glAccum
[New
- Windows 95, OEM Service Release 2]
The glAccum
function operates on the accumulation buffer.
void glAccum(
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GLenum op, |
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GLfloat value |
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); |
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Parameters
op
The
accumulation buffer operation. The accepted symbolic constants are:
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 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 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.
value
A
floating-point value used in the accumulation buffer operation. The op
parameter determines how value is used.
Remarks
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. You can create effects
such as antialiasing (of points, lines, and polygons), motion blur, and depth
of field 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 the
arguments GL_ACCUM_RED_BITS, GL_ACCUM_GREEN_BITS, GL_ACCUM_BLUE_BITS, and
GL_ACCUM_ALPHA_BITS, respectively. Regardless of the number of bits per
component, however, the range of values stored by each component is [ - 1, 1]. The accumulation buffer pixels are mapped
one-to-one with frame buffer pixels.
The glAccum
function 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 are applied identically to the red, green, blue, and alpha components
of each pixel. The contents of an accumulation buffer pixel component are
undefined if the glAccum operation results in a value outside the range
[ - 1,1]. The operations are as
follows:
To clear the
accumulation buffer, use the glClearAccum function to specify R, G, B, and A values to
set it to, and issue a glClear function with the accumulation buffer enabled.
Only those
pixels within the current scissor box are updated by any glAccum
operation.
The following
functions retrieve information related to the glAccum function:
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
Error Codes
The following
are the error codes generated and their conditions.
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Error
Code |
Condition |
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GL_INVALID_ENUM
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op was not an accepted value. |
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GL_INVALID_OPERATION
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There was
no accumulation buffer. |
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GL_INVALID_OPERATION
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glAccum was called between a call to glBegin and the
corresponding call to glEnd. |
See Also