GLACCUM(3G)GLACCUM(3G)NAMEglAccum - 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 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
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 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:
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, multi‐
plied 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 corre‐
sponding 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 frag‐
ment 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, glClearAccum, glCopyPixels, glDrawBuffer, glGet, glReadBuffer,
glReadPixels, glScissor, glStencilOp
GLACCUM(3G)
