NAME

glBlendFunc - specify pixel arithmetic

C SPECIFICATION

void ggllBBlleennddFFuunncc( GLenum _s_f_a_c_t_o_r,
                  GLenum _d_f_a_c_t_o_r )

delim $$

PARAMETERS

_s_f_a_c_t_o_r
Specifies how the red, green, blue, and alpha source blending factors are computed. The following symbolic constants are accepted: GGLL__ZZEERROO, GGLL__OONNEE, GGLL__DDSSTT__CCOOLLOORR, GGLL__OONNEE__MMIINNUUSS__DDSSTT__CCOOLLOORR, GGLL__SSRRCC__AALLPPHHAA, GGLL__OONNEE__MMIINNUUSS__SSRRCC__AALLPPHHAA, GGLL__DDSSTT__AALLPPHHAA, GGLL__OONNEE__MMIINNUUSS__DDSSTT__AALLPPHHAA, and GGLL__SSRRCC__AALLPPHHAA__SSAATTUURRAATTEE. The initial value is GGLL__OONNEE.

Additionally, if the GGLL__AARRBB__iimmaaggiinngg extension is supported, the following constants are accepted: GGLL__CCOONNSSTTAANNTT__CCOOLLOORR, GGLL__OONNEE__MMIINNUUSS__CCOONNSSTTAANNTT__CCOOLLOORR, GGLL__CCOONNSSTTAANNTT__AALLPPHHAA, GGLL__OONNEE__MMIINNUUSS__CCOONNSSTTAANNTT__AALLPPHHAA.

_d_f_a_c_t_o_r
Specifies how the red, green, blue, and alpha destination blending factors are computed. Eight symbolic constants are accepted: GGLL__ZZEERROO, GGLL__OONNEE, GGLL__SSRRCC__CCOOLLOORR, GGLL__OONNEE__MMIINNUUSS__SSRRCC__CCOOLLOORR, GGLL__SSRRCC__AALLPPHHAA, GGLL__OONNEE__MMIINNUUSS__SSRRCC__AALLPPHHAA, GGLL__DDSSTT__AALLPPHHAA, and GGLL__OONNEE__MMIINNUUSS__DDSSTT__AALLPPHHAA. The initial value is GGLL__ZZEERROO.

Additionally, if the GGLL__AARRBB__iimmaaggiinngg extension is supported, the following constants are accepted: GGLL__CCOONNSSTTAANNTT__CCOOLLOORR, GGLL__OONNEE__MMIINNUUSS__CCOONNSSTTAANNTT__CCOOLLOORR, GGLL__CCOONNSSTTAANNTT__AALLPPHHAA, GGLL__OONNEE__MMIINNUUSS__CCOONNSSTTAANNTT__AALLPPHHAA.

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 ggllEEnnaabbllee and ggllDDiissaabbllee with argument GGLL__BBLLEENNDD to enable and disable blending.

ggllBBlleennddFFuunncc defines the operation of blending when it is enabled. _s_f_a_c_t_o_r specifies which of nine methods is used to scale the source color components. _d_f_a_c_t_o_r 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 ggllBBlleennddCCoolloorr 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].

