1GLBLENDFUNC(3G) GLBLENDFUNC(3G)
2
6 glBlendFunc - specify pixel arithmetic
7
10 void glBlendFunc( GLenum sfactor,
11 GLenum dfactor )
12
15 sfactor Specifies how the red, green, blue, and alpha source blending
16 factors are computed. The following symbolic constants are
17 accepted: GL_ZERO, GL_ONE, GL_DST_COLOR,
18 GL_ONE_MINUS_DST_COLOR, GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA,
19 GL_DST_ALPHA, GL_ONE_MINUS_DST_ALPHA, and
20 GL_SRC_ALPHA_SATURATE. The initial value is GL_ONE.
21 Additionally, if the GL_ARB_imaging extension is supported,
23 the following constants are accepted: GL_CONSTANT_COLOR,
24 GL_ONE_MINUS_CONSTANT_COLOR, GL_CONSTANT_ALPHA,
25 GL_ONE_MINUS_CONSTANT_ALPHA.
26 dfactor Specifies how the red, green, blue, and alpha destination
28 blending factors are computed. Eight symbolic constants are
29 accepted: GL_ZERO, GL_ONE, GL_SRC_COLOR,
30 GL_ONE_MINUS_SRC_COLOR, GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA,
31 GL_DST_ALPHA, and GL_ONE_MINUS_DST_ALPHA. The initial value
32 is GL_ZERO.
33 Additionally, if the GL_ARB_imaging extension is supported,
35 the following constants are accepted: GL_CONSTANT_COLOR,
36 GL_ONE_MINUS_CONSTANT_COLOR, GL_CONSTANT_ALPHA,
37 GL_ONE_MINUS_CONSTANT_ALPHA.
38
40 In RGBA mode, pixels can be drawn using a function that blends the
41 incoming (source) RGBA values with the RGBA values that are already in
42 the frame buffer (the destination values). Blending is initially dis‐
43 abled. Use glEnable and glDisable with argument GL_BLEND to enable and
44 disable blending.
45 glBlendFunc defines the operation of blending when it is enabled.
47 sfactor specifies which of nine methods is used to scale the source
48 color components. dfactor specifies which of eight methods is used to
49 scale the destination color components. The eleven possible methods
50 are described in the following table. Each method defines four scale
51 factors, one each for red, green, blue, and alpha.
52 In the table and in subsequent equations, source and destination color
54 components are referred to as (Rs,Gs,Bs,As) and (Rd,Gd,Bd,Ad). The
55 color specified by glBlendColor is referred to as (Rc,Gc,Bc,Ac). They
56 are understood to have integer values between 0 and (kR,kG,kB,kA),
57 where
58 kc=2mc−1
60 and (mR,mG,mB,mA) is the number of red, green, blue, and alpha bit‐
62 planes.
63 Source and destination scale factors are referred to as (sR,sG,sB,sA)
65 and (dR,dG,dB,dA). The scale factors described in the table, denoted
66 (fR,fG,fB,fA), represent either source or destination factors. All
67 scale factors have range [0, 1].
