Hi ivan 1-(1-0.52)^3.137169 = 0.9 1-(1-0.4)^3.137169 = 0.798617 1-(1-0.2)^3.137169 = 0.503434
In other words given an accumulated alpha and a target alpha, you can calculate one exponent that can then be applied to every sample as above. Colin On 18 Dec 2013 01:10, "Ivan Busquets" <[email protected]> wrote: > Hi, > > Sorry for the repost. I sent this to the development list yesterday, but > posting over here as well to cast a broader net. > > Has anyone dug around the "target input alpha" option in the DeepRecolor > node, and has some insight on how it is retargetting each sample internally? > > Long story short, I'm trying to implement a procedure to re-target opacity > of each sample in a deep pixel, akin to what's happening in a DeepRecolor > node when "target input alpha" is checked. > > I've got this to a point where it's working ok, but I think I might be > missing something, as my results differ from those you'd get in a > DeepRecolor. > > My re-targetting algorithm is based on the assumption that the relative > opacity between samples should be preserved, but DeepRecolor clearly uses a > different approach. > > Example: > > Say you have a deep pixel with 2 samples, and the following opacities: > > Samp1 : 0.4 > Samp2 : 0.2 > > The accumulated opacity is 0.52 (Samp1 over Samp2). Note that Samp1 > deliberately has an opacity of 2 times Samp2. > > Now, let's say we want to re-target those samples to an accumulated > opacity of 0.9. > > What I am doing is trying to calculate new opacities for Samp1 and Samp2 > in such a way that both of these conditions are met. > > a) Samp1 == 2 * Samp2 > b) Samp1 over Samp2 == 0.9 > > This gives me the following re-targeted values: > > Samp1 : 0.829284 > > Samp2 : 0.414642 > > > I'm happy with those, but it bugs me that DeepRecolor throws different > results: > > > Samp1 : 0.798617 > Samp2 : 0.503434 > > Which meets the second condition (Samp1 over Samp2 == 0.9), but does not > preserve the relative opacities of the original samples. > > It seems to me like DeepRecolor is applying some sort of non-linear > function to apply a different weight to each of the original samples, but I > haven't been able to figure out the logic of that weighting, or a reason > why it's done that way. > > Does anyone have any insight/ideas on what DeepRecolor might be doing > internally? > Or a reason why you might want to distribute the target alpha in a > non-linear way? > > Thanks in advance, > Ivan > > _______________________________________________ > Nuke-users mailing list > [email protected], http://forums.thefoundry.co.uk/ > http://support.thefoundry.co.uk/cgi-bin/mailman/listinfo/nuke-users >
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