There are three key things to understand about the way RGB Math components handle their inputs.
First, most RGB Math components process their inputs on a channel-wise basis. That is, they accept RGB colors and perform their titular operation on all color channels separately. For example, if you're using the Add component to add two colors (denoted below as A and B) to produce the output color (denoted below as Out), the result is calculated as follows:
Rout = Ra + Rb
Gout = Ga + Gb
Bout = Ba + Bb
To illustrate the following example, when the Add component is given two colors, R = 0.5, G = 1, B = 2 and R = 10, G = 20, B = 30, the resulting color will be R = 10.5, G = 21, B = 32.
Unlike the RGB channels, the alpha channel (opacity) is usually not processed in a channel-wise manner – most components just use the intact alpha from the first operand. You can use the Get Alpha and Set Alpha components to process the alpha channel in any way you want.
Second, RGB Math components treat RGB channels as floating-point numbers. Essentially, each RGB channel in a Filter Forge color is a floating-point number. In LDR colors, RGB channels are limited to the range of 0…1 (displayed as 0…255 by the Color Picker), and in HDR colors they can have unlimited values ranging from tiny fractions to trillions, positive or negative.
This means that you can literally enter numbers like -1200 or 0.00032 or 750000 into RGB Math components (via the RGB channels in the color picker in HDR mode), and the components will treat these numbers exactly as your pocket calculator would. There's no capping or wrapping when the result of calculations exceeds some arbitrary threshold, be it 255 or whatever else.
And third, RGB Math components use the base range of 0…1, not the display range of 0…255. When working with RGB Math components, it is important to understand that RGB channels in LDR colors, despite having the display range of 0…255, have actual values in the 0…1 range – and Filter Forge components use this latter representation, ignoring the display range.
For example, suppose that you’re adding an LDR color with the RGB channels values of R=0, G=128.5, B=255, and an HDR color with RGB channels of R=1000, G=2000 and B=3000. The result will be R=1000, G=2000.5 B=30001. If you expected the answer to be R=1000, G=2128.5 and B=3255, you're confusing the display range of 0…255 with the actual range of 0…1. To clear the confusion, read the section titled 'RGB ranges of 0…1 and 0…255 – what's the difference?' in the article on HDR Colors.
