Explaining Premultiplicaiton and unpremultiplication. Breakdowns of Over and Addmix, and tricks to gamma the alpha before overing. Also at the end talking about the importance of unpremulting before color corrections (wrong and right ways to CC).
This tutorial uses basic colors with a transition to teach these merging techniques and the math behind them. The math and techniques are the same for complex images and alphas, but I felt it was easier to demonstrate this way. If you are bored by the beginning sections, I’d recommend skipping ahead to the “gamma-ing the alpha before an over” section. Hope this de-mystifies some complex concept of merging images.
0:00 Intro
2:01 Multiplying (how alphas are applied, premult and unpremult)
4:56 Adding
5:47 Over – Breakdown (math explained)
8:28 Overing an unpremultiplied image
10:21 Overing a double premultiplied image
13:42 Gamma-ing the Alpha before an Over
18:36 Gamma-ing the Alpha (practical example)
19:55 Addmix Breakdown
26:41 Addmix Techniques
30:25 Unpremulting before Color Corrections
42:03 Outro
