sync code with last improvements from OpenBSD
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doc/gl-docs/GL/gl/blendfunc.3gl
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doc/gl-docs/GL/gl/blendfunc.3gl
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'\" te
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'\"! tbl|eqn | mmdoc
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'\"macro stdmacro
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.ds Vn Version 1.2
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.ds Dt 24 September 1999
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.ds Re Release 1.2.1
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.ds Dp Jan 14 18:30
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.ds Dm 01 blendfunc
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.ds Xs 18970 10 blendfunc.gl
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.TH GLBLENDFUNC 3G
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.SH NAME
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.B "glBlendFunc
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\- specify pixel arithmetic
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.SH C SPECIFICATION
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void \f3glBlendFunc\fP(
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GLenum \fIsfactor\fP,
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.nf
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.ta \w'\f3void \fPglBlendFunc( 'u
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GLenum \fIdfactor\fP )
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.fi
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.EQ
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delim $$
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.EN
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.SH PARAMETERS
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.TP \w'\f2sfactor\fP\ \ 'u
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\f2sfactor\fP
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Specifies how the red, green, blue,
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and alpha source blending factors are computed.
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The following symbolic constants are accepted:
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\%\f3GL_ZERO\fP,
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\%\f3GL_ONE\fP,
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\%\f3GL_DST_COLOR\fP,
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\%\f3GL_ONE_MINUS_DST_COLOR\fP,
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\%\f3GL_SRC_ALPHA\fP,
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\%\f3GL_ONE_MINUS_SRC_ALPHA\fP,
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\%\f3GL_DST_ALPHA\fP,
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\%\f3GL_ONE_MINUS_DST_ALPHA\fP, and
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\%\f3GL_SRC_ALPHA_SATURATE\fP.
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The initial value is \%\f3GL_ONE\fP.
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.TE
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Additionally, if the \%\f3GL_ARB_imaging\fP extension is supported, the
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following constants are accepted:
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\%\f3GL_CONSTANT_COLOR\fP,
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\%\f3GL_ONE_MINUS_CONSTANT_COLOR\fP,
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\%\f3GL_CONSTANT_ALPHA\fP,
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\%\f3GL_ONE_MINUS_CONSTANT_ALPHA\fP.
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.TP
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\f2dfactor\fP
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Specifies how the red, green, blue,
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and alpha destination blending factors are computed.
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Eight symbolic constants are accepted:
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\%\f3GL_ZERO\fP,
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\%\f3GL_ONE\fP,
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\%\f3GL_SRC_COLOR\fP,
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\%\f3GL_ONE_MINUS_SRC_COLOR\fP,
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\%\f3GL_SRC_ALPHA\fP,
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\%\f3GL_ONE_MINUS_SRC_ALPHA\fP,
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\%\f3GL_DST_ALPHA\fP, and
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\%\f3GL_ONE_MINUS_DST_ALPHA\fP.
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The initial value is \%\f3GL_ZERO\fP.
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.TE
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Additionally, if the \%\f3GL_ARB_imaging\fP extension is supported, the
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following constants are accepted:
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\%\f3GL_CONSTANT_COLOR\fP,
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\%\f3GL_ONE_MINUS_CONSTANT_COLOR\fP,
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\%\f3GL_CONSTANT_ALPHA\fP,
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\%\f3GL_ONE_MINUS_CONSTANT_ALPHA\fP.
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.SH DESCRIPTION
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In RGBA mode, pixels can be drawn using a function that blends
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the incoming (source) RGBA values with the RGBA values
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that are already in the frame buffer (the destination values).
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Blending is initially disabled.
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Use \%\f3glEnable\fP and \%\f3glDisable\fP with argument \%\f3GL_BLEND\fP
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to enable and disable blending.
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.P
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\%\f3glBlendFunc\fP defines the operation of blending when it is enabled.
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\f2sfactor\fP specifies which of nine methods is used to scale the
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source color components.
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\f2dfactor\fP specifies which of eight methods is used to scale the
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destination color components.
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The eleven possible methods are described in the following table.
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Each method defines four scale factors,
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one each for red, green, blue, and alpha.
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.P
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In the table and in subsequent equations, source and destination
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color components are referred to as
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$(R sub s , G sub s , B sub s , A sub s )$ and
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$(R sub d , G sub d , B sub d , A sub d )$.
