fglAccum(3G) OpenGL Reference fglAccum(3G)NAME
fglAccum - operate on the accumulation buffer
FORTRAN SPECIFICATION
SUBROUTINE fglAccum( INTEGER*4 op,
REAL*4 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
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 fglGetIntegerv 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.
fglAccum 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 fglAccum 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 fglReadBuffer). Each component
n
value is divided by 2 -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
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fglAccum(3G) OpenGL Reference fglAccum(3G)
[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
n
currently selected buffer are divided by 2 -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
n n
2 -1, clamped to the range [0, 2 -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 fglClearAccum with R, G, B, and A
values to set it to, then call fglClear with the accumulation buffer
enabled.
NOTES
Only pixels within the current scissor box are updated by a fglAccum
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 or
if the draw drawable is not identical to the read drawable (see
fglXMakeCurrentReadSGI).
GL_INVALID_OPERATION is generated if fglAccum is executed between the
execution of fglBegin and the corresponding execution of fglEnd.
ASSOCIATED GETS
fglGet with argument GL_ACCUM_RED_BITS
fglGet with argument GL_ACCUM_GREEN_BITS
fglGet with argument GL_ACCUM_BLUE_BITS
fglGet with argument GL_ACCUM_ALPHA_BITS
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fglAccum(3G) OpenGL Reference fglAccum(3G)MACHINE DEPENDENCIES
On InfiniteReality systems the following restrictions apply:
1. When op is GL_RETURN, value must be greater than or equal to
-4.0 and strictly less than 4.0. If value is outside this
range, the results are undefined.
2. For other values of op, value must be greater than or equal to
-1.0 and strictly less than 1.0. If value is outside this
range, the results are undefined.
RealityEngine, RealityEngine2, and VTX systems do not support negative
values in the accumulation buffer.
SEE ALSO
fglBlendFunc, fglClear, fglClearAccum, fglCopyPixels, fglDrawBuffer,
fglGet, fglLogicOp,
fglPixelStore, fglPixelTransfer, fglReadBuffer, fglReadPixels,
fglScissor, fglStencilOp
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