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GLBLENDFUNC(3G)                                                                              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
                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_CONSTANT_ALPHA,
                GL_ONE_MINUS_CONSTANT_ALPHA.

       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.

                Additionally,  if  the  GL_ARB_imaging  extension  is supported, the following constants are
                accepted:      GL_CONSTANT_COLOR,      GL_ONE_MINUS_CONSTANT_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
       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 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
       (Rs , Gs , Bs , As ) and (Rd , Gd , Bd , Ad ).  The color specified by glBlendColor is referred to as
       (Rc , Gc , Bc , Ac ).  They are understood to have integer values between 0 and (kR , kG , kB , kA ),
       where

                                                     kc = (2^^ mc) -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 (sR , sG , sB , sA ) and (dR , dG , dB , dA
       ).  The scale factors described in the table, denoted (fR , fG , fB , fA ), represent  either  source
       or destination factors.  All scale factors have range [0, 1].



        Parameter                                    (fR ,  fG ,  fB ,  fA )
        GL_ZERO                        (0, 0, 0, 0 )
        GL_ONE                                   (1, 1, 1, 1 )
        GL_SRC_COLOR                     (Rs / kR , Gs / kG , Bs / kB , As / kA )
        GL_ONE_MINUS_SRC_COLOR        (1, 1, 1, 1 ) - (Rs / kR , Gs / kG , Bs / kB , As / kA )
        GL_DST_COLOR                     (Rd / kR , Gd / kG , Bd / kB , Ad / kA )
        GL_ONE_MINUS_DST_COLOR        (1, 1, 1, 1 ) - (Rd / kR , Gd / kG , Bd / kB , Ad / kA )
        GL_SRC_ALPHA                     (As / kA , As / kA , As / kA , As / kA )
        GL_ONE_MINUS_SRC_ALPHA        (1, 1, 1, 1 ) - (As / kA , As / kA , As / kA , As / kA )
        GL_DST_ALPHA                     (Ad / kA , Ad / kA , Ad / kA , Ad / kA )
        GL_ONE_MINUS_DST_ALPHA        (1, 1, 1, 1 ) - (Ad / kA , Ad / kA , Ad / kA , Ad / kA )
        GL_SRC_ALPHA_SATURATE        (i, i, i, 1 )
        GL_CONSTANT_COLOR            (Rc, Gc, Bc, Ac)
        GL_ONE_MINUS_CONSTANT_COLOR(1, 1, 1, 1 ) - (Rc, Gc, Bc, Ac)
        GL_CONSTANT_ALPHA            (Ac, Ac, Ac, Ac)
        GL_ONE_MINUS_CONSTANT_ALPHA(1, 1, 1, 1 ) - (Ac, Ac, Ac, Ac)


       In the table,


                     i =  min (As , kA - Ad ) / kA

       To  determine  the blended RGBA values of a pixel when drawing in RGBA mode, the system uses the fol-lowing following
       lowing equations:


                     Rd = min ( kR, Rs sR + Rd dR )
                     Gd = min ( kG, Gs sG + Gd dG )
                     Bd = min ( kB, Bs sB + Bd dB )
                     Ad = min ( kA, As sA + Ad dA )

       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
       As is equal to kA, the equations reduce to simple replacement:


                     Rd = Rs
                     Gd = Gs
                     Bd = Bs
                     Ad = As


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 (kA),  repre-senting representing
       senting 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.

       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 GETS
       glGet with argument GL_BLEND_SRC
       glGet with argument GL_BLEND_DST
       glIsEnabled with argument GL_BLEND

SEE ALSO
       glAlphaFunc(3G), glBlendColor(3G), glBlendEquation(3G), glClear(3G), glDrawBuffer(3G), glEnable(3G), glLogicOp(3G), glStencilFunc(3G)




                                                                                             GLBLENDFUNC(3G)