OpenGL man pages
glAccum - operate on the accumulation buffer
void glAccum( GLenum op,
GLfloat value )
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.
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,
respectively. Regardless of the number of bits per component, however, 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 are applied identically to the red, green, blue, and alpha
components of each pixel. The contents of an accumulation buffer pixel
component are undefined if the glAccum operation results in a value outside
the range [-1,1]. 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.
The accumulation buffer is cleared by specifying R, G, B, and A values to
set it to with the glClearAccum directive, and issuing a glClear command
with the accumulation buffer enabled.
Only those pixels within the current scissor box are updated by any glAccum
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.
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
glBlendFunc, glClear, glClearAccum, glCopyPixels, glGet, glLogicOp,
glPixelStore, glPixelTransfer, glReadPixels, glReadBuffer, glScissor,
Fri Dec 6 11:18:03 EST 1996
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