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 ren‐
       dering.  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 matrices.

       Each pixel in the accumulation buffer consists of red, green, blue,  and
       alpha values.  The number of bits per component in the accumulation buf‐
       fer depends on the implementation. You can examine this number by  call‐
       ing   glGetIntegerv   four   times,  with  arguments  GL_ACCUM_RED_BITS,
       GL_ACCUM_GREEN_BITS, GL_ACCUM_BLUE_BITS, and  GL_ACCUM_ALPHA_BITS.   Re‐
       gardless 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 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 op‐
       eration.  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 cur‐
       rent 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 2n−1, where n is the number of bits
                     allocated to each color component  in  the  currently  se‐
                     lected  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
                     2n−1,  clamped  to  the  range [0,2n−1], and stored in the
                     corresponding display buffer cell.  The only fragment  op‐
                     erations  that are applied to this transfer are pixel own‐
                     ership, 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 en‐
       abled.

NOTES
       Only pixels within the current scissor box are updated by a glAccum  op‐
       eration.

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 ex‐
       ecution 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, glClearAccum, glCopyPixels, glDrawBuffer, glGet,  glReadBuffer,
       glReadPixels, glScissor, glStencilOp

                                                                    GLACCUM(3G)