glBlendFunc(3G)glBlendFunc(3G)NAMEglBlendFunc - specify pixel arithmetic
SYNOPSIS
void glBlendFunc(
GLenum sfactor,
GLenum dfactor );
PARAMETERS
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_CON‐
STANT_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 follow‐
ing constants are accepted: GL_CONSTANT_COLOR, GL_ONE_MINUS_CON‐
STANT_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 dis‐
abled. 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 fac‐
tors, 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 arith‐
metic is not exactly specified, because blending operates with impre‐
cise 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 ren‐
derers.
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 GETSglGet() 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 near‐
est. Note that this transparency calculation does not require the pres‐
ence 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 func‐
tion to operate correctly, store the accumulated coverage.
SEE ALSOglAlphaFunc(3), glBlendColor(3), glBlendEquation(3), glClear(3),
glDrawBuffer(3), glEnable(3), glLogicOp(3), glStencilFunc(3)glBlendFunc(3G)