[FFmpeg-devel] [PATCH] Revert "avfilter/vf_palette(gen|use): support palettes with alpha"

[Michael Niedermayer]

On Mon, Oct 31, 2022 at 11:57:16AM +0100, Clément Bœsch wrote:
> On Mon, Oct 31, 2022 at 01:43:11AM +0000, Soft Works wrote:
> [...]
> > > > > The patch I had submitted doesn't change the previous behavior
> > > > > without the use_alpha parameter.
> > > 
> > > Yes I noticed, but unfortunately I'm reworking the color distance to
> > > work
> > > in perceptual color space, and the way that alpha is mixed up in the
> > > equation just doesn't make any sense at all and prevents me from
> > > doing
> > > these changes. 
> > 
> > If you want to implement a new color distance algorithm, it should 
> > be either a new filter or a new (switchable) mode for the existing 
> > filter.
> 
> Why?
> 
> > Photoshop has these different modes as well and it would 
> > surely be useful, but I don't think it should be replacing the
> > existing behavior.
> > 
> 
> There is no point in keeping a ton of complexity exposed as user options
> for something implementation specific. We offer no guarantee over how the
> quantization is expected to run.
> 
> > When it turns out that the use_alpha implementation doesn't fit
> > with your new color distance calculation and you add it as 
> > an additional mode, then it would be fine IMO when the filter
> > errors in case it would be attempted to use that mode in 
> > combination with use_alpha.
> 
> IMO The use_alpha option shouldn't exist in the first place, it should be
> the default behaviour because honoring the alpha is the correct thing to
> do. That's not what the option is currently doing though.
> 
> > > > Do you think it might make sense to put more weight on the
> > > > alpha value by tripling it? So it would be weighted equally to the
> > > > RGB value?
> > > 
> > > You cannot mix alpha with colors at all, they are separate domains
> > > and you
> > > need to treat them as such.
> > 
> > What's interesting is that I've followed the same (simplified)
> > way for adding a use_alpha option to vf_elbg and it provides excellent
> > results without treating alpha separately.
> 
> I don't know how the filter works and what it's supposed to do, but if
> it's indeed using the same approach as the palette ones, it cannot work.
> 
> > > From paletteuse perspective what you need to do is first choose the
> > > colors
> > > in the palette that match exactly the alpha (or at least the closest
> > > if
> > > and only there is no exact match). Then within that set, and only
> > > within
> > > that one, you'd pick the closest color.
> > > 
> > > From palettegen perspective, you need to split the colors in
> > > different
> > > transparency domain (a first dimensional quantization), then quantize
> > > the
> > > colors in each quantized alpha dimension. And when you have all your
> > > quantized palettes for each level of alpha, you find an algorithm to
> > > reduce the number of transparency dimensions or the number of colors
> > > per
> > > dimension to make it fit inside a single palette. But you can't just
> > > do
> > > the alpha and the colors at the same time, it cannot work, whatever
> > > weights you choose.
> > 
> > I would be curious to see how well that would work, especially
> > in cases when the target palettes have just a few number of colors.
> > 
> 
> You have to make a call between whether you want to preserve the
> transparency or the color while constructing the palette, but when
> choosing a color you must absolutely not choose a color with a different
> transparency, you must pick amongst the closest alpha, with a particular
> attention to extreme alphas: an opaque colors must stay opaque, and fully
> transparent one as well:
> - rounding a color with 43% alpha into 50% alpha is acceptable
> - rounding a color with 100% alpha into a 99% alpha is not acceptable in
>   any way because you're starting to make transparent areas that weren't
> - rounding a color with 0% alpha into a 1% alpha is not acceptable because
>   some areas of the images are not starting to blend into an area that was
>   supposedly non-existent

really ?
so if i have all shades of green available for all transparencies from 1% to 99%
i "must" make my plants all use 0% trasparency even if i only have a single color and
that is bright pink 
I dont think that is the best choice

There are perceptual differences between the different areas of the RGBA hypercube
though. Hardly anyone would notice the difference between a 255 and 254 blue but
having some slight transparency might be noticable.
These different weights in different areas could maybe be considered in palette*
and elbg, it likely would improve things. OTOH heuristics like always and never
feels like that might become alot of work to tune. I think its better to attemt
to achieve a similar goal with less hard and more perceptual scoring

thx

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Its not that you shouldnt use gotos but rather that you should write
readable code and code with gotos often but not always is less readable
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