visible low-level grid in images

[jannefoo]

See attached image for example. A very faint grid-like pattern emerges after stacking lots of very dark images - the example is after stacking and pushing of 10 stops. Thus far I've ruled out my camera (pattern not present with ACR converted files), DNG conversion (pattern present with out-of-camera NEF files), use of manual CA correction in raw tab, use of flat field images were ruled out previously.

My educated guess for the culprit would be parallelization of (RCD) demosaicing, there being nine boxes on eight-core system.

No sharpening, lens correction nor noise reduction was done.

Version: 5.4 Branch: 5.4 Commit: a5e8eb9c8 Commit date: 2018-03-20 Compiler: gcc 7.3.0 Processor: generic x86 System: Windows Bit depth: 64 bits Gtkmm: V3.22.0 Lensfun: V0.3.2.0 Build type: release Build flags: -m64 -mwin32 -mthreads -Wno-aggressive-loop-optimizations -std=c++11 -mtune=generic -Werror=unused-label -fopenmp -Werror=unknown-pragmas -Wall -Wno-unused-result -Wno-deprecated-declarations -DNDEBUG -O3 Link flags: -m64 -mthreads -static-libgcc -mtune=generic -s -O3 -fno-use-linker-plugin OpenMP support: ON MMAP support: ON

I'll try this with other demosaicing methods, but it'll take some time because of the large number of images.

[heckflosse]

@jannefoo Did you rule out auto CA correction in raw tab?

[jannefoo]

auto ca correction was not used

[jannefoo]

ok, found the culprit. it's actually the manual ca correction from raw tab that's causing this. the number of squares increases as the values on the ca correction sliders increase.

Slider at zero, no artifacts:

Big squares appear at slider position of 1:

Then they get smaller and more numerous as the slider position increases, 2:

3:

and finally 4: