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Table of Contents
glBlendFunc - specify pixel arithmetic
void glBlendFunc(
GLenum sfactor,
GLenum dfactor )
eqn not supported
- 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_DST_COLOR, GL_ONE_MINUS_DST_COLOR, GL_SRC_ALPHA,
GL_ONE_MINUS_SRC_ALPHA, GL_DST_ALPHA, GL_ONE_MINUS_DST_ALPHA, and GL_SRC_ALPHA_SATURATE.
The initial value is GL_ONE.
Additionally, if the GL_ARB_imaging extension
is supported, the following constants are accepted: GL_CONSTANT_COLOR,
GL_ONE_MINUS_CONSTANT_COLOR, GL_CONSTANT_ALPHA, GL_ONE_MINUS_CONSTANT_ALPHA.
- dfactor
- Specifies how the red, green, blue, and alpha destination blending
factors are computed. Eight symbolic constants are accepted: GL_ZERO, GL_ONE,
GL_SRC_COLOR, GL_ONE_MINUS_SRC_COLOR, GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA,
GL_DST_ALPHA, and GL_ONE_MINUS_DST_ALPHA. The initial value is GL_ZERO.
Additionally, if the GL_ARB_imaging extension is supported, the following
constants are accepted: GL_CONSTANT_COLOR, GL_ONE_MINUS_CONSTANT_COLOR,
GL_CONSTANT_ALPHA, GL_ONE_MINUS_CONSTANT_ALPHA.
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 of
nine methods is used to scale the source color components. dfactor specifies
which of eight methods is used to scale the destination color components.
The eleven 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 $(R sub s , G sub s , B sub s , A sub s )$ and $(R sub
d , G sub d , B sub d , A sub d )$. The color specified by glBlendColor
is referred to as $(R sub c , G sub c , B sub c , A sub c )$. They are understood
to have integer values between 0 and $(k sub R , k sub G , k sub B , k
sub A )$, where
$k sub c ~=~ 2 sup m sub c - 1$
and $(m sub R , m sub G
, m sub B , m sub A )$ is the number of red, green, blue, and alpha bitplanes.
Source and destination scale factors are referred to as $(s sub R , s sub
G , s sub B , s sub A )$ and $(d sub R , d sub G , d sub B , d sub A )$.
The scale factors described in the table, denoted $(f sub R , f sub G ,
f sub B , f sub A )$, represent either source or destination factors. All
scale factors have range [0, 1].
| Parameter $(f sub R , ~~ f sub G , ~~ f
sub B , ~~ f sub A )$ |
| GL_ZERO $(0, ~0, ~0, ~0 )$ |
| GL_ONE $(1, ~1, ~1, ~1 )$ |
| GL_SRC_COLOR $(R
sub s / k sub R , ~G sub s / k sub G , ~B sub s / k sub B , ~A sub s /
k sub A )$ |
| GL_ONE_MINUS_SRC_COLOR $(1, ~1, ~1, ~1 ) ~-~ (R sub s / k sub R
, ~G sub s / k sub G , ~B sub s / k sub B , ~A sub s / k sub A )$ |
| GL_DST_COLOR $(R
sub d / k sub R , ~G sub d / k sub G , ~B sub d / k sub B , ~A sub d /
k sub A )$ |
| GL_ONE_MINUS_DST_COLOR $(1, ~1, ~1, ~1 ) ~-~ (R sub d / k sub R
, ~G sub d / k sub G , ~B sub d / k sub B , ~A sub d / k sub A )$ |
| GL_SRC_ALPHA $(A
sub s / k sub A , ~A sub s / k sub A , ~A sub s / k sub A , ~A sub s /
k sub A )$ |
| GL_ONE_MINUS_SRC_ALPHA $(1, ~1, ~1, ~1 ) ~-~ (A sub s / k sub A
, ~A sub s / k sub A , ~A sub s / k sub A , ~A sub s / k sub A )$ |
| GL_DST_ALPHA $(A
sub d / k sub A , ~A sub d / k sub A , ~A sub d / k sub A , ~A sub d /
k sub A )$ |
| GL_ONE_MINUS_DST_ALPHA $(1, ~1, ~1, ~1 ) ~-~ (A sub d / k sub A
, ~A sub d / k sub A , ~A sub d / k sub A , ~A sub d / k sub A )$ |
| GL_SRC_ALPHA_SATURATE $(i,
~i, ~i, ~1 )$ |
| GL_CONSTANT_COLOR $(R sub c, G sub c, B sub c, A sub c)$ |
| GL_ONE_MINUS_CONSTANT_COLOR $(1,
~1, ~1, ~1 ) ~-~ (R sub c, G sub c, B sub c, A sub c)$ |
| GL_CONSTANT_ALPHA $(A
sub c, A sub c, A sub c, A sub c)$ |
| GL_ONE_MINUS_CONSTANT_ALPHA $(1, ~1, ~1,
~1 ) ~-~ (A sub c, A sub c, A sub c, A sub c)$ |
In the table,
- $i ~=~ min (A sub s , ~k sub A ~-~ A sub d ) ~/~ k sub A$To determine the
blended RGBA values of a pixel when drawing in RGBA mode,
- the system uses
the following equations:
- $R sub d ~=~ mark min ( k sub R, ~R sub s~s sub R~+~R sub d~d sub R )$$G
sub d ~=~ lineup min ( k sub G, ~G sub s~s sub G~+~G sub d~d sub G )$$B
sub d ~=~ lineup min ( k sub B, ~B sub s~s sub B~+~B sub d~d sub B )$$A
sub d ~=~ lineup min ( k sub A, ~A sub s~s sub A~+~A sub d~d sub A )$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 $A sub s$ is equal to $k sub A$, the equations
reduce to simple replacement:
- 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 ($K sub A$), representing complete
opacity, to 0.0 (0), representing complete transparency.
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 only available if the
GL_ARB_imaging is supported by your implementation.
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(3G)
, glBlendColor(3G)
, glBlendEquation(3G)
, glClear(3G)
,
glDrawBuffer(3G)
, glEnable(3G)
, glLogicOp(3G)
, glStencilFunc(3G)
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