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<STRONG>NAME</STRONG>
<STRONG>glBlendFunc</STRONG> - specify pixel arithmetic
<STRONG>C</STRONG> <STRONG>SPECIFICATION</STRONG>
void <STRONG>glBlendFunc</STRONG>( GLenum <EM>sfactor</EM>,
GLenum <EM>dfactor</EM> )
<STRONG>PARAMETERS</STRONG>
<EM>sfactor</EM> Specifies how the red, green, blue, and alpha
source blending factors are computed. Nine
symbolic constants are accepted: <STRONG>GL_ZERO</STRONG>, <STRONG>GL_ONE</STRONG>,
<STRONG>GL_DST_COLOR</STRONG>, <STRONG>GL_ONE_MINUS_DST_COLOR</STRONG>, <STRONG>GL_SRC_ALPHA</STRONG>,
<STRONG>GL_ONE_MINUS_SRC_ALPHA</STRONG>, <STRONG>GL_DST_ALPHA</STRONG>,
<STRONG>GL_ONE_MINUS_DST_ALPHA</STRONG>, and <STRONG>GL_SRC_ALPHA_SATURATE</STRONG>.
The initial value is <STRONG>GL_ONE</STRONG>.
<EM>dfactor</EM> Specifies how the red, green, blue, and alpha
destination blending factors are computed. Eight
symbolic constants are accepted: <STRONG>GL_ZERO</STRONG>, <STRONG>GL_ONE</STRONG>,
<STRONG>GL_SRC_COLOR</STRONG>, <STRONG>GL_ONE_MINUS_SRC_COLOR</STRONG>, <STRONG>GL_SRC_ALPHA</STRONG>,
<STRONG>GL_ONE_MINUS_SRC_ALPHA</STRONG>, <STRONG>GL_DST_ALPHA</STRONG>, and
<STRONG>GL_ONE_MINUS_DST_ALPHA</STRONG>. The initial value is
<STRONG>GL_ZERO</STRONG>.
<STRONG>DESCRIPTION</STRONG>
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 <STRONG>glEnable</STRONG> and
<STRONG>glDisable</STRONG> with argument <STRONG>GL_BLEND</STRONG> to enable and disable
blending.
<STRONG>glBlendFunc</STRONG> defines the operation of blending when it is
enabled. <EM>sfactor</EM> specifies which of nine methods is used to
scale the source color components. <EM>dfactor</EM> 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 ,G ,B ,A ) and (R ,G ,B ,A ). They are understood to
ha<STRONG>v</STRONG>e <STRONG>i</STRONG>nt<STRONG>e</STRONG>ge<STRONG>r</STRONG> values <STRONG>b</STRONG>et<STRONG>w</STRONG>ee<STRONG>n</STRONG> 0dand (k ,k ,k ,k ), where
R G B A
mc
kc = 2 -1
and (mR,mG,mB,mA) is the number of red, green, blue, and
alpha bitplanes.
Source and destination scale factors are referred to as
(s ,s ,s ,s ) and (d ,d ,d ,d ). The scale factors
de<STRONG>s</STRONG>cr<STRONG>i</STRONG>be<STRONG>d</STRONG> i<STRONG>n</STRONG> the tab<STRONG>l</STRONG>e,Gde<STRONG>n</STRONG>ot<STRONG>e</STRONG>d (f ,f ,f ,f ), represent
either source or destination facto<STRONG>r</STRONG>s.G A<STRONG>l</STRONG>l <STRONG>s</STRONG>cale factors
have range [0,1].
