A premultiplied color already includes alpha weighting. Applying that weighting again changes the result.
Name the stored representation
Straight color components and alpha are separate quantities. Premultiplied components already contain their color multiplied by alpha. Compositing formulas must match that representation; otherwise the foreground contribution can be weighted twice or used without the required weighting. Primary reference: W3C simple alpha compositing and premultiplied values.
For an original worksheet, label each input as straight or premultiplied and retain the backdrop alpha. Keep the compositing color domain specified separately. A translucent appearance alone does not reveal how stored components were represented or which arithmetic generated the displayed result.
Compare fictional scalar contributions
In a defined scalar color domain, use straight foreground value 0.8, alpha 0.25 and opaque backdrop value 0.2. Source-over gives 0.8 × 0.25 + 0.2 × 0.75 = 0.35. The foreground's premultiplied value is already 0.2, so the equivalent calculation is 0.2 + 0.2 × 0.75.
Incorrectly multiplying that premultiplied 0.2 by 0.25 again would produce 0.20 instead of 0.35. These invented values isolate representation arithmetic, not a light-measurement or display prediction. The assumed opaque backdrop makes output alpha one in this illustration.
Review contribution definitions
Prepare an original table with straight value, alpha, premultiplied value and the composited output. If an image changes representation during export, preserve that conversion rather than copying components into another formula unchanged. Treat zero-alpha unpremultiplication as a separately defined case instead of dividing blindly.
The guide claims no HM artwork or camera compositing behavior. Its reader decision is whether stored color was already alpha-weighted before the next calculation. Keep that evidence with transparent overlays used in technical image records so a representation mismatch is not mistaken for an observed feature change.
Customer Questions
What is the fictional premultiplied foreground value?
Zero point two.
What is the correct stated composite?
Zero point thirty-five.
Should premultiplied foreground be weighted by alpha again here?
No. That would count the weighting twice.
Primary References
These references support the technical principles discussed in this guide. The worked examples and review questions are educational.
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