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GLACCUM(3G)                                                                                      GLACCUM(3G)



NAME
       glAccum - operate on the accumulation buffer


C SPECIFICATION
       void glAccum( GLenum op,
                     GLfloat value )


PARAMETERS
       op     Specifies  the  accumulation  buffer operation.  Symbolic constants GL_ACCUM, GL_LOAD, GL_ADD,
              GL_MULT, and GL_RETURN are accepted.

       value  Specifies a floating-point value used in the accumulation buffer operation.  op determines how
              value is used.

DESCRIPTION
       The accumulation buffer is an extended-range color buffer.  Images are not rendered into it.  Rather,
       images rendered into one of the color buffers are added to the contents of  the  accumulation  buffer
       after  rendering.   Effects  such  as antialiasing (of points, lines, and polygons), motion blur, and
       depth of field can be created by accumulating images generated with different  transformation  matri-ces. matrices.
       ces.

       Each  pixel in the accumulation buffer consists of red, green, blue, and alpha values.  The number of
       bits per component in the accumulation buffer depends on the implementation.  You  can  examine  this
       number  by  calling  glGetIntegerv four times, with arguments GL_ACCUM_RED_BITS, GL_ACCUM_GREEN_BITS,
       GL_ACCUM_BLUE_BITS, and GL_ACCUM_ALPHA_BITS.  Regardless of the number of  bits  per  component,  the
       range  of values stored by each component is [-1, 1].  The accumulation buffer pixels are mapped one-to-one oneto-one
       to-one with frame buffer pixels.

       glAccum operates on the accumulation buffer.  The first argument, op, is  a  symbolic  constant  that
       selects  an  accumulation buffer operation.  The second argument, value, is a floating-point value to
       be used in that operation.  Five operations are specified: GL_ACCUM, GL_LOAD,  GL_ADD,  GL_MULT,  and
       GL_RETURN.

       All  accumulation  buffer  operations  are limited to the area of the current scissor box and applied
       identically to the red, green, blue, and alpha components of each  pixel.   If  a  glAccum  operation
       results  in a value outside the range [-1, 1], the contents of an accumulation buffer pixel component
       are undefined.

       The operations are as follows:

       GL_ACCUM      Obtains R, G, B, and A values from the  buffer  currently  selected  for  reading  (see
                     glReadBuffer).   Each  component  value is divided by (2^n)-1, where n is the number of
                     bits allocated to each color component in the currently selected buffer.  The result is
                     a  floating-point  value in the range [0, 1], which is multiplied by value and added to
                     the corresponding pixel component in the  accumulation  buffer,  thereby  updating  the
                     accumulation buffer.

       GL_LOAD       Similar  to  GL_ACCUM,  except that the current value in the accumulation buffer is not
                     used in the calculation of the new value.  That is, the R, G, B, and A values from  the
                     currently  selected buffer are divided by (2^n)-1, multiplied by value, and then stored
                     in the corresponding accumulation buffer cell, overwriting the current value.

       GL_ADD        Adds value to each R, G, B, and A in the accumulation buffer.

       GL_MULT       Multiplies each R, G, B, and A in the accumulation buffer  by  value  and  returns  the
                     scaled component to its corresponding accumulation buffer location.

       GL_RETURN     Transfers  accumulation buffer values to the color buffer or buffers currently selected
                     for writing.  Each R, G, B, and A component is multiplied by value, then multiplied  by
                     (2^n)-1,  clamped  to  the  range  [0,(2^n)-1], and stored in the corresponding display
                     buffer cell.  The only fragment operations that are applied to this transfer are  pixel
                     ownership, scissor, dithering, and color writemasks.

       To  clear  the  accumulation  buffer, call glClearAccum with R, G, B, and A values to set it to, then
       call glClear with the accumulation buffer enabled.

NOTES
       Only pixels within the current scissor box are updated by a glAccum operation.

ERRORS
       GL_INVALID_ENUM is generated if op is not an accepted value.

       GL_INVALID_OPERATION is generated if there is no accumulation buffer.

       GL_INVALID_OPERATION is generated if glAccum is executed between the execution  of  glBegin  and  the
       corresponding execution of glEnd.

ASSOCIATED GETS
       glGet with argument GL_ACCUM_RED_BITS
       glGet with argument GL_ACCUM_GREEN_BITS
       glGet with argument GL_ACCUM_BLUE_BITS
       glGet with argument GL_ACCUM_ALPHA_BITS

SEE ALSO
       glClear(3G),  glClearAccum(3G),  glCopyPixels(3G),  glDrawBuffer(3G),  glGet(3G),  glReadBuffer(3G), glReadPixels(3G), glScissor(3G),
       glStencilOp(3G)




                                                                                                 GLACCUM(3G)