glAccum
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 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 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 2n-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 2n-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
2n-1, clamped to the range
[0,2n-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
glBlendFunc,
glClear,
glClearAccum,
glCopyPixels,
glGet,
glLogicOp,
glPixelStore,
glPixelTransfer,
glReadBuffer,
glReadPixels,
glScissor,
glStencilOp