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GLCOPYPIXELS(3G)                                                                            GLCOPYPIXELS(3G)



NAME
       glCopyPixels - copy pixels in the frame buffer


C SPECIFICATION
       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  nonnega-tive. nonnegative.
            tive.

       type Specifies whether color values, depth values, or stencil values are to be copied.  Symbolic con-stants constants
            stants 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 parame-ters parameters
       ters 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 refer-
                      ences 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 fil-ters. filters.
                      ters.

                      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 (xr + i , yr + j), where (xr , yr) 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  poly-gons. polygons.
                      gons.   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 cur-rent current
                      rent raster position color or color index and texture coordinates to each pixel,  then
                      assigning  window coordinates (xr + i , yr + j), where (xr , yr) 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 poly-gons. polygons.
                      gons.  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 fixedpoint
                      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^b)-1, where b is the  num-ber number
                      ber  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 (xr + i , yr + j), where (xr , yr) 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 (xr, yr) 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

                                           (xr + zoomx*i, yr + zoomy*j)

                                                        and

                                       (xr + zoomx*(i+1), yr + zoomy*(j+1))

       where zoomx is the value of GL_ZOOM_X and zoomy 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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