Making the Applications Interesting - Essentials of Interactive Computer Graphics - page 330

When the square is rendered, the RGB values of the source color S are assigned
to the destination pixels D :

⎧
⎨

D r ← S r ,

D ← S or D g ← S g ,

⎩

D b ← S b .

In this way, the destination pixel color is overwritten by the source primitive color.
Modern graphics hardware generalizes this color assignment operation with a
color blending function. Instead of simply overwriting the destination pixel color,
the blending function combines the destination pixel colors ( D ) with the source
primitive colors ( S ) to create new destination colors:

D ← f ( D × A d , S × A s ) , (12.3)

where

⎧
⎨

D − destination pixel color ,

A d − destination blending factor ,

S − source primitive color ,

⎩

A s − source blending factor ,

() − .

f blending function

For example, with blending factors
A d =(

. , . , . ) ,

0 2 0 2 0 2

A s =( 0 . 8 , 0 . 8 , 0 . 8 ) ,

and a blending function of addition

D ← f ( D × A d , S × A s ) ,

D ← ( D × 0 . 2 )+( S × 0 . 8 ) ,

we get

⎧
⎨

← × . + × . ,

D r D r 0 2 S r 0 8

← × . + × . ,

D g D g 0 2 S g 0 8

⎩

D b ← D b × 0 . 2 + S b × 0 . 8 .

In this case, the final destination pixel colors are a blend of 20% (0 . 2) original

destination pixel colors with 80% (0 8) source primitive colors.

To properly support this blending functionality, modern graphics hardware
introduced a forth channel to the color representation, the

.

α channel:

color =( r , g , b , α ) ,