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<STRONG>NAME</STRONG>
<STRONG>glAccum</STRONG> - operate on the accumulation buffer
<STRONG>C</STRONG> <STRONG>SPECIFICATION</STRONG>
void <STRONG>glAccum</STRONG>( GLenum <EM>op</EM>,
GLfloat <EM>value</EM> )
<STRONG>PARAMETERS</STRONG>
<EM>op</EM> Specifies the accumulation buffer operation.
Symbolic constants <STRONG>GL_ACCUM</STRONG>, <STRONG>GL_LOAD</STRONG>, <STRONG>GL_ADD</STRONG>,
<STRONG>GL_MULT</STRONG>, and <STRONG>GL_RETURN</STRONG> are accepted.
<EM>value</EM> Specifies a floating-point value used in the
accumulation buffer operation. <EM>op</EM> determines how
<EM>value</EM> is used.
<STRONG>DESCRIPTION</STRONG>
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
<STRONG>glGetIntegerv</STRONG> four times, with arguments <STRONG>GL_ACCUM_RED_BITS</STRONG>,
<STRONG>GL_ACCUM_GREEN_BITS</STRONG>, <STRONG>GL_ACCUM_BLUE_BITS</STRONG>, and
<STRONG>GL_ACCUM_ALPHA_BITS</STRONG>. 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.
<STRONG>glAccum</STRONG> operates on the accumulation buffer. The first
argument, <EM>op</EM>, is a symbolic constant that selects an
accumulation buffer operation. The second argument, <EM>value</EM>,
is a floating-point value to be used in that operation.
Five operations are specified: <STRONG>GL_ACCUM</STRONG>, <STRONG>GL_LOAD</STRONG>, <STRONG>GL_ADD</STRONG>,
<STRONG>GL_MULT</STRONG>, and <STRONG>GL_RETURN</STRONG>.
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
<STRONG>glAccum</STRONG> 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:
<STRONG>GL_ACCUM</STRONG> Obtains R, G, B, and A values from the buffer
currently selected for reading (see
<STRONG>glReadBuffer</STRONG>). 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 <EM>value</EM> and added to the
corresponding pixel component in the
accumulation buffer, thereby updating the
accumulation buffer.
<STRONG>GL_LOAD</STRONG> Similar to <STRONG>GL_ACCUM</STRONG>, 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 <EM>value</EM>, and then stored in the
corresponding accumulation buffer cell,
overwriting the current value.
<STRONG>GL_ADD</STRONG> Adds <EM>value</EM> to each R, G, B, and A in the
accumulation buffer.
<STRONG>GL_MULT</STRONG> Multiplies each R, G, B, and A in the
accumulation buffer by <EM>value</EM> and returns the
scaled component to its corresponding
accumulation buffer location.
<STRONG>GL_RETURN</STRONG> Transfers accumulation buffer values to the
color buffer or buffers currently selected for
writing. Each R, G, B, and A component is
multiplied by <EM>value</EM>, 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 <STRONG>glClearAccum</STRONG> with R,
G, B, and A values to set it to, then call <STRONG>glClear</STRONG> with the
accumulation buffer enabled.
<STRONG>NOTES</STRONG>
Only pixels within the current scissor box are updated by a
<STRONG>glAccum</STRONG> operation.
<STRONG>ERRORS</STRONG>
<STRONG>GL_INVALID_ENUM</STRONG> is generated if <EM>op</EM> is not an accepted value.
<STRONG>GL_INVALID_OPERATION</STRONG> is generated if there is no
accumulation buffer.
<STRONG>GL_INVALID_OPERATION</STRONG> is generated if <STRONG>glAccum</STRONG> is executed
between the execution of <STRONG>glBegin</STRONG> and the corresponding
execution of <STRONG>glEnd</STRONG>.
<STRONG>ASSOCIATED</STRONG> <STRONG>GETS</STRONG>
<STRONG>glGet</STRONG> with argument <STRONG>GL_ACCUM_RED_BITS</STRONG>
<STRONG>glGet</STRONG> with argument <STRONG>GL_ACCUM_GREEN_BITS</STRONG>
<STRONG>glGet</STRONG> with argument <STRONG>GL_ACCUM_BLUE_BITS</STRONG>
<STRONG>glGet</STRONG> with argument <STRONG>GL_ACCUM_ALPHA_BITS</STRONG>
<STRONG>SEE</STRONG> <STRONG>ALSO</STRONG>
<STRONG>glBlendFunc</STRONG>, <STRONG>glClear</STRONG>, <STRONG>glClearAccum</STRONG>, <STRONG>glCopyPixels</STRONG>, <STRONG>glGet</STRONG>,
<STRONG>glLogicOp</STRONG>, <STRONG>glPixelStore</STRONG>, <STRONG>glPixelTransfer</STRONG>, <STRONG>glReadBuffer</STRONG>,
<STRONG>glReadPixels</STRONG>, <STRONG>glScissor</STRONG>, <STRONG>glStencilOp</STRONG>
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