Any excuse will serve a tyrant

Concept 1.4 : A Simple Example
==============================

Idea
====
Two rectangles are drawn on the screen to your left.
One is a yellow rectangle, and the other one is
colored cyan.  By default the following happens :

A yellow triangle is drawn first with an alpha value
of 1.0.  Next the cyan triangle is drawn with an initial
alpha value of 1.0 and obscures part of the yellow triangle.

Click on the rendering window and select the menu options 
to change the alpha values for the cyan triangle.

The source and destination (Yellow triangle on
a Black Background):
===============================================
Initially the source is the yellow triangle and the
destination is the black background.
Here Color components are glColor3f(1, 1, 0, 1)
and blend factor GL_ONE is (1, 1, 1, 1) - (Concept 1.2)
Multiplying color components and blend factor we get :
(RsSr, GsSg, BsSb) = (1*1, 1*1, 0*1, 1*1) = (1, 1, 0, 1)
Hence the yellow triangle gets rendered and it obscures 
the black background since alpha value is 1.

Rendering the Cyan Triangle
===========================
Here the source is the Cyan triangle.  The destination
is part yellow (yellow triangle just rendered) and part
black (background). Using similar calculations we get

glBlendFunc (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glColor4f(0.0, 1.0, 1.0, alpha_factor);
where GL_SRC_ALPHA = (As, As, As, As) 
and GL_ONE_MINUS_SRC_ALPHA = (1, 1, 1, 1)-(As, As, As, As)
{From Concept 1.2}
Initially As = 1 (default value for cyan triangle)
Multiplying by the color components we get
(0.0, 1.0, 1.0, 1.0) * ( 1, 1, 1, 1) = (0, 1, 1, 1)

Now take a look at the destination.  Consider the
areas of the screen which have their pixels turned
to a yellow color.  These areas have a glColor4f() value 
of (1.0, 1.0, 0.0, 1.0)
The blend factor in this case is GL_ONE_MINUS_SRC_
ALPHA i.e. (1,1,1,1)-(As,As,As,As)
i.e. (1-1, 1-1, 1-1, 1-1)
This is equal to (0, 0, 0, 0)
The color components are (1, 1, 0, 1). Multiplying we get 
(1, 1, 0, 1) * (0, 0, 0, 0) = (0 ,0 ,0 ,0)

Adding up the source and destination factors where
the cyan and yellow triangles overlap leads to the
following :
(0, 1, 1, 1) + (0, 0, 0, 0) = (0.0, 1.0, 1.0, 1.0).

Now the cyan triangle is rendered with the above
components which account for a cyan color and an
alpha value of 1.0 i.e. total opacity.  Therefore
where the cyan triangle overlaps the yellow only the
cyan is visible. 

A similar justification can be provided for where
the cyan overlaps the black background.

It is obvious that changing alpha values for the cyan
triangle changes the final computed value and hence we
see different degrees of translucency.