Add 2.6 dithering filter for viewer

[cedricp]

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PR Description

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
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What does this pull request do?

Add a dithering filter to avoid color banding

Show a few screenshots (if this is a visual change)

[Your answer, delete this section if it is not a visual change.]

Have you tested your changes (if applicable)? If so, how?

Yes

Futher details of this pull request

Only works when the viewer is set to 32 bits FP

[devernay]

I'm pretty sure we already have software (Floyd Steinberg) dithering, and dithered tiles are cached in the viewer cache. Did you see that in the code?
Did you observe color banding, and can you show before/after this change?

[cedricp]

Oh, really, I haven't found any dithering code, maybe I overlooked it... This PR removes banding, though. I'll make a with/without image ASAP

[cedricp]

Here's a ramp without dithering filter :

and with :

I searched for a dithering algo in the code, but I couldn't find it.

[cedricp]

BTW, is there a release planned soon ?

[cedricp]

There's a typo in the shader code I fixed here : b992cd8

[cedricp]

It seems that there's a dithering filter when working in 8 bits mode in the viewer, I didn't check that before, this patch remains valid as I can't find dithering filter in 32 bits mode