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GLBLENDFUNC(3G) OpenGL 3.3 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 initial
value is GL_ONE.
dfactor
Specifies how the red, green, blue, and alpha destination blending factors are computed. The
following symbolic constants are accepted: GL_ZERO, GL_ONE, GL_SRC_COLOR, GL_ONE_MINUS_SRC_COLOR,
GL_DST_COLOR, GL_ONE_MINUS_DST_COLOR, GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA, GL_DST_ALPHA,
GL_ONE_MINUS_DST_ALPHA. GL_CONSTANT_COLOR, GL_ONE_MINUS_CONSTANT_COLOR, GL_CONSTANT_ALPHA, and
GL_ONE_MINUS_CONSTANT_ALPHA. The initial value is GL_ZERO.
DESCRIPTION
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 method is
used to scale the source color components. dfactor specifies which method is used to scale the
destination color components. Both parameters must be one of the following symbolic constants:
GL_ZERO, GL_ONE, GL_SRC_COLOR, GL_ONE_MINUS_SRC_COLOR, GL_DST_COLOR, GL_ONE_MINUS_DST_COLOR,
GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA, GL_DST_ALPHA, GL_ONE_MINUS_DST_ALPHA, GL_CONSTANT_COLOR,
GL_ONE_MINUS_CONSTANT_COLOR, GL_CONSTANT_ALPHA, GL_ONE_MINUS_CONSTANT_ALPHA, GL_SRC_ALPHA_SATURATE,
GL_SRC1_COLOR, GL_ONE_MINUS_SRC1_COLOR, GL_SRC1_ALPHA, and GL_ONE_MINUS_SRC1_ALPHA. The 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, first source, second source
and destination color components are referred to as R s0 G s0 B s0 A s0, R s1 G s1 B s1 A s1 and R d
G d B d A d, respectively. 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
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 |
| | s0 |
| | |
| | k |
| | R |
| | |
| | |
| | |
| | G |
| | s0 |
| | |
| | k |
| | G |
| | |
| | |
| | |
| | B |
| | s0 |
| | |
| | k |
| | B |
| | |
| | |
| | |
| | A |
| | s0 |
| | |
| | k |
| | A |
+----------------------------------+--------------------------------------------+
|GL_ONE_MINUS_SRC_COLOR | 1 |
| | 1 |
| | 1 |
| | 1 |
| | |
| | - |
| | |
| | |
| | R |
| | s0 |
| | |
| | k |
| | R |
| | |
| | |
| | |
| | G |
| | s0 |
| | |
| | k |
| | G |
| | |
| | |
| | |
| | B |
| | s0 |
| | |
| | k |
| | B |
| | |
| | |
| | |
| | A |
| | s0 |
| | |
| | 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 |
| | s0 |
| | |
| | k |
| | A |
| | |
| | |
| | |
| | A |
| | s0 |
| | |
| | k |
| | A |
| | |
| | |
| | |
| | A |
| | s0 |
| | |
| | k |
| | A |
| | |
| | |
| | |
| | A |
| | s0 |
| | |
| | k |
| | A |
+----------------------------------+--------------------------------------------+
|GL_ONE_MINUS_SRC_ALPHA | 1 |
| | 1 |
| | 1 |
| | 1 |
| | |
| | - |
| | |
| | |
| | A |
| | s0 |
| | |
| | k |
| | A |
| | |
| | |
| | |
| | A |
| | s0 |
| | |
| | k |
| | A |
| | |
| | |
| | |
| | A |
| | s0 |
| | |
| | k |
| | A |
| | |
| | |
| | |
| | A |
| | s0 |
| | |
| | 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_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 |
+----------------------------------+--------------------------------------------+
|GL_SRC_ALPHA_SATURATE | i |
| | i |
| | i |
| | 1 |
+----------------------------------+--------------------------------------------+
|GL_SRC1_COLOR | R |
| | s1 |
| | |
| | k |
| | R |
| | |
| | |
| | |
| | G |
| | s1 |
| | |
| | k |
| | G |
| | |
| | |
| | |
| | B |
| | s1 |
| | |
| | k |
| | B |
| | |
| | |
| | |
| | A |
| | s1 |
| | |
| | k |
| | A |
+----------------------------------+--------------------------------------------+
|GL_ONE_MINUS_SRC1_COLOR | 1 |
| | 1 |
| | 1 |
| | 1 |
| | |
| | - |
| | |
| | |
| | R |
| | s1 |
| | |
| | k |
| | R |
| | |
| | |
| | |
| | G |
| | s1 |
| | |
| | k |
| | G |
| | |
| | |
| | |
| | B |
| | s1 |
| | |
| | k |
| | B |
| | |
| | |
| | |
| | A |
| | s1 |
| | |
| | k |
| | A |
+----------------------------------+--------------------------------------------+
|GL_SRC1_ALPHA | A |
| | s1 |
| | |
| | k |
| | A |
| | |
| | |
| | |
| | A |
| | s1 |
| | |
| | k |
| | A |
| | |
| | |
| | |
| | A |
| | s1 |
| | |
| | k |
| | A |
| | |
| | |
| | |
| | A |
| | s1 |
| | |
| | k |
| | A |
+----------------------------------+--------------------------------------------+
|GL_ONE_MINUS_SRC1_ALPHA | 1 |
| | 1 |
| | 1 |
| | 1 |
| | |
| | - |
| | |
| | |
| | A |
| | s1 |
| | |
| | k |
| | A |
| | |
| | |
| | |
| | A |
| | s1 |
| | |
| | k |
| | A |
| | |
| | |
| | |
| | A |
| | s1 |
| | |
| | k |
| | A |
| | |
| | |
| | |
| | A |
| | s1 |
| | |
| | k |
| | A |
+----------------------------------+--------------------------------------------+
In the table,
To determine the blended RGBA values of a pixel, the system uses the following equations:
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:
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 (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().)
When dual source blending is enabled (i.e., one of the blend factors requiring the second color input
is used), the maximum number of enabled draw buffers is given by GL_MAX_DUAL_SOURCE_DRAW_BUFFERS,
which may be lower than GL_MAX_DRAW_BUFFERS.
ERRORS
GL_INVALID_ENUM is generated if either sfactor or dfactor is not an accepted value.
ASSOCIATED GETS
glGet() with argument GL_BLEND_SRC
glGet() with argument GL_BLEND_DST
glIsEnabled() with argument GL_BLEND
SEE ALSO
glBlendColor(), glBlendEquation(), glBlendFuncSeparate(), glClear(), glDrawBuffer(), glEnable(),
glLogicOp(), glStencilFunc()
COPYRIGHT
Copyright (C) 1991-2006 Silicon Graphics, Inc. This document is licensed under the SGI Free Software
B License. For details, see http://oss.sgi.com/projects/FreeB/.
OpenGL 3.3 03/08/2011 GLBLENDFUNC(3G)
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