Banding from Lumagen CMS settings
I'm creating 3D LUTS for my Lumagen 2143 / JVC X30. Lumagen is configured for 422 in / 422 out.
I'm struggling with quite a lot of banding in the blue channel, particularly visible in the RGB ramps in the Spears and Munsil 2nd edition disc. The banding is in the form of repeating mini-ramps over the length of a blue channel ramp from 0,0,0 to 0,0,255 - it looks like some kind of mathematical artifact. They're quite distinct.
I've worked out so far that there are two sources of banding:
1) Having Darbee enabled. Not a problem, disable it. (this actually causes banding in all the ramps)
2) Having the CMS do any addition of green to the blue primary position.
I proved this by taking the CMS setting down from 17x17x17 to 8 point, which kept the same primary / secondary locations. The blue primary is then (64,270,920), which is what I measure as being necessary to have blue in the right position. This has the same banding - so the banding is not an artefact from the LUT itself. Changing the value of the blue primary down to (64,64,920) removes the banding.
As I say, this looks like some kind of maths-type artefact. Has anyone else seen this? Is this a settings issue or inherent to the CMS implementation? Thanks!