GLBLENDFUNC(3G) 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 destination blending factors are computed.
Eight 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. 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 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
(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
kc = (2^^ mc) -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 fol-lowing following
lowing 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 1 is guaranteed not to 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 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_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), repre-senting representing
senting 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 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 implementation.
ERRORS
GL_INVALID_ENUM is generated if either sfactor or dfactor is not an accepted value.
GL_INVALID_OPERATION is generated if glBlendFunc is executed between the execution of glBegin and the
corresponding execution 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)
GLBLENDFUNC(3G)
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