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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 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
       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 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 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
		     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 2n−1, multiplied
		     by value, and then stored in the corresponding  accumula‐
		     tion 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 corre‐
		     sponding 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 2n−1, clamped to the range [0,2n−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)
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