GLBLENDFUNC(3G) UNIX Programmer's Manual GLBLENDFUNC(3G)
NAME
glBlendFunc - specify pixel arithmetic
C SPECIFICATION
void glBlendFunc( GLenum sfactor,
GLenum dfactor )
PARAMETERS
sfactor Specifies how the red, green, blue, and alpha
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, and GL_SRC_ALPHA_SATURATE.
The initial value is GL_ONE.
Additionally, if the GL_ARB_imaging extension is
supported, the following constants are accepted:
GL_CONSTANT_COLOR, GL_ONE_MINUS_CONSTANT_COLOR,
GL_CONSTANT_ALPHA, GL_ONE_MINUS_CONSTANT_ALPHA.
dfactor Specifies how the red, green, blue, and alpha des-
tination blending factors are computed. Eight sym-
bolic 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. The initial value is
GL_ZERO.
Additionally, if the GL_ARB_imaging extension is
supported, the following constants are accepted:
GL_CONSTANT_COLOR, GL_ONE_MINUS_CONSTANT_COLOR,
GL_CONSTANT_ALPHA, GL_ONE_MINUS_CONSTANT_ALPHA.
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 glEnable and
glDisable with argument GL_BLEND to enable and disable
blending.
glBlendFunc defines the operation of blending when it is
enabled. sfactor specifies which of nine methods is used to
scale the source color components. dfactor specifies which
of eight methods is used to scale the destination color com-
ponents. The eleven possible methods are described in the
following table. Each method defines four scale factors, one
each for red, green, blue, and alpha.
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In the table and in subsequent equations, source and desti-
nation color components are referred to as (Rs,Gs,Bs,As) and
(Rd,Gd,Bd,Ad). The color specified by glBlendColor is
referred to as (Rc,Gc,Bc,Ac). They are understood to have
integer values between 0 and (kR,kG,kB,kA), where
mc
kc = 2 -1
and (mR,mG,mB,mA) is the number of red, green, blue, and
alpha bitplanes.
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 (Rc,Gc,Bc,Ac)
GL_ONE_MINUS_CONSTANT_COLOR(1, 1, 1, 1) - (Rc,Gc,Bc,Ac)
GL_CONSTANT_ALPHA (Ac,Ac,Ac,Ac)
GL_ONE_MINUS_CONSTANT_ALPHA(1, 1, 1, 1) - (Ac,Ac,Ac,Ac)
_________________________
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 following equations:
Rd = min(kR, Rs sR + Rd dR)
Gd = min(kG, Gs sG + Gd dG)
Bd = min(kB, Bs sB + Bd dB)
Ad = min(kA, As sA + Ad dA)
Despite the apparent precision of the above equations,
blending arithmetic is not exactly specified, because blend-
ing operates with imprecise integer color values. However, a
blend factor that should be equal to 1 is guaranteed not to
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modify its multiplicand, and a blend factor equal to 0
reduces its multiplicand to 0. For example, when sfactor is
GL_SRC_ALPHA, dfactor is GL_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 nearest. Note that this transparency calcu-
lation 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_POLYGON_SMOOTH 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 (KA), representing com-
plete opacity, to 0.0 (0), representing complete tran-
sparency.
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.
GL_CONSTANT_COLOR, GL_ONE_MINUS_CONSTANT_COLOR,
GL_CONSTANT_ALPHA, GL_ONE_MINUS_CONSTANT_ALPHA are only
available if the GL_ARB_imaging is supported by your imple-
mentation.
ERRORS
GL_INVALID_ENUM is generated if either sfactor or dfactor is
not an accepted value.
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GL_INVALID_OPERATION is generated if glBlendFunc is executed
between the execution of glBegin and the corresponding exe-
cution of glEnd.
ASSOCIATED GETS
glGet with argument GL_BLEND_SRC
glGet with argument GL_BLEND_DST
glIsEnabled with argument GL_BLEND
SEE ALSO
glAlphaFunc(3G), glBlendColor(3G), glBlendEquation(3G),
glClear(3G), glDrawBuffer(3G), glEnable(3G), glLogicOp(3G),
glStencilFunc(3G)
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