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The color specified by \%\f3glBlendColor\fP is referred to as
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$(R sub c , G sub c , B sub c , A sub c )$.
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They are understood to have integer values between 0 and
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$(k sub R , k sub G , k sub B , k sub A )$,
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where
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.P
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.RS
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.ce
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$k sub c ~=~ 2 sup m sub c - 1$
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.RE
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.P
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and
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$(m sub R , m sub G , m sub B , m sub A )$
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is the number of red,
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green,
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blue,
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and alpha bitplanes.
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.P
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Source and destination scale factors are referred to as
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$(s sub R , s sub G , s sub B , s sub A )$ and
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$(d sub R , d sub G , d sub B , d sub A )$.
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The scale factors described in the table,
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denoted $(f sub R , f sub G , f sub B , f sub A )$,
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represent either source or destination factors.
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All scale factors have range [0,\ 1].
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.P
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.TS
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center;
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lb cb
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l c .
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_
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Parameter $(f sub R , ~~ f sub G , ~~ f sub B , ~~ f sub A )$
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_
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\%\f3GL_ZERO\fP $(0, ~0, ~0, ~0 )$
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\%\f3GL_ONE\fP $(1, ~1, ~1, ~1 )$
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\%\f3GL_SRC_COLOR\fP $(R sub s / k sub R , ~G sub s / k sub G , ~B sub s / k sub B , ~A sub s / k sub A )$
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\%\f3GL_ONE_MINUS_SRC_COLOR\fP $(1, ~1, ~1, ~1 ) ~-~ (R sub s / k sub R , ~G sub s / k sub G , ~B sub s / k sub B , ~A sub s / k sub A )$
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\%\f3GL_DST_COLOR\fP $(R sub d / k sub R , ~G sub d / k sub G , ~B sub d / k sub B , ~A sub d / k sub A )$
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\%\f3GL_ONE_MINUS_DST_COLOR\fP $(1, ~1, ~1, ~1 ) ~-~ (R sub d / k sub R , ~G sub d / k sub G , ~B sub d / k sub B , ~A sub d / k sub A )$
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\%\f3GL_SRC_ALPHA\fP $(A sub s / k sub A , ~A sub s / k sub A , ~A sub s / k sub A , ~A sub s / k sub A )$
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\%\f3GL_ONE_MINUS_SRC_ALPHA\fP $(1, ~1, ~1, ~1 ) ~-~ (A sub s / k sub A , ~A sub s / k sub A , ~A sub s / k sub A , ~A sub s / k sub A )$
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\%\f3GL_DST_ALPHA\fP $(A sub d / k sub A , ~A sub d / k sub A , ~A sub d / k sub A , ~A sub d / k sub A )$
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\%\f3GL_ONE_MINUS_DST_ALPHA\fP $(1, ~1, ~1, ~1 ) ~-~ (A sub d / k sub A , ~A sub d / k sub A , ~A sub d / k sub A , ~A sub d / k sub A )$
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\%\f3GL_SRC_ALPHA_SATURATE\fP $(i, ~i, ~i, ~1 )$
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\%\f3GL_CONSTANT_COLOR\fP $(R sub c, G sub c, B sub c, A sub c)$
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\%\f3GL_ONE_MINUS_CONSTANT_COLOR\fP $(1, ~1, ~1, ~1 ) ~-~ (R sub c, G sub c, B sub c, A sub c)$
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\%\f3GL_CONSTANT_ALPHA\fP $(A sub c, A sub c, A sub c, A sub c)$
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\%\f3GL_ONE_MINUS_CONSTANT_ALPHA\fP $(1, ~1, ~1, ~1 ) ~-~ (A sub c, A sub c, A sub c, A sub c)$
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_
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.TE
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.sp
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In the table,
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.P
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.RS
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.nf
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.IP
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$i ~=~ min (A sub s , ~k sub A ~-~ A sub d ) ~/~ k sub A$
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.fi
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.RE
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.P
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To determine the blended RGBA values of a pixel when drawing in RGBA mode,
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the system uses the following equations:
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.P
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.RS
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.nf
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.IP
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$R sub d ~=~ mark min ( k sub R, ~R sub s~s sub R~+~R sub d~d sub R )$
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$G sub d ~=~ lineup min ( k sub G, ~G sub s~s sub G~+~G sub d~d sub G )$
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$B sub d ~=~ lineup min ( k sub B, ~B sub s~s sub B~+~B sub d~d sub B )$
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$A sub d ~=~ lineup min ( k sub A, ~A sub s~s sub A~+~A sub d~d sub A )$
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.fi
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.RE
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.P
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Despite the apparent precision of the above equations,
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blending arithmetic is not exactly specified,
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because blending operates with imprecise integer color values.
