GLBLENDFUNC()GLBLENDFUNC()NAMEglBlendFunc - specify pixel arithmetic
C SPECIFICATION
void glBlendFunc( GLenum sfactor,
GLenum dfactor )
delim $$
PARAMETERS
sfactor Specifies how the red, green, blue, and alpha source blending
factors are computed. Nine 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.
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.
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
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 $(R sub s , G sub s , B sub s , A sub s
)$ and $(R sub d , G sub d , B sub d , A sub d )$. They are understood
to have integer values between 0 and $(k sub R , k sub G , k sub B , k
sub A )$, where
$k sub c ~=~ 2 sup m sub c - 1$
and $(m sub R , m sub G , m sub B , m sub A )$ is the number of red,
green, blue, and alpha bitplanes.
Source and destination scale factors are referred to as $(s sub R , s
sub G , s sub B , s sub A )$ and $(d sub R , d sub G , d sub B , d sub
A )$. The scale factors described in the table, denoted $(f sub R , f
sub G , f sub B , f sub A )$, represent either source or destination
factors. All scale factors have range [0,1].
┌───────────────────────┬────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│ parameter │ $(f sub R , ~~ f sub G , ~~ f sub B , ~~ f sub A )$ │
├───────────────────────┼────────────────────────────────────────────────────────────────────────────────────────────────────────────┤
│ GL_ZERO │ $(0, ~0, ~0, ~0 )$ │
│ GL_ONE │ $(1, ~1, ~1, ~1 )$ │
│ GL_SRC_COLOR │ $(R sub s / k sub R , ~G sub s / k sub G , ~B sub s / k sub B , ~A sub s / k sub A )$ │
│GL_ONE_MINUS_SRC_COLOR │ $(1, ~1, ~1, ~1 ) ~-~ (R sub s / k sub R , ~G sub s / k sub G , ~B sub s / k sub B , ~A sub s / k sub A )$ │
│ GL_DST_COLOR │ $(R sub d / k sub R , ~G sub d / k sub G , ~B sub d / k sub B , ~A sub d / k sub A )$ │
│GL_ONE_MINUS_DST_COLOR │ $(1, ~1, ~1, ~1 ) ~-~ (R sub d / k sub R , ~G sub d / k sub G , ~B sub d / k sub B , ~A sub d / k sub A )$ │
│ GL_SRC_ALPHA │ $(A sub s / k sub A , ~A sub s / k sub A , ~A sub s / k sub A , ~A sub s / k sub A )$ │
│GL_ONE_MINUS_SRC_ALPHA │ $(1, ~1, ~1, ~1 ) ~-~ (A sub s / k sub A , ~A sub s / k sub A , ~A sub s / k sub A , ~A sub s / k sub A )$ │
│ GL_DST_ALPHA │ $(A sub d / k sub A , ~A sub d / k sub A , ~A sub d / k sub A , ~A sub d / k sub A )$ │
│GL_ONE_MINUS_DST_ALPHA │ $(1, ~1, ~1, ~1 ) ~-~ (A sub d / k sub A , ~A sub d / k sub A , ~A sub d / k sub A , ~A sub d / k sub A )$ │
│GL_SRC_ALPHA_SATURATE │ $(i, ~i, ~i, ~1 )$ │
└───────────────────────┴────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
In the table,
$i ~=~ min (A sub s , ~k sub A - A sub d ) ~/~ k sub A$
To determine the blended RGBA values of a pixel when drawing in RGBA
mode, the system uses the following equations:
$R sub d ~=~ min ( k sub R , ~~ R sub s s sub R + R sub d d sub R )$
$G sub d ~=~ min ( k sub G , ~~ G sub s s sub G + G sub d d sub G )$
$B sub d ~=~ min ( k sub B , ~~ B sub s s sub B + B sub d d sub B )$
$A sub d ~=~ min ( k sub A , ~~ A sub s s sub A + A sub d d sub A )$
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 $A sub
s$ is equal to $k sub A$, the equations reduce to simple replacement:
$R sub d ~=~ R sub s$
$G sub d ~=~ G sub s$
$B sub d ~=~ B sub s$
$A sub d ~=~ A sub s$
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
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_POLY‐
GON_SMOOTH argument for information on polygon antialiasing.) Destina‐
tion 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 ($K sub A$), 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.
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
SEE ALSO
glAlphaFunc, glClear, glDrawBuffer, glEnable, glLogicOp, glStencilFunc
GLBLENDFUNC()