______________________________________________________________________
| <EM>parameter</EM> | (f , f , f , f ) |
<EM>|</EM>_______________________<EM>|</EM>______________<STRONG>_</STRONG>____<STRONG>_</STRONG>____<STRONG>_</STRONG>____<STRONG>_</STRONG>_______________|
| <STRONG>GL_ZERO</STRONG> | (0, 0, 0, 0) |
| <STRONG>GL_ONE</STRONG> | (1, 1, 1, 1) |
| <STRONG>GL_SRC_COLOR</STRONG> | (R /k , G /k , B /k , A /k ) |
|<STRONG>GL_ONE_MINUS_SRC_COLOR</STRONG> | (1, 1, 1,s1)R- (<STRONG>R</STRONG> /<STRONG>k</STRONG> , <STRONG>G</STRONG> /<STRONG>k</STRONG> , <STRONG>B</STRONG> /<STRONG>k</STRONG> , A /k ) |
| <STRONG>GL_DST_COLOR</STRONG> | (R /k , G <STRONG>/</STRONG>k <STRONG>,</STRONG> B <STRONG>/</STRONG>k <STRONG>,</STRONG> A <STRONG>/</STRONG>k <STRONG>)</STRONG> s A |
|<STRONG>GL_ONE_MINUS_DST_COLOR</STRONG> | (1, 1, 1,d1)R- (<STRONG>R</STRONG> /<STRONG>k</STRONG> , <STRONG>G</STRONG> /<STRONG>k</STRONG> , <STRONG>B</STRONG> /<STRONG>k</STRONG> , A /k ) |
| <STRONG>GL_SRC_ALPHA</STRONG> | (A /k , A <STRONG>/</STRONG>k <STRONG>,</STRONG> A <STRONG>/</STRONG>k <STRONG>,</STRONG> A <STRONG>/</STRONG>k <STRONG>)</STRONG> d A |
|<STRONG>GL_ONE_MINUS_SRC_ALPHA</STRONG> | (1, 1, 1,s1)A- (<STRONG>A</STRONG> /<STRONG>k</STRONG> , <STRONG>A</STRONG> /<STRONG>k</STRONG> , <STRONG>A</STRONG> /<STRONG>k</STRONG> , A /k ) |
| <STRONG>GL_DST_ALPHA</STRONG> | (A /k , A <STRONG>/</STRONG>k <STRONG>,</STRONG> A <STRONG>/</STRONG>k <STRONG>,</STRONG> A <STRONG>/</STRONG>k <STRONG>)</STRONG> s A |
|<STRONG>GL_ONE_MINUS_DST_ALPHA</STRONG> | (1, 1, 1,d1)A- (<STRONG>A</STRONG> /<STRONG>k</STRONG> , <STRONG>A</STRONG> /<STRONG>k</STRONG> , <STRONG>A</STRONG> /<STRONG>k</STRONG> , A /k ) |
|<STRONG>GL_SRC_ALPHA_SATURATE</STRONG> | (i<STRONG>,</STRONG> i<STRONG>,</STRONG> i,d1)A d A d A |
<EM>|</EM>_______________________<EM>|</EM>_____________________________________________|
In the table,
i = min(A , k -A ) / k
s A d A
To determine the blended RGBA values of a pixel when drawing
in RGBA mode, the system uses the following equations:
R = min(k , R s +R d )
Gd = min(kR, GssR+GddR)
Bd = min(kG, BssG+BddG)
Ad = min(kB, AssB+AddB)
d A s A 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 <EM>sfactor</EM> is <STRONG>GL_SRC_ALPHA</STRONG>, <EM>dfactor</EM> is
<STRONG>GL_ONE_MINUS_SRC_ALPHA</STRONG>, and A is equal to k , the equations
reduce to simple replacement:s A
R = R
Gd = Gs
Bd = Bs
Ad = As
d s
<STRONG>EXAMPLES</STRONG>
Transparency is best implemented using blend function
(<STRONG>GL_SRC_ALPHA</STRONG>, <STRONG>GL_ONE_MINUS_SRC_ALPHA</STRONG>) 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 (<STRONG>GL_SRC_ALPHA</STRONG>, <STRONG>GL_ONE_MINUS_SRC_ALPHA</STRONG>) is
also useful for rendering antialiased points and lines in
arbitrary order.
Polygon antialiasing is optimized using blend function
(<STRONG>GL_SRC_ALPHA_SATURATE</STRONG>, <STRONG>GL_ONE</STRONG>) with polygons sorted from
nearest to farthest. (See the <STRONG>glEnable</STRONG>, <STRONG>glDisable</STRONG> reference
page and the <STRONG>GL_POLYGON_SMOOTH</STRONG> argument for information on
polygon antialiasing.) Destination alpha bitplanes, which
must be present for this blend function to operate
correctly, store the accumulated coverage.
<STRONG>NOTES</STRONG>
Incoming (source) alpha is correctly thought of as a
material opacity, ranging from 1.0 (K ), representing
complete opacity, to 0.0 (0), represe<STRONG>n</STRONG>ting 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 <STRONG>glDrawBuffer</STRONG>.)
Blending affects only RGBA rendering. It is ignored by
color index renderers.
<STRONG>ERRORS</STRONG>
<STRONG>GL_INVALID_ENUM</STRONG> is generated if either <EM>sfactor</EM> or <EM>dfactor</EM> is
not an accepted value.
<STRONG>GL_INVALID_OPERATION</STRONG> is generated if <STRONG>glBlendFunc</STRONG> is executed
between the execution of <STRONG>glBegin</STRONG> and the corresponding
execution of <STRONG>glEnd</STRONG>.
<STRONG>ASSOCIATED</STRONG> <STRONG>GETS</STRONG>
<STRONG>glGet</STRONG> with argument <STRONG>GL_BLEND_SRC</STRONG>
<STRONG>glGet</STRONG> with argument <STRONG>GL_BLEND_DST</STRONG>
<STRONG>glIsEnabled</STRONG> with argument <STRONG>GL_BLEND</STRONG>
<STRONG>SEE</STRONG> <STRONG>ALSO</STRONG>
<STRONG>glAlphaFunc</STRONG>, <STRONG>glClear</STRONG>, <STRONG>glDrawBuffer</STRONG>, <STRONG>glEnable</STRONG>, <STRONG>glLogicOp</STRONG>,
<STRONG>glStencilFunc</STRONG>
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