GLACCUM() MachTen Programmer’s Manual GLACCUM()

NAME
glAccum - operate on the accumulation buffer

C SPECIFICATION
void glAccum( GLenum op,
GLfloat value )

delim $$

PARAMETERS
op Specifies the accumulation buffer operation. Sym-
bolic constants GL_ACCUM, GL_LOAD, GL_ADD, GL_MULT,
and GL_RETURN are accepted.

value Specifies a floating-point value used in the accu-
mulation 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 imple-
mentation. You can examine this number by calling glGet-
Integerv 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 accu-
mulation 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 com-
ponent 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 compo-
nent 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 cur-
rently 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 accu-
mulation buffer by value and returns the
scaled component to its corresponding accu-
mulation 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 correspond-
ing 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 accumula-
tion 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
glBlendFunc, glClear, glClearAccum, glCopyPixels, glGet,
glLogicOp, glPixelStore, glPixelTransfer, glReadBuffer,
glReadPixels, glScissor, glStencilOp

MachTen 2