glBlendFunc - specify pixel arithmetic
void glBlendFunc(
GLenum sfactor,
GLenum dfactor );
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.
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[s], G[s],
B[s], A[s] ) and (R[d], G[d], B[d], A[d] ). The color
specified by glBlendColor() is referred to as (R[c], G[c],
B[c], A[c] ). They are understood to have integer values
between 0 and (k[R], k[G], k[B], k[A] ), where
k[c] = 2^m[c] - 1
and (m[R], m[G], m[B], m[A] ) is the number of red, green,
blue, and alpha bitplanes.
Source and destination scale factors are referred to as
(s[R], s[G], s[B], s[A] ) and (d[R], d[G], d[B], d[A] ).
The scale factors described in the table, denoted (f[R],
f[G], f[B], f[A] ), represent either source or destination
factors. All scale factors have range [0, 1].
Parameter (f[R], f[G], f[B], f[A] )
GL_ZERO (0, 0, 0, 0 )
GL_ONE (1, 1, 1, 1 )
GL_SRC_COLOR (R[s] / k[R], G[s] / k[G], B[s] /
k[B], A[s] / k[A] )
GL_ONE_MINUS_SRC_COLOR (1, 1, 1, 1 ) - (R[s] / k[R], G[s] /
k[G], B[s] / k[B], A[s] / k[A] )
GL_DST_COLOR (R[d] / k[R], G[d] / k[G], B[d] /
k[B], A[d] / k[A] )
GL_ONE_MINUS_DST_COLOR (1, 1, 1, 1 ) - (R[d] / k[R], G[d] /
k[G], B[d] / k[B], A[d] / k[A] )
GL_SRC_ALPHA (A[s] / k[A], A[s] / k[A], A[s] /
k[A], A[s] / k[A] )
GL_ONE_MINUS_SRC_ALPHA (1, 1, 1, 1 ) - (A[s] / k[A], A[s] /
k[A], A[s] / k[A], A[s] / k[A] )
GL_DST_ALPHA (A[d] / k[A], A[d] / k[A], A[d] /
k[A], A[d] / k[A] )
GL_ONE_MINUS_DST_ALPHA (1, 1, 1, 1 ) - (A[d] / k[A], A[d] /
k[A], A[d] / k[A], A[d] / k[A] )
GL_SRC_ALPHA_SATURATE (i, i, i, 1 )
GL_CONSTANT_COLOR (R[c], G[c], B[c], A[c])
GL_ONE_MINUS_CONSTANT_COLOR (1, 1, 1, 1 ) - (R[c], G[c], B[c],
A[c])
GL_CONSTANT_ALPHA (A[c], A[c], A[c], A[c])
GL_ONE_MINUS_CONSTANT_ALPHA (1, 1, 1, 1 ) - (A[c], A[c], A[c],
A[c])
In the table,
i = min (A[s], k[A] - A[d] ) / k[A]
To determine the blended RGBA values of a pixel when drawing
in RGBA mode, the system uses the following equations:
R[d] min ( k[R], R[s]s[R]+R[d]d[R] )
G[d]( k[G], G[s]s[G]+G[d]d[G] )
B[d]( k[B], B[s]s[B]+B[d]d[B] )
A[d]( k[A], A[s]s[A]+A[d]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[s] is equal to k[A], the
equations reduce to simple replacement:
R[d] = mark R[s] G[d] = lineup G[s] B[d] = lineup B[s]
A[d] = lineup A[s]
Incoming (source) alpha is correctly thought of as a material
opacity, ranging from 1.0 (K[A]), representing 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.
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
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.
glAlphaFunc(3), glBlendColor(3), glBlendEquation(3),
glClear(3), glDrawBuffer(3), glEnable(3), glLogicOp(3),
glStencilFunc(3)
glBlendFunc(3G)
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