glBlendFunc — specify pixel arithmetic
void glBlendFunc( | GLenum | sfactor, |
GLenum | dfactor) ; |
sfactor
Specifies how the red, green, blue,
and alpha source blending factors are computed.
The following symbolic constants are accepted:
GL_ZERO
,
GL_ONE
,
GL_SRC_COLOR
,
GL_ONE_MINUS_SRC_COLOR
,
GL_DST_COLOR
,
GL_ONE_MINUS_DST_COLOR
,
GL_SRC_ALPHA
,
GL_ONE_MINUS_SRC_ALPHA
,
GL_DST_ALPHA
,
GL_ONE_MINUS_DST_ALPHA
,
GL_CONSTANT_COLOR
,
GL_ONE_MINUS_CONSTANT_COLOR
,
GL_CONSTANT_ALPHA
,
GL_ONE_MINUS_CONSTANT_ALPHA
, and
GL_SRC_ALPHA_SATURATE
.
The initial value is GL_ONE
.
dfactor
Specifies how the red, green, blue,
and alpha destination blending factors are computed.
The following symbolic constants are accepted:
GL_ZERO
,
GL_ONE
,
GL_SRC_COLOR
,
GL_ONE_MINUS_SRC_COLOR
,
GL_DST_COLOR
,
GL_ONE_MINUS_DST_COLOR
,
GL_SRC_ALPHA
,
GL_ONE_MINUS_SRC_ALPHA
,
GL_DST_ALPHA
,
GL_ONE_MINUS_DST_ALPHA
.
GL_CONSTANT_COLOR
,
GL_ONE_MINUS_CONSTANT_COLOR
,
GL_CONSTANT_ALPHA
, and
GL_ONE_MINUS_CONSTANT_ALPHA
.
The initial value is GL_ZERO
.
In RGBA mode, pixels can be drawn using a function that blends
the incoming (source) RGBA values with the RGBA values
that are already in the frame buffer (the destination values).
Blending is initially disabled.
Use glEnable and glDisable with argument GL_BLEND
to enable and disable blending.
glBlendFunc
defines the operation of blending when it is enabled.
sfactor
specifies which method is used to scale the
source color components.
dfactor
specifies which method is used to scale the
destination color components.
The possible methods are described in the following table.
Each method defines four scale factors,
one each for red, green, blue, and alpha.
In the table and in subsequent equations, source and destination
color components are referred to as
and
Source and destination scale factors are referred to as
Parameter |
|
---|---|
GL_ZERO
|
|
GL_ONE
|
|
GL_SRC_COLOR
|
|
GL_ONE_MINUS_SRC_COLOR
|
|
GL_DST_COLOR
|
|
GL_ONE_MINUS_DST_COLOR
|
|
GL_SRC_ALPHA
|
|
GL_ONE_MINUS_SRC_ALPHA
|
|
GL_DST_ALPHA
|
|
GL_ONE_MINUS_DST_ALPHA
|
|
GL_CONSTANT_COLOR
|
|
GL_ONE_MINUS_CONSTANT_COLOR
|
|
GL_CONSTANT_ALPHA
|
|
GL_ONE_MINUS_CONSTANT_ALPHA
|
|
GL_SRC_ALPHA_SATURATE
|
|
In the table,
To determine the blended RGBA values of a pixel when drawing in RGBA mode, the system uses the following equations:
Despite the apparent precision of the above equations,
blending arithmetic is not exactly specified,
because blending operates with imprecise integer color values.
However,
a blend factor that should be equal to 1
is guaranteed not to modify its multiplicand,
and a blend factor equal to 0 reduces its multiplicand to 0.
For example,
when sfactor
is GL_SRC_ALPHA
,
dfactor
is GL_ONE_MINUS_SRC_ALPHA
,
and
Transparency is best implemented using blend function
(GL_SRC_ALPHA
, GL_ONE_MINUS_SRC_ALPHA
)
with primitives sorted from farthest to nearest.
Note that this transparency calculation does not require
the presence of alpha bitplanes in the frame buffer.
Blend function
(GL_SRC_ALPHA
, GL_ONE_MINUS_SRC_ALPHA
)
is also useful for rendering antialiased points and lines
in arbitrary order.
Polygon antialiasing is optimized using blend function
(GL_SRC_ALPHA_SATURATE
, GL_ONE
)
with polygons sorted from nearest to farthest.
(See the glEnable, glDisable reference page and the
GL_POLYGON_SMOOTH
argument for information on polygon antialiasing.)
Destination alpha bitplanes,
which must be present for this blend function to operate correctly,
store the accumulated coverage.
Incoming (source) alpha is correctly thought of as a material opacity,
ranging from 1.0
(
When more than one color buffer is enabled for drawing, the GL performs blending separately for each enabled buffer, using the contents of that buffer for destination color. (See glDrawBuffer.)
Blending affects only RGBA rendering. It is ignored by color index renderers.
GL_CONSTANT_COLOR
, GL_ONE_MINUS_CONSTANT_COLOR
,
GL_CONSTANT_ALPHA
, GL_ONE_MINUS_CONSTANT_ALPHA
are available only
if the GL version is 1.4 or greater or if the ARB_imaging
is
supported by your implementation.
GL_SRC_COLOR
and GL_ONE_MINUS_SRC_COLOR
are valid only for
sfactor
if the GL version is 1.4 or greater.
GL_DST_COLOR
and GL_ONE_MINUS_DST_COLOR
are valid only for
dfactor
if the GL version is 1.4 or greater.
GL_INVALID_ENUM
is generated if either sfactor
or dfactor
is not an
accepted value.
GL_INVALID_OPERATION
is generated if glBlendFunc
is executed between the execution of glBegin
and the corresponding execution of glEnd.
glGet with argument GL_BLEND_SRC
glGet with argument GL_BLEND_DST
glIsEnabled with argument GL_BLEND
glAlphaFunc, glBlendColor, glBlendEquation, glBlendFuncSeparate, glClear, glDrawBuffer, glEnable, glLogicOp, glStencilFunc
Copyright © 1991-2006 Silicon Graphics, Inc. This document is licensed under the SGI Free Software B License. For details, see http://oss.sgi.com/projects/FreeB/.