GLACCUM(3G) UNIX Programmer's Manual GLACCUM(3G)
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 accumu-
lation 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 com-
ponent in the accumulation buffer depends on the implementa-
tion. 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 argu-
ment, op, is a symbolic constant that selects an accumula-
tion 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.
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GLACCUM(3G) UNIX Programmer's Manual GLACCUM(3G)
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 allo-
cated to each color component in the currently
selected buffer. The result is a floating-
point value in the range [0, 1], which is mul-
tiplied by value and added to the correspond-
ing 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, 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 accu-
mulation buffer.
GL_MULT Multiplies each R, G, B, and A in the accumu-
lation 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 mul-
tiplied 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.
Only pixels within the current scissor box are updated by a
glAccum operation.
GL_INVALID_ENUM is generated if op is not an accepted value.
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GLACCUM(3G) UNIX Programmer's Manual GLACCUM(3G)
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 exe-
cution 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
glClear(3G), glClearAccum(3G), glCopyPixels(3G),
glDrawBuffer(3G), glGet(3G), glReadBuffer(3G),
glReadPixels(3G), glScissor(3G), glStencilOp(3G)
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