
GLint y,
GLsizei width,
GLsizei height,
GLenum type );
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
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 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 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:
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 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 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 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.
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.
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 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.
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
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 GETS
glGet() with argument GL_CURRENT_RASTER_POSITION
glGet() with argument GL_CURRENT_RASTER_POSITION_VALID
SEE ALSO
glColorTable(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), glStencilFunc(3)
|
Index for
Section 3G |
|
|
Alphabetical
listing for G |
|
 |
Top of
page |
|