GLBLENDFUNC

NAME

glBlendFunc - specify pixel arithmetic

C SPECIFICATION

void f3glBlendFuncfP( GLenum fIsfactorfP,

f2sfactorfP
Specifies how the red, green, blue,
and alpha source blending factors are computed.
The following symbolic constants are accepted:
%f3GL_ZEROfP,
%f3GL_ONEfP,
%f3GL_DST_COLORfP,
%f3GL_ONE_MINUS_DST_COLORfP,
%f3GL_SRC_ALPHAfP,
%f3GL_ONE_MINUS_SRC_ALPHAfP,
%f3GL_DST_ALPHAfP,
%f3GL_ONE_MINUS_DST_ALPHAfP, and
%f3GL_SRC_ALPHA_SATURATEfP.
The initial value is %f3GL_ONEfP.

Additionally, if the %f3GL_ARB_imagingfP extension is supported, the
following constants are accepted:
%f3GL_CONSTANT_COLORfP,
%f3GL_ONE_MINUS_CONSTANT_COLORfP,
%f3GL_CONSTANT_ALPHAfP,
%f3GL_ONE_MINUS_CONSTANT_ALPHAfP.

f2dfactorfP
Specifies how the red, green, blue,
and alpha destination blending factors are computed.
Eight symbolic constants are accepted:
%f3GL_ZEROfP,
%f3GL_ONEfP,
%f3GL_SRC_COLORfP,
%f3GL_ONE_MINUS_SRC_COLORfP,
%f3GL_SRC_ALPHAfP,
%f3GL_ONE_MINUS_SRC_ALPHAfP,
%f3GL_DST_ALPHAfP, and
%f3GL_ONE_MINUS_DST_ALPHAfP.
The initial value is %f3GL_ZEROfP.

Additionally, if the %f3GL_ARB_imagingfP extension is supported, the
following constants are accepted:
%f3GL_CONSTANT_COLORfP,
%f3GL_ONE_MINUS_CONSTANT_COLORfP,
%f3GL_CONSTANT_ALPHAfP,
%f3GL_ONE_MINUS_CONSTANT_ALPHAfP.

DESCRIPTION

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 %f3glEnablefP and %f3glDisablefP with argument %f3GL_BLENDfP to enable and disable blending. %f3glBlendFuncfP defines the operation of blending when it is enabled. f2sfactorfP specifies which of nine methods is used to scale the source color components. f2dfactorfP 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 %f3glBlendColorfP 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	$(f sub R , ~~ f sub G , ~~ f sub B , ~~ f sub A )$

%f3GL_ZEROfP	$(0, ~0, ~0, ~0 )$
%f3GL_ONEfP	$(1, ~1, ~1, ~1 )$
%f3GL_SRC_COLORfP	$(R sub s / k sub R , ~G sub s / k sub G , ~B sub s / k sub B , ~A sub s / k sub A )$
%f3GL_ONE_MINUS_SRC_COLORfP	$(1, ~1, ~1, ~1 ) ~-~ (R sub s / k sub R , ~G sub s / k sub G , ~B sub s / k sub B , ~A sub s / k sub A )$
%f3GL_DST_COLORfP	$(R sub d / k sub R , ~G sub d / k sub G , ~B sub d / k sub B , ~A sub d / k sub A )$
%f3GL_ONE_MINUS_DST_COLORfP	$(1, ~1, ~1, ~1 ) ~-~ (R sub d / k sub R , ~G sub d / k sub G , ~B sub d / k sub B , ~A sub d / k sub A )$
%f3GL_SRC_ALPHAfP	$(A sub s / k sub A , ~A sub s / k sub A , ~A sub s / k sub A , ~A sub s / k sub A )$
%f3GL_ONE_MINUS_SRC_ALPHAfP	$(1, ~1, ~1, ~1 ) ~-~ (A sub s / k sub A , ~A sub s / k sub A , ~A sub s / k sub A , ~A sub s / k sub A )$
%f3GL_DST_ALPHAfP	$(A sub d / k sub A , ~A sub d / k sub A , ~A sub d / k sub A , ~A sub d / k sub A )$
%f3GL_ONE_MINUS_DST_ALPHAfP	$(1, ~1, ~1, ~1 ) ~-~ (A sub d / k sub A , ~A sub d / k sub A , ~A sub d / k sub A , ~A sub d / k sub A )$
%f3GL_SRC_ALPHA_SATURATEfP	$(i, ~i, ~i, ~1 )$
%f3GL_CONSTANT_COLORfP	$(R sub c, G sub c, B sub c, A sub c)$
%f3GL_ONE_MINUS_CONSTANT_COLORfP	$(1, ~1, ~1, ~1 ) ~-~ (R sub c, G sub c, B sub c, A sub c)$
%f3GL_CONSTANT_ALPHAfP	$(A sub c, A sub c, A sub c, A sub c)$
%f3GL_ONE_MINUS_CONSTANT_ALPHAfP	$(1, ~1, ~1, ~1 ) ~-~ (A sub c, A sub c, A sub c, A sub c)$

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 f2sfactorfP is %f3GL_SRC_ALPHAfP, f2dfactorfP is %f3GL_ONE_MINUS_SRC_ALPHAfP, and $A sub s$ is equal to $k sub A$, the equations reduce to simple replacement:


$R sub d ~=~ mark   R sub s$
$G sub d ~=~ lineup G sub s$
$B sub d ~=~ lineup B sub s$
$A sub d ~=~ lineup A sub s$

EXAMPLES

Transparency is best implemented using blend function (%f3GL_SRC_ALPHAfP, %f3GL_ONE_MINUS_SRC_ALPHAfP) 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 (%f3GL_SRC_ALPHAfP, %f3GL_ONE_MINUS_SRC_ALPHAfP) is also useful for rendering antialiased points and lines in arbitrary order. Polygon antialiasing is optimized using blend function
(%f3GL_SRC_ALPHA_SATURATEfP, %f3GL_ONEfP) with polygons sorted from nearest to farthest. (See the %f3glEnablefP, %f3glDisablefP reference page and the %f3GL_POLYGON_SMOOTHfP argument for information on polygon antialiasing.) Destination alpha bitplanes, which must be present for this blend function to operate correctly, store the accumulated coverage.

NOTES

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 %f3glDrawBufferfP.) Blending affects only RGBA rendering. It is ignored by color index renderers. %f3GL_CONSTANT_COLORfP, %f3GL_ONE_MINUS_CONSTANT_COLORfP, %f3GL_CONSTANT_ALPHAfP, %f3GL_ONE_MINUS_CONSTANT_ALPHAfP are only available if the %f3GL_ARB_imagingfP is supported by your implementation.

ERRORS

%f3GL_INVALID_ENUMfP is generated if either f2sfactorfP or f2dfactorfP is not an accepted value. %f3GL_INVALID_OPERATIONfP is generated if %f3glBlendFuncfP is executed between the execution of %f3glBeginfP and the corresponding execution of %f3glEndfP.

ASSOCIATED GETS

%f3glGetfP with argument %f3GL_BLEND_SRCfP
%f3glGetfP with argument %f3GL_BLEND_DSTfP
%f3glIsEnabledfP with argument %f3GL_BLENDfP