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 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 matri-ces. matrices.
ces.
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 oneto-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^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^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^n)-1, clamped to the range [0,(2^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)
GLACCUM(3G)
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