GLBLENDFUNCSEPARATEEXT(3gl)GLBLENDFUNCSEPARATEEXT(3gl)NAMEglBlendFuncSeparateEXT - specify RGB and Alpha blending arithmetic
C SPECIFICATION
void glBlendFuncSeparateEXT( GLenum sfactorRGB,
GLenum dfactorRGB,
GLenum sfactorAlpha,
GLenum dfactorAlpha )
PARAMETERS
sfactorRGB Specifies how the red, green, and blue 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, GL_SRC_ALPHA_SATURATE, GL_CON‐
STANT_COLOR_EXT, GL_ONE_MINUS_CONSTANT_COLOR_EXT, GL_CON‐
STANT_ALPHA_EXT, GL_ONE_MINUS_CONSTANT_ALPHA_EXT. If the
device supports GL_SUN_blend_src_mult_dst_alpha exten‐
sion, the following symbolic constants are also accepted:
GL_SRC_ALPHA_MULT_DST_ALPHA_SUN,
GL_SRC_ALPHA_MULT_ONE_MINUS_DST_ALPHA_SUN. The initial
value is GL_ONE.
dfactorRGB Specifies how the red, green, and blue destination blend‐
ing factors are computed. The following symbolic con‐
stants 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, GL_CONSTANT_COLOR_EXT,
GL_ONE_MINUS_CONSTANT_COLOR_EXT, GL_CONSTANT_ALPHA_EXT,
GL_ONE_MINUS_CONSTANT_ALPHA_EXT. The initial value is
GL_ZERO.
sfactorAlpha Specifies how the alpha source blending factors are com‐
puted. 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, GL_SRC_ALPHA_SATURATE, GL_CON‐
STANT_COLOR_EXT, GL_ONE_MINUS_CONSTANT_COLOR_EXT, GL_CON‐
STANT_ALPHA_EXT, GL_ONE_MINUS_CONSTANT_ALPHA_EXT. If the
device supports GL_SUN_blend_src_mult_dst_alpha exten‐
sion, the following symbolic constants are also accepted:
GL_SRC_ALPHA_MULT_DST_ALPHA_SUN,
GL_SRC_ALPHA_MULT_ONE_MINUS_DST_ALPHA_SUN. The initial
value is GL_ONE.
dfactorAlpha Specifies how the alpha destination blending factors are
computed. The following 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, GL_CONSTANT_COLOR_EXT,
GL_ONE_MINUS_CONSTANT_COLOR_EXT, GL_CONSTANT_ALPHA_EXT,
GL_ONE_MINUS_CONSTANT_ALPHA_EXT. The initial value is
GL_ZERO.
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 dis‐
abled. Use glEnable and glDisable with argument GL_BLEND to enable and
disable blending.
glBlendFuncSeparateEXT defines the operation of blending when it is
enabled. sfactorRGB and sfactorAlpha specify which of the methods is
used to scale the source color components. dfactorRGB and dfactorAlpha
specify which of the methods is used to scale the destination color
components. The 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, destination, and con‐
stant color components are referred to as (Rs,Gs,Bs,As) , (Rd,Gd,Bd,Ad)
and (Rc,Gc,Bc,Ac) . The source and destination color components are
understood to have integer values between 0 and (kR,kG,kB,kA), where
kc=2mc−1
and (mR,mG,mB,mA) is the number of red, green, blue, and alpha bit‐
planes.
The constant color components are the color components specified by
glBlendColorEXT. These color components are understood to be in range
[0,1], and are not scaled by (kR,kG,kB,kA).
Source and destination scale factors are referred to as (sR,sG,sB,sA)
and (dR,dG,dB,dA). The scale factors described in the table, denoted
(fR,fG,fB,fA), represent either source or destination factors. All
scale factors have range [0,1].
