glAccum man page on OpenBSD

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GLACCUM(3G)							   GLACCUM(3G)

NAME
       glAccum - operate on the accumulation buffer

C SPECIFICATION
       void glAccum( GLenum op,
	    GLfloat value )

       delim $$

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 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
       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 $2 sup n^-^1$, 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 $2 sup n^-^1$,
		     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 $2 sup n^-^1$, clamped to the range [0,$~2 sup 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)

								 March 1, 2011
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