Weird grid after using rotate and perspective

Software darktable
1

When trying to analyze noise reduction settings with forensically web tool, I notices that rotate and perspective module produces weird grid on exported photos.

This seems to happen for all images I use rotate and perspective. I have tried different pixel interpolators from settings without help. Also different export settings doesn’t seem to get rid of this. Problem can be seen more easily when using multiple instances of diffuse or sharpen module.

This is before rotate with multiple diffuse and sharpen instances.

before rotate and perspective
before rotate and perspective2068×1380 2.45 MB

This is after rotate with multiple diffuse and sharpen instances.

after rotate and perspective
after rotate and perspective2070×1387 2.39 MB

And this is after rotate and no diffuse and sharpen module.

after rotate and perspective no diffuse and sharpen
after rotate and perspective no diffuse and sharpen2068×1378 2.36 MB

Any ideas? Files I have edited recently are from Canon 50D. Here is one example cr2 and xmp file. CC-BY-NC

2011_07_07_IMG_4698.CR2 (17.4 MB)
2011_07_07_IMG_4698.CR2.xmp (11.3 KB)

2

That looks remarkably like a moiré effect, when overlaying two grids with a small difference in frequency or orientation.
The two grids you have here are:

  • the “noise grid” caused by the multiple “diffuse or sharpen” modules,
  • and the sampling from the “rotate and perspective” module.

Getting that first grid from the “diffuse or sharpen” modules means for me that you have overdone the number of iterations on that module (perhaps on purpose?)