Image Arithmetic
Add

Addition

When you use a divisor of 1 and a bias of 0, adding two colors simply adds the values of the red, green and blue components as three separate additions. For example, if you add (15,30,90) to (90,20,10), you get (15+90,30+20,90+10), or (105,50,100).

Clipping

Since each of the individual values ranges from 0 through 255, each result can range from 0 through 510. You have two choices in how you handle results larger than 255:

• Clip - Any value larger than 255 is simply set to 255.

• No clip - The value "wraps around": 253, 254, 255, 0, 1, 2, ... For any value larger than 255, 256 is subtracted. Another way to think of this is that the result is the remainder after dividing by 256.

You'll get quite different results depending on whether you choose to clip:

Adding Black

The color black has RGB values of (0,0,0). Therefore, adding black to any color does not change that color.

Adding White

The color white has RGB values of (255,255,255). What happens when you add white to another color depending on whether you're using clipping:

• Clipped: adding white to any color results in white.

• No clip: adding white to any color that does not have any RGB values of 0 has very little effect on the color. It darkens it just a teeny little bit because it subtracts 1 (eg., 255 + 3 = 258, which becomes 2 after subtracting 256). But any component that has a value of 0 becomes 255 (255 + 0 = 255). Black becomes white: (255,255,255) + (0,0,0) = (255,255,255). But (255,255,255) + (0,1,1) (which is very nearly black) becomes red: (255+0,255+1,255+1) = (255,256,256) which becomes (255,0,0).

Here are a few examples of addition: