NAMEglBlendFunc -- specify pixel arithmetic
void glBlendFunc(GLenum sfactor,
- Specifies how the red, green, blue and alpha source
blending factors are computed. Nine symbolic constants
are accepted: GL_ZERO, GL_ONE,
GL_DST_ALPHA, GL_ONE_MINUS_DST_ALPHA, and
- Specifies how the red, green, blue and alpha
destination blending factors are computed. Eight symbolic
constants are accepted: GL_ZERO, GL_ONE,
GL_DST_ALPHA, and GL_ONE_MINUS_DST_ALPHA.
In RGB 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). By default, blending is disabled. Use
glDisable with argument GL_BLEND
to enable and disable blending.
glBlendFunc defines the operation whenbelnding 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 components. The eleven possible methods are described in the
table below. Each method defines four scale factors, one for red, green, blue
In the table and in subsequent equations, source and destination color
components are referred to as (Rs, Gs, Bs, As) and
(Rd, Gd, Bd, Ad). They are understood to have
integer values between zero and (kR, kG, kB, kA),
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].
In the table,i = min(As, kA-Ad)/kA
To determine the blended RGBA values of a pixel when drawing in RGB 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 blending operates with imprecise integer color
values. However, a blend factor that should be equal to one is guaranteed not
to modify its multiplicand, and a blend factor equal to zero reduces its
multiplicand to zero. Thus, 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
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 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
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.
Incoming (source) alpha is correctly thought of as material opacity, ranging
from 1.0 (kA), representing complete opacity, to 0.0 (0), representing
When more than one color buffer is enabled for drawing, blending is done
separately for each enabled buffer, using for destination color the contents
of that buffer.
Blending affects only RGB rendering. It is ignored by color index renderers.
GL_INVALID_ENUM is generated if either sfactor or dfactor
is not an accepted value.
GL_INVALID_OPERATION is generated if glBlendFunc is called
between a call to glBegin and the
corresponding call to glEnd.
glGet with argument GL_BLEND_SRC
glGet with argument GL_BLEND_DST
glIsEnabled with argument
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© 1995 Uwe Behrens. All rights reserved.