glBlendFunc(3G) OpenGL Reference glBlendFunc(3G)
glBlendFunc - specify pixel arithmetic
void glBlendFunc( GLenum sfactor,
GLenum dfactor )
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.
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 (R ,G ,B ,A ) and (R ,G ,B ,A ). The color
s s s s d d d d
specified by glBlendColor is referred to as (R ,G ,B ,A ). They are
c c c c
understood to have integer values between 0 and (k ,k ,k ,k ), where
R G B A
Page 1
glBlendFunc(3G) OpenGL Reference glBlendFunc(3G)
m
c
k = 2 -1
c
and (m ,m ,m ,m ) is the number of red, green, blue, and alpha bitplanes.
R G B A
Source and destination scale factors are referred to as (s ,s ,s ,s ) and
R G B A
(d ,d ,d ,d ). The scale factors described in the table, denoted
R G B A
(f ,f ,f ,f ), represent either source or destination factors. All scale
R G B A
factors have range [0, 1].
_________________________________________________________________________
Parameter (f , f , f , f )
R G B A
_________________________________________________________________________
GL_ZERO (0, 0, 0, 0)
GL_ONE (1, 1, 1, 1)
GL_SRC_COLOR (R /k , G /k , B /k , A /k )
s R s G s B s A
GL_ONE_MINUS_SRC_COLOR (1, 1, 1, 1) - (R /k , G /k , B /k , A /k )
s R s G s B s A
GL_DST_COLOR (R /k , G /k , B /k , A /k )
d R d G d B d A
GL_ONE_MINUS_DST_COLOR (1, 1, 1, 1) - (R /k , G /k , B /k , A /k )
d R d G d B d A
GL_SRC_ALPHA (A /k , A /k , A /k , A /k )
s A s A s A s A
GL_ONE_MINUS_SRC_ALPHA (1, 1, 1, 1) - (A /k , A /k , A /k , A /k )
s A s A s A s A
GL_DST_ALPHA (A /k , A /k , A /k , A /k )
d A d A d A d A
GL_ONE_MINUS_DST_ALPHA (1, 1, 1, 1) - (A /k , A /k , A /k , A /k )
d A d A d A d A
GL_SRC_ALPHA_SATURATE (i, i, i, 1)
GL_CONSTANT_COLOR (R G B A
c, c, c, c)
GL_ONE_MINUS_CONSTANT_COLOR (1, 1, 1, 1) - (R G B A
c, c, c, c)
GL_CONSTANT_ALPHA (A A A A
c, c, c, c)
GL_ONE_MINUS_CONSTANT_ALPHA (1, 1, 1, 1) - (A A A A
c, c, c, c)
_________________________________________________________________________
In the table,
i = min(A , k - A ) / k
s A d A
To determine the blended RGBA values of a pixel when drawing in RGBA
mode, the system uses the following equations:
R = min(k R s + R d )
d R, s R d R
G = min(k G s + G d )
d G, s G d G
B = min(k B s + B d )
d B, s B d B
A = min(k A s + A d )
d A, s A d A
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 A is
s
equal to k , the equations reduce to simple replacement:
A
Page 2
glBlendFunc(3G) OpenGL Reference glBlendFunc(3G)
R = R
d s
G = G
d s
B = B
d s
A = A
d s
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.
Incoming (source) alpha is correctly thought of as a material opacity,
ranging from 1.0 (K ), representing complete opacity, to 0.0 (0),
A
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.
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.
glGet with argument GL_BLEND_SRC
glGet with argument GL_BLEND_DST
glIsEnabled with argument GL_BLEND
Page 3
glBlendFunc(3G) OpenGL Reference glBlendFunc(3G)
MACHINE DEPENDENCIES
RealityEngine, RealityEngine2, and VTX systems do not support the use of
source blend factor GL_SRC_ALPHA_SATURATE and a destination factor based
on the constant blend color (GL_.._CONSTANT_.._EXT); such combinations
produce incorrect results.
glAlphaFunc, glBlendColor, glBlendEquation, glClear, glDrawBuffer,
glEnable, glLogicOp, glStencilFunc
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