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When I use "Wavelet Decompose" to make a frequency separation on Gimp, I use, let's say 3 levels, which generate 3 layers of detail. But IMHO they are too many (On Ps I am used to having only 1 layer for details)

So I have tried different methods to merge these layers into only 1.

The first method is leaving on only the "details" layers and right-click selecting "New from Visible".

enter image description here

The other method is "Merge Visible Layers" (1).

Both methods render acceptable results, but they are not precise. If I generate a "Difference" layer (2) comparing the resulting image vs the original, some slight differences appear (3 and 4).

enter image description here

Which would be a more precise method to merge the layers?

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  • There will always be rounding errors. Use higher color depth.
    – joojaa
    Jun 29, 2021 at 6:51

1 Answer 1

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Very likely round-off errors(*) because if I can reproduce the problem in 8-bit mode, changing the image to high-precision (32-bit floating point) (Image > Precision) at the beginning of the process makes the problem disappear:

enter image description here

Btw, as shown above, using the Histogram dialog is better than Levels. The screenshot shows that the count of pixels with a value above 0.001 (actually between 0.001 and 1.000) is indeed 0.

(*) Because even if all computations are done in floating-point in Gimp 2.10, using 8-bit mode means that the results are still stored on 8-bit, which is slightly worse than 3 decimal digits.

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  • One should avoid doing very computationally big things in 8 bit mode. Agter all 256 values is not very much as far as computation goes.
    – joojaa
    Jun 29, 2021 at 9:08
  • Yes but going to 32-bit FP requires four times the RAM. Otherwise you can tell Gimp to promote all images to 32-bit FP on load.
    – xenoid
    Jun 29, 2021 at 9:44
  • Sure, but hey its 2020 everybody has atleast 32 Gb of memory by now. Hell, even my graphics card has more than that
    – joojaa
    Jun 29, 2021 at 9:46
  • Yes but they run WIndows on that :)
    – xenoid
    Jun 29, 2021 at 9:47
  • Well to be honest all the macs in this room also have 32 Gb of memory. But obvioulsy the windows and linux machines in the next room have even more.
    – joojaa
    Jun 29, 2021 at 9:49

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