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,
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 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, 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
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.
The accumulation buffer is cleared by specifying R, G, B, and A values to set it to with the
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glClearAccum directive, and issuing a glClear command with the accumulation buffer enabled.
NOTES
Only those pixels within the current scissor box are updated by any 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 called between a call to glBegin and the
corresponding call to 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" , "glReadPixels" , "glReadBuffer" , "glScissor" , "glStencilOp"
glAlphaFunc
NAME
glAlphaFunc − specify the alpha test function
C SPECIFICATION
void glAlphaFunc( GLenum func, GLclampf ref )
PARAMETERS
func Specifies the alpha comparison function. Symbolic constants GL_NEVER, GL_LESS
, GL_EQUAL, GL_LEQUAL, GL_GREATER, GL_NOTEQUAL, GL_GEQUAL, and
GL_ALWAYS are accepted. The default function is GL_ALWAYS.
ref Specifies the reference value that incoming alpha values are compared to. This value
is clamped to the range 0 through 1, where 0 represents the lowest possible alpha
value and 1 the highest possible value. The default reference is 0.
DESCRIPTION
The alpha test discards fragments depending on the outcome of a comparison between the incoming
fragment’s alpha value and a constant reference value. glAlphaFunc specifies the reference and
comparison function. The comparison is performed only if alpha testing is enabled. (See "glEnable"
and glDisable of GL_ALPHA_TEST.)
func and ref specify the conditions under which the pixel is drawn. The incoming alpha value is
compared to ref using the function specified by func. If the comparison passes, the incoming fragment
is drawn, conditional on subsequent stencil and depth buffer tests. If the comparison fails, no change is
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