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However,
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a blend factor that should be equal to 1
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is guaranteed not to modify its multiplicand,
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and a blend factor equal to 0 reduces its multiplicand to 0.
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For example,
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when \f2sfactor\fP is \%\f3GL_SRC_ALPHA\fP,
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\f2dfactor\fP is \%\f3GL_ONE_MINUS_SRC_ALPHA\fP,
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and $A sub s$ is equal to $k sub A$,
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the equations reduce to simple replacement:
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.P
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.RS
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.nf
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.IP
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$R sub d ~=~ mark R sub s$
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$G sub d ~=~ lineup G sub s$
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$B sub d ~=~ lineup B sub s$
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$A sub d ~=~ lineup A sub s$
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.fi
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.RE
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.P
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.SH EXAMPLES
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.P
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Transparency is best implemented using blend function
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(\%\f3GL_SRC_ALPHA\fP, \%\f3GL_ONE_MINUS_SRC_ALPHA\fP)
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with primitives sorted from farthest to nearest.
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Note that this transparency calculation does not require
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the presence of alpha bitplanes in the frame buffer.
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.P
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Blend function
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(\%\f3GL_SRC_ALPHA\fP, \%\f3GL_ONE_MINUS_SRC_ALPHA\fP)
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is also useful for rendering antialiased points and lines
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in arbitrary order.
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.P
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Polygon antialiasing is optimized using blend function
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.br
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(\%\f3GL_SRC_ALPHA_SATURATE\fP, \%\f3GL_ONE\fP)
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with polygons sorted from nearest to farthest.
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(See the \%\f3glEnable\fP, \%\f3glDisable\fP reference page and the
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\%\f3GL_POLYGON_SMOOTH\fP argument for information on polygon antialiasing.)
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Destination alpha bitplanes,
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which must be present for this blend function to operate correctly,
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store the accumulated coverage.
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.SH NOTES
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Incoming (source) alpha is correctly thought of as a material opacity,
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ranging from 1.0 ($K sub A$), representing complete opacity,
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to 0.0 (0), representing complete
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transparency.
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.P
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When more than one color buffer is enabled for drawing,
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the GL performs blending separately for each enabled buffer,
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using the contents of that buffer for destination color.
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(See \%\f3glDrawBuffer\fP.)
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.P
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Blending affects only RGBA rendering.
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It is ignored by color index renderers.
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.P
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\%\f3GL_CONSTANT_COLOR\fP, \%\f3GL_ONE_MINUS_CONSTANT_COLOR\fP,
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\%\f3GL_CONSTANT_ALPHA\fP, \%\f3GL_ONE_MINUS_CONSTANT_ALPHA\fP are only
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available if the \%\f3GL_ARB_imaging\fP is supported by your implementation.
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.SH ERRORS
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\%\f3GL_INVALID_ENUM\fP is generated if either \f2sfactor\fP or \f2dfactor\fP is not an
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accepted value.
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.P
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\%\f3GL_INVALID_OPERATION\fP is generated if \%\f3glBlendFunc\fP
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is executed between the execution of \%\f3glBegin\fP
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and the corresponding execution of \%\f3glEnd\fP.
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.SH ASSOCIATED GETS
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\%\f3glGet\fP with argument \%\f3GL_BLEND_SRC\fP
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.br
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\%\f3glGet\fP with argument \%\f3GL_BLEND_DST\fP
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.br
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\%\f3glIsEnabled\fP with argument \%\f3GL_BLEND\fP
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.br
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.SH SEE ALSO
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\%\f3glAlphaFunc(3G)\fP,
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\%\f3glBlendColor(3G)\fP,
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\%\f3glBlendEquation(3G)\fP,
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\%\f3glClear(3G)\fP,
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\%\f3glDrawBuffer(3G)\fP,
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\%\f3glEnable(3G)\fP,
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\%\f3glLogicOp(3G)\fP,
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\%\f3glStencilFunc(3G)\fP
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