68 ──────────────────────────────────────────────────────────────────
70 Parameter (fR,fG,fB,fA)
71 ──────────────────────────────────────────────────────────────────
72 GL_ZERO (0,0,0,0)
73 GL_ONE (1,1,1,1)
74 GL_SRC_COLOR (Rs/kR,Gs/kG,Bs/kB,As/kA)
75 GL_ONE_MINUS_SRC_COLOR (1,1,1,1)−(Rs/kR,Gs/kG,Bs/kB,As/kA)
76 GL_DST_COLOR (Rd/kR,Gd/kG,Bd/kB,Ad/kA)
77 GL_ONE_MINUS_DST_COLOR (1,1,1,1)−(Rd/kR,Gd/kG,Bd/kB,Ad/kA)
78 GL_SRC_ALPHA (As/kA,As/kA,As/kA,As/kA)
79 GL_ONE_MINUS_SRC_ALPHA (1,1,1,1)−(As/kA,As/kA,As/kA,As/kA)
80 GL_DST_ALPHA (Ad/kA,Ad/kA,Ad/kA,Ad/kA)
81 GL_ONE_MINUS_DST_ALPHA (1,1,1,1)−(Ad/kA,Ad/kA,Ad/kA,Ad/kA)
82 GL_SRC_ALPHA_SATURATE (i,i,i,1)
83 GL_CONSTANT_COLOR (Rc,Gc,Bc,Ac)
84 GL_ONE_MINUS_CONSTANT_COLOR (1,1,1,1)−(Rc,Gc,Bc,Ac)
85 GL_CONSTANT_ALPHA (Ac,Ac,Ac,Ac)
86 GL_ONE_MINUS_CONSTANT_ALPHA (1,1,1,1)−(Ac,Ac,Ac,Ac)
87 ──────────────────────────────────────────────────────────────────
88 In the table,
90 i=min(As,kA−Ad)/kA
92 To determine the blended RGBA values of a pixel when drawing in RGBA
94 mode, the system uses the following equations:
95 Rd=min(kR,RssR+RddR)
97 Gd=min(kG,GssG+GddG)
98 Bd=min(kB,BssB+BddB)
99 Ad=min(kA,AssA+AddA)
100 Despite the apparent precision of the above equations, blending arith‐
102 metic is not exactly specified, because blending operates with impre‐
103 cise integer color values. However, a blend factor that should be
104 equal to 1 is guaranteed not to modify its multiplicand, and a blend
105 factor equal to 0 reduces its multiplicand to 0. For example, when
106 sfactor is GL_SRC_ALPHA, dfactor is GL_ONE_MINUS_SRC_ALPHA, and As is
107 equal to kA, the equations reduce to simple replacement:
108 Rd=Rs
110 Gd=Gs
111 Bd=Bs
112 Ad=As
113
115 Transparency is best implemented using blend function (GL_SRC_ALPHA,
116 GL_ONE_MINUS_SRC_ALPHA) with primitives sorted from farthest to near‐
117 est. Note that this transparency calculation does not require the
118 presence of alpha bitplanes in the frame buffer.
119 Blend function (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA) is also useful
121 for rendering antialiased points and lines in arbitrary order.
122 Polygon antialiasing is optimized using blend function
124 (GL_SRC_ALPHA_SATURATE, GL_ONE) with polygons sorted from nearest to
125 farthest. (See the glEnable, glDisable reference page and the
126 GL_POLYGON_SMOOTH argument for information on polygon antialiasing.)
127 Destination alpha bitplanes, which must be present for this blend func‐
128 tion to operate correctly, store the accumulated coverage.
129
131 Incoming (source) alpha is correctly thought of as a material opacity,
132 ranging from 1.0 (KA), representing complete opacity, to 0.0 (0), rep‐
133 resenting complete transparency.
134 When more than one color buffer is enabled for drawing, the GL performs
136 blending separately for each enabled buffer, using the contents of that
137 buffer for destination color. (See glDrawBuffer.)
138 Blending affects only RGBA rendering. It is ignored by color index
140 renderers.
141 GL_CONSTANT_COLOR, GL_ONE_MINUS_CONSTANT_COLOR, GL_CONSTANT_ALPHA,
143 GL_ONE_MINUS_CONSTANT_ALPHA are only available if the GL_ARB_imaging is
144 supported by your implementation.
145
147 GL_INVALID_ENUM is generated if either sfactor or dfactor is not an
148 accepted value.
149 GL_INVALID_OPERATION is generated if glBlendFunc is executed between
151 the execution of glBegin and the corresponding execution of glEnd.
152
154 glGet with argument GL_BLEND_SRC
155 glGet with argument GL_BLEND_DST
156 glIsEnabled with argument GL_BLEND
157
159 glAlphaFunc(3G), glBlendColor(3G), glBlendEquation(3G), glClear(3G),
160 glDrawBuffer(3G), glEnable(3G), glLogicOp(3G), glStencilFunc(3G)
161 GLBLENDFUNC(3G)