And on the seventh Day, He exited from append mode

Concept 1.2 - Combining Color - The hows!
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Blending of incoming pixel color values with
the existing ones is done as follows:

1 First specify how to compute the source and 
  destination factors.  Specify RGBA quadrapulets for 
  both the source and the destination.
2 Multiply the RGBA quadrapulets with the corresponding 
  RGBA values in the source and destination
3 Add up the corresponding components of the source and
  destination.

eg.  Say Source has the following values (Rs, Gs, Bs, As)
     Destination has the following (Rd, Gd, Bd, Ad)
     Source factor is the following (Sr, Sg, Sb, As)
     Destination factor is the following (Dr, Dg, Db, Ad)

Then the above computation would result in the following :

(RsSr + RdDr, GsSg + GdDg, BsSb + BdDb, AsSa + AdDa)

Each component of this quadrupulet is eventually clamped 
to [0,1]


The OpenGL function calls:
==========================
void glBlendFunc(GLenum sfactor, GLenu dfactor);
specify the source and the destination factor.
To enable blending call :
glEnable(GL_BLEND); 

Similarly use glDiable(GL_BLEND) to disable blending in
your programs.  

The following table shows possible values for the
glBendFunc.

The RGBA values of the source and destination are 
indicated with the subscripts s and d, respectively.  
Subtraction of quadrupulets means subtracting them 
componentwise.  The Relevant Factor column indicates 
whether the corresponding constant can be used to specify
the source or destination blend factor or used for both.

---------------------------------------------------------
Constant		  Relevant       Computed
to use in		  Factor         Blend Factor
GLenum sfactor
and dfactor
=========================================================

GL_ZERO			  source or      (0,0,0,0)
			  destination


GL_ONE			  source or      (1,1,1,1)
			  destination

GL_DST_COLOR		  source         (Rd,Gd,Bd,Ad)

GL_SRC_COLOR		  destination    (Rs,Gs,Bs,As)

GL_ONE_MINUS_DST_COLOR	  source         (1,1,1,1) -
				         (Rd,Gd,Bd,Ad)

GL_ONE_MINUS_SRC_COLOR    destination    (1,1,1,1) - 
				         (Rs,Gs,Bs,As)

GL_SRC_ALPHA		  source or      (Ad,Ad,Ad,Ad)
			  destination    

GL_ONE_MINUS_SRC_ALPHA    source or      (1,1,1,1) - 
			  destination    (As,As,As,As)
                            

GL_DST_ALPHA		  source or      (Ad,Ad,Ad,Ad)
		          destination    

GL_ONE_MINUS_DST_ALPHA    source or      (1,1,1,1) - 
			  destination    (Ad,Ad,Ad,Ad)
                                                                       

GL_SRC_ALPHA_SATURATE     source         (f,f,f,1);  
				         f=min(As, 1-Ad)

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