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Index for Section 3G |
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Alphabetical listing for G |
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glBlendFunc(3G)
NAME
glBlendFunc - specify pixel arithmetic
SYNOPSIS
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 (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]
NOTES
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.
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
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.
SEE ALSO
glAlphaFunc(3), glBlendColor(3), glBlendEquation(3), glClear(3),
glDrawBuffer(3), glEnable(3), glLogicOp(3), glStencilFunc(3)
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Index for Section 3G |
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Alphabetical listing for G |
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