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In GIMP, is adjusting the opacity of a layer directly above a white layer identical to making some combination of adjustments to brightness and contrast, or do the two achieve different ends? If they are interchangeable, how can one go back and forth between the two?

Similarly for modes such as Hard Light and Grain Merge, is there a way to replicate these effects using some combination of brightness, contrast, and opacity?

EDIT: The reason for the question is to learn whether fine-tuning the values for each of these metrics when editing an image is redundant (so that the same result is achievable in fewer steps), or whether they each contribute irreducibly to the result.

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    Hi. Welcome to GDSE. What problem are you actually trying to solve here? If I'm honest I don't really understand the question, or more importantly why it would matter. If it's just out of curiosity, then you should probably just do the experiments yourself. – Billy Kerr Sep 12 '18 at 23:22
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The math is different.

The math for the blend modes is here. In your case, I=255.

The math for alpha compositing is here.

Edit: since this has been bumped, a little experimental set up so that you can try for yourself:

  • At the bottom, a white layer
  • Above it, a Black-to-white gradient. Set opacity and mode to your heart's desire. The result (visible in bottom half) is your target
  • On top, covering the top half of the image, is you test gradient. It is the same gradient you used for the layer below.

Now, the game is to play with Levels, Curves or Whatever to make the top layer look identical to the rest, the criterion being that the boundary between the two becomes invisible.

enter image description here

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