glAccum

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 con-
	      stants 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 poly-
       gons), 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  accumula-
       tion 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	accu-
		     mulation	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  correspond-
		     ing  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 multi-
		     plied 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)