┌──────────────────────────────────────────┬───────────────────────────────────────────────────────────────────┐
│ parameter │ (fR,fG,fB,fA) │
├──────────────────────────────────────────┼───────────────────────────────────────────────────────────────────┤
│ GL_ZERO │ (0,0,0,0) │
│ GL_ONE │ (1,1,1,1) │
│ GL_SRC_COLOR │ (Rs/kR,Gs/kG,Bs/kB,As/kA) │
│ GL_ONE_MINUS_SRC_COLOR │ (1,1,1,1)−(Rs/kR,Gs/kG,Bs/kB,As/kA) │
│ GL_DST_COLOR │ (Rd/kR,Gd/kG,Bd/kB,Ad/kA) │
│ GL_ONE_MINUS_DST_COLOR │ (1,1,1,1)−(Rd/kR,Gd/kG,Bd/kB,Ad/kA) │
│ GL_SRC_ALPHA │ (As/kA,As/kA,As/kA,As/kA) │
│ GL_ONE_MINUS_SRC_ALPHA │ (1,1,1,1)−(As/kA,As/kA,As/kA,As/kA) │
│ GL_DST_ALPHA │ (Ad/kA,Ad/kA,Ad/kA,Ad/kA) │
│ GL_ONE_MINUS_DST_ALPHA │ (1,1,1,1)−(Ad/kA,Ad/kA,Ad/kA,Ad/kA) │
│ GL_SRC_ALPHA_SATURATE │ (i,i,i,1) │
│ GL_CONSTANT_COLOR_EXT │ (Rc,Gc,Bc,Ac) │
│ GL_ONE_MINUS_CONSTANT_COLOR_EXT │ (1,1,1,1)−(Rc,Gc,Bc,Ac) │
│ GL_CONSTANT_ALPHA_EXT │ (Ac,Ac,Ac,Ac) │
│ GL_ONE_MINUS_CONSTANT_ALPHA_EXT │ (1,1,1,1)−(Ac,Ac,Ac,Ac) │
│ GL_SRC_ALPHA_MULT_DST_ALPHA_SUN │ (As/kA*Ad/kA,As/kA*Ad/kA,As/kA*Ad/kA,As/kA*Ad/kA) │
│GL_SRC_ALPHA_MULT_ONE_MINUS_DST_ALPHA_SUN │ (As/kA*(1−Ad/kA),As/kA*(1−Ad/kA),As/kA*(1−Ad/kA),As/kA*(1−Ad/kA)) │
└──────────────────────────────────────────┴───────────────────────────────────────────────────────────────────┘
In the table,
i=min(As,kA−Ad)/kA
To determine the blended RGBA values of a pixel when drawing in RGBA
mode, the system uses the blend equation specified by glBlendEqua‐
tionEXT:
Rr=blend(RssR,RddR)
Gr=blend(GssG,GddG)
Br=blend(BssB,BddB)
Ar=blend(AssA,AddA)
and the following equations:
Rd=min(kR,Rr)
Gd=min(kG,Gr)
Bd=min(kB,Br)
Ad=min(kA,Ar)
Despite the apparent precision of the above equations, blending arith‐
metic is not exactly specified, because blending operates with impre‐
cise 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
sfactorRGB and sfactorAlpha are GL_SRC_ALPHA, dfactorRGB and dfactorAl‐
pha areGL_ONE_MINUS_SRC_ALPHA, and As is equal to kA, the equations
reduce to simple replacement:
Rd=Rs
Gd=Gs
Bd=Bs
Ad=As
EXAMPLES
Transparency is best implemented using blend function (GL_SRC_ALPHA,
GL_ONE_MINUS_SRC_ALPHA) with primitives sorted from farthest to near‐
est. 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_POLY‐
GON_SMOOTH argument for information on polygon antialiasing.) Destina‐
tion 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 (KA), representing complete opacity, to 0.0 (0), rep‐
resenting 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.
ERRORS
GL_INVALID_ENUM is generated if either sfactorRGB, dfactorRGB sfac‐
torAlpha, dfactorAlpha is not an accepted value.
GL_INVALID_OPERATION is generated if glBlendFuncSeparateEXT is executed
between the execution of glBegin and the corresponding execution of
glEnd.
ASSOCIATED GETS
glGet with argument GL_BLEND_SRC_RGB_EXT
glGet with argument GL_BLEND_DST_RGB_EXT
glGet with argument GL_BLEND_SRC_ALPHA_EXT
glGet with argument GL_BLEND_DST_ALPHA_EXT
glIsEnabled with argument GL_BLEND
SEE ALSO
glAlphaFunc, glBlendFunc, glBlendColorEXT, glBlendEquationEXT, glClear,
glDrawBuffer, glEnable, glLogicOp, glStencilFunc
21 Aug 2002 GLBLENDFUNCSEPARATEEXT(3gl)