center;
lb cb
l c .
_
Parameter         $(f sub R , ~~ f sub G , ~~ f sub B , ~~ f sub A )$
_
GGLL__ZZEERROO           $(0, ~0, ~0, ~0 )$
GGLL__OONNEE            $(1, ~1, ~1, ~1 )$
GGLL__SSRRCC__CCOOLLOORR      $(R sub s / k sub R , ~G sub s / k sub G , ~B sub s / k sub B , ~A sub s / k sub A )$
GGLL__OONNEE__MMIINNUUSS__SSRRCC__CCOOLLOORR$(1, ~1, ~1, ~1 ) ~-~ (R sub s / k sub R , ~G sub s / k sub G , ~B sub s / k sub B , ~A sub s / k sub A )$
GGLL__DDSSTT__CCOOLLOORR      $(R sub d / k sub R , ~G sub d / k sub G , ~B sub d / k sub B , ~A sub d / k sub A )$
GGLL__OONNEE__MMIINNUUSS__DDSSTT__CCOOLLOORR$(1, ~1, ~1, ~1 ) ~-~ (R sub d / k sub R , ~G sub d / k sub G , ~B sub d / k sub B , ~A sub d / k sub A )$
GGLL__SSRRCC__AALLPPHHAA      $(A sub s / k sub A , ~A sub s / k sub A , ~A sub s / k sub A , ~A sub s / k sub A )$
GGLL__OONNEE__MMIINNUUSS__SSRRCC__AALLPPHHAA$(1, ~1, ~1, ~1 ) ~-~ (A sub s / k sub A , ~A sub s / k sub A , ~A sub s / k sub A , ~A sub s / k sub A )$
GGLL__DDSSTT__AALLPPHHAA      $(A sub d / k sub A , ~A sub d / k sub A , ~A sub d / k sub A , ~A sub d / k sub A )$
GGLL__OONNEE__MMIINNUUSS__DDSSTT__AALLPPHHAA$(1, ~1, ~1, ~1 ) ~-~ (A sub d / k sub A , ~A sub d / k sub A , ~A sub d / k sub A , ~A sub d / k sub A )$
GGLL__SSRRCC__AALLPPHHAA__SSAATTUURRAATTEE$(i, ~i, ~i, ~1 )$
GGLL__CCOONNSSTTAANNTT__CCOOLLOORR $(R sub c, G sub c, B sub c, A sub c)$
GGLL__OONNEE__MMIINNUUSS__CCOONNSSTTAANNTT__CCOOLLOORR$(1, ~1, ~1, ~1 ) ~-~ (R sub c, G sub c, B sub c, A sub c)$
GGLL__CCOONNSSTTAANNTT__AALLPPHHAA $(A sub c, A sub c, A sub c, A sub c)$
GGLL__OONNEE__MMIINNUUSS__CCOONNSSTTAANNTT__AALLPPHHAA$(1, ~1, ~1, ~1 ) ~-~ (A sub c, A sub c, A sub c, A sub c)$
_


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 _s_f_a_c_t_o_r is GGLL__SSRRCC__AALLPPHHAA, _d_f_a_c_t_o_r is GGLL__OONNEE__MMIINNUUSS__SSRRCC__AALLPPHHAA, 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 (GGLL__SSRRCC__AALLPPHHAA, GGLL__OONNEE__MMIINNUUSS__SSRRCC__AALLPPHHAA) 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 (GGLL__SSRRCC__AALLPPHHAA, GGLL__OONNEE__MMIINNUUSS__SSRRCC__AALLPPHHAA) is also useful for rendering antialiased points and lines in arbitrary order.

Polygon antialiasing is optimized using blend function
(GGLL__SSRRCC__AALLPPHHAA__SSAATTUURRAATTEE, GGLL__OONNEE) with polygons sorted from nearest to farthest. (See the ggllEEnnaabbllee, ggllDDiissaabbllee reference page and the GGLL__PPOOLLYYGGOONN__SSMMOOOOTTHH 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 ggllDDrraawwBBuuffffeerr.)

Blending affects only RGBA rendering. It is ignored by color index renderers.

GGLL__CCOONNSSTTAANNTT__CCOOLLOORR, GGLL__OONNEE__MMIINNUUSS__CCOONNSSTTAANNTT__CCOOLLOORR, GGLL__CCOONNSSTTAANNTT__AALLPPHHAA, GGLL__OONNEE__MMIINNUUSS__CCOONNSSTTAANNTT__AALLPPHHAA are only available if the GGLL__AARRBB__iimmaaggiinngg is supported by your implementation.

ERRORS

GGLL__IINNVVAALLIIDD__EENNUUMM is generated if either _s_f_a_c_t_o_r or _d_f_a_c_t_o_r is not an accepted value.

GGLL__IINNVVAALLIIDD__OOPPEERRAATTIIOONN is generated if ggllBBlleennddFFuunncc is executed between the execution of ggllBBeeggiinn and the corresponding execution of ggllEEnndd.

ASSOCIATED GETS

ggllGGeett with argument GGLL__BBLLEENNDD__SSRRCC
ggllGGeett with argument GGLL__BBLLEENNDD__DDSSTT
ggllIIssEEnnaabblleedd with argument GGLL__BBLLEENNDD

SEE ALSO

ggllAAllpphhaaFFuunncc((33GG)), ggllBBlleennddCCoolloorr((33GG)), ggllBBlleennddEEqquuaattiioonn((33GG)), ggllCClleeaarr((33GG)), ggllDDrraawwBBuuffffeerr((33GG)), ggllEEnnaabbllee((33GG)), ggllLLooggiiccOOpp((33GG)), ggllSStteenncciillFFuunncc((33GG))