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Thursday, December 18, 2014

Resurrecting an old idea...

Several years ago I have developed some code that adds natural-looking grain to a black and white image. The code was never released so far, it just stayed on my hard drive and I've been using it from time to time to add grain to my images. Now that PhotoFlow is getting more solid and stable, it is maybe time to resurrect this old project and try to integrate it with the rest of PhotoFlow's tools.

Here are a couple of examples of how the added grain looks like (they correspond to two possible choices of the grain size), compared to the original image.

The way the grain is generated is quite different from conventional approaches: instead of overlaying a "grain field" over the original image, the final result is gnerated by adding the individual grains one-by-one. The density at which the grains are locally distributed is such that, on average, the final tonal value matches the original one.

The difference with respect to a classic grain overlay might not be huge, but is visible. As an example, you can find below a comparison with the result of Darktable's grain filter at 6400 ISO and 50% strength (mouseover to see the result of my method):

All that might be just over-complicated, but it was an intresting intellectual exercise which brings (at least to my taste) some visual benefits. Probably it will land into PhotoFlow sooner or later, also depending upon the feedback I get from possibly interested users...

UPDATE: G'MIC also provides a nice grain simulation filter, and it was interesting to compare it with my own recipy.
Surprisingly, my "large grain setting" shown above matches very closely G'MIC's "TMAX 3200" preset at 80% of opacity, 100% scale and "grain merge" blending. The comparison is show below (left: G'MIC, right: my code):

Below  you can see the test image rendered by G'MIC using the same settings (mouse over to see my own result). As one can see, while the grain rendering in the mid tones is very similar, the two methods give quite different results in dark and light areas, as well as regions of high contrast.

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