GLBLENDFUNC() MachTen Programmer’s Manual GLBLENDFUNC()

NAME
glBlendFunc - 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 sym-
bolic 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 destina-
tion values). Blending is initially disabled. Use glEn-
able 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 des-
tination 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 num-
ber 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].

center box ; ci | ci c | c . 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 draw-
ing 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 arithmetic is not exactly specified, because
blending operates with imprecise integer color values.
However, a blend factor that should be equal to 1 is guar-
anteed 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 nearest. Note that this trans-
parency 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 refer-
ence page and the GL_POLYGON_SMOOTH argument for informa-
tion on polygon antialiasing.) Destination alpha bit-
planes, 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 mate-
rial 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 exe-
cuted between the execution of glBegin and the correspond-
ing 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

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