GLenum dfactor )

PARAMETERS
       sfactor  Specifies  how  the red, green, blue, and alpha source blending
                factors are computed.  The following symbolic constants are ac‐
                cepted:  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 in‐
       coming (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.  sfac‐
       tor 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  de‐
       scribed 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=2mc−1

       and  (mR,mG,mB,mA)  is  the  number  of red, green, blue, and alpha bit‐
       planes.

       Source and destination scale factors are referred  to  as  (sR,sG,sB,sA)
          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 following equations:

                     Rd=min(kR,RssR+RddR)
                     Gd=min(kG,GssG+GddG)
                     Bd=min(kB,BssB+BddB)
                     Ad=min(kA,AssA+AddA)

       Despite the apparent precision of the above equations,  blending  arith‐
       metic 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 func‐
       tion to operate correctly, store the accumulated coverage.

NOTES

ERRORS
       GL_INVALID_ENUM  is generated if either sfactor or dfactor is not an ac‐
       cepted 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,   glBlendColor,   glBlendEquation,  glClear,  glDrawBuffer,
       glEnable, glLogicOp, glStencilFunc

                                                                GLBLENDFUNC(3G)