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> They're not terribly intuitive for designers, and they don't even attempt to simulate the effects of combining light. Can you expand on this? I thought that

• pavlov 3y ago
  Yes, that’s true if you have samples that represent actual measured luminosity (a.k.a. linear light). But most digital images are not that. Instead they are gamma-corrected to make better use of the available encoding range. Gamma correction is effectively a 1/2.2 power function applied to each RGB component (normalized and clamped to a 0-1 range), so more bits are used to encode darker values. If you were to store raw linear values in a typical image format that uses 8 bits per component (i.e. 24 bits for RGB), you’d get very visible banding in dark areas of the image. So this is the problem for Photoshop’s blending modes. They want to simulate how light works, but they operate on gamma-corrected images so you can’t just do the simple linear math directly. And converting to a higher-precision linear light representation would have been expensive on 1990s CPUs. Instead the PS blending mode algorithms are more like approximated hacks to do visually interesting combinations of gamma-corrected layers “on the cheap.” As for Porter-Duff, they don’t even pretend to simulate light in any way. The article explains this pretty well below the example images of the twelve operations: “Porter/Duff is not a way to blend the source and destination shapes. It is way to overlay, combine and trim them as if they were pieces of cardboard. The only place where source and destination pixels are actually blended is along the antialiased edges.“