Making the Applications Interesting - Essentials of Interactive Computer Graphics - page 331

so

S =( S r , S g , S b , S α ) ,

D α ) .
Notice that although defined as the fourth channel of color, the

D =( D r , D g , D b ,

channel cannot
affect the color of a pixel directly. Only the R , G ,and B channels can change
the color of a pixel. The

α

channel is designed to be used as the blending factor
in achieving transparency effects. For example, Figure 12.8 shows a semitrans-

α

parent circle drawn over a square. Notice that we can see through and observe
colors/objects behind the circle. The final color of pixels covered by the circle is
actually a blend of the circle color and that of the original destination pixel color:
the colors of the square and the grid. In this case, the circle is the source primitive
with color S ; before rendering the circle, the destination pixels' colors are D .In
this example, the source blending factor A s is

Figure 12.8. Asemi-

transparent circle.

A s = S α ,

and the destination blending factor A d is

A d =( 1 − S α ) ,

whereas the blending function is the addition function

← ×

A d +

× ,

D D S A s

← × ( −

S α )+

×

S α , D D 1 S

or

⎧
⎨

← × ( −

S α )+

×

S α , D r D r 1 S r

D g ← D g × ( 1 − S α )+ S g × S α ,

(12.4)

⎩ D b ← D b × ( 1 − S α )+ S b × S α ,

D α ← D α × ( 1 − S α )+ S α × S α .

In this way, S α approximates the opacity effect of the circle: when α = 1, we will

not see any color behind the circle; when α = 0 . 5, we will see an equal blend;

0, the circle will be completely transparent.
Typical modern graphics hardware supports the following blending factors
( A d and A s ) for source and destination:

and when α =

High-end hardware. Special-
ized graphics hardware may
support additional options for
blending factor and function
types.

Constant 0 ( 0 , 0 , 0 , 0 )

Constant 1 ( 1 , 1 , 1 , 1 )

Source color ( S r , S g , S b , S α )

Inverse source color ( 1 − S r , 1 − S g , 1 − S b , 1 − S α )

Source alpha ( S α , S α , S α , S α )

Inverse source alpha ( 1 − S α , 1 − S α , 1 − S α , 1 − S α )

Destination alpha ( D α , D α , D α , D α )

Inverse destination alpha ( 1 − D α , 1 − D α , 1 − D α , 1 − D α )