glAccum(3G)glAccum(3G)NAMEglAccum - operate on the accumulation buffer
SYNOPSIS
void glAccum(
GLenum op,
GLfloat value );
PARAMETERS
Specifies the accumulation buffer operation. Symbolic constants
GL_ACCUM, GL_LOAD, GL_ADD, GL_MULT, and GL_RETURN are accepted. Speci‐
fies 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 ren‐
dering. 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 buf‐
fer depends on the implementation. You can examine this number by call‐
ing 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 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 cur‐
rent 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: Obtains R, G, B, and A values from the
buffer currently selected for reading (see glReadBuffer()). Each compo‐
nent 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 accumula‐
tion buffer, thereby updating the accumulation buffer. 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, mul‐
tiplied by value, and then stored in the corresponding accumulation
buffer cell, overwriting the current value. Adds value to each R, G,
B, and A in the accumulation buffer. Multiplies each R, G, B, and A in
the accumulation buffer by value and returns the scaled component to
its corresponding accumulation buffer location. 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 GETSglGet() 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 ALSOglClear(3), glClearAccum(3), glCopyPixels(3), glDrawBuffer(3),
glGet(3), glReadBuffer(3), glReadPixels(3), glScissor(3), glSten‐
cilOp(3)glAccum(3G)
