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0 pointsdrcode9y ago0 comments

As a comic artist working with scanned images, the "divide by blur" trick is going to save me significant effort in the future.

THIS is exactly the sort of insight I wish I would get when I click one of those "one weird trick" links!

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tripzilch9y ago

Then you might also like the Grain Extract and Grain Merge layer modes in GIMP (not sure if they exist in photoshop).

They're a bit obscure if you don't know how they work, basically Grain Extract subtracts the layers and adds 128, so you get sort of a fake 8-bit signed integer. Grain Merge does the opposite (add two layers, subtract 128).

I haven't tried to divide-by-blur (but I'm going to :) ). Grain Extract on the other hand, allows to subtract-with-blur, which is more like what I'd do if I were to code such an algorithm myself (the operation is roughly what the Unsharp Mask filter does, but you get a bit more control). Still I'm curious to see how divide-by-blur differs in its results.

Check this Flickr-post which has a nice explanation of using Grain Extract (used for a different purpose here, it's really a very interesting and versatile trick, only discovered it last week myself):

https://www.flickr.com/groups/48637250@N00/discuss/721576192...

kazinator9y ago

I should mention that I know about Grain Extract's subtraction operation. I've used it for "leveling" also.

Subtraction would be fine if the data were logarithmic: that is to say, if the intensity information we are operating on represents decibels. We figure out the low frequency signal's "floor" over each area of the image, and simply subtract those decibels.

Division is better on the assumption that the intensity values of the pixels are linear. Division of linear samples is subtraction, in the decibel scale.

The right way is somewhere in between, because in RGB displays, pixels are put through some gamma curve. They are neither linear nor logarithmic.

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