Nuke assumes associated alpha all over the place, no? I don't see why it doesn't also assume associated alpha when it's writes PNG files so it can write correct ones.
But you are right, if you assume that Nuke doesn't know if alpha is associated or not, and some nodes assumes it's not and some assume that it is, then the situation can't really be improved other than perhaps a option on the file writer to make it more obvious. --jono On Thu, Jul 24, 2014 at 5:02 PM, Rangi Sutton <[email protected]> wrote: > Hey Jono, > > So, if you're working with un-premultiplied alpha in your scripts then you > would need to put down a pre-multiply node before writing out you're pngs. > Which is counter intuitive and potentially lossy. > > Which one is it? Should nuke guess? Or should it just do what it's told.. > ie, write the colour values it's given. I like that it does what it's told. > > Cheers, > r. > > > Rangi Sutton > VFX Supervisor > Cutting Edge > > > On 25 July 2014 09:38, Jonathan Gibbs <[email protected]> wrote: > >> Well, that's pretty unfriendly on Nuke's part. PNG is pretty clear what >> is and isn't supposed to be in the file. >> --jono >> >> >> >> On Thu, Jul 24, 2014 at 3:46 PM, Rangi Sutton <[email protected] >> > wrote: >> >>> Hey, >>> >>> I think it's a good thing Nuke doesn't try and second guess you and make >>> "intelligent" decisions about what you're trying to achieve. For this case, >>> if the nuke script contains pre-mulitplied alpha, then un-premultiply it >>> (divide by alpha) before writing out your png files. >>> >>> Unless I'm missing something! >>> >>> Cheers, >>> r. >>> >>> >>> >>> Rangi Sutton >>> VFX Supervisor >>> Cutting Edge >>> >>> >>> On 25 July 2014 04:32, Mikael Sundell <[email protected]> wrote: >>> >>>> Aaaargggh :-) >>>> >>>> Skickat från min iPhone >>>> >>>> 24 Jul 2014 kl. 19:49 skrev Larry Gritz <[email protected]>: >>>> >>>> The fun with PNG never seems to end. And by "fun", I mean a hot >>>> poker in the eye. >>>> >>>> >>>> On Jul 24, 2014, at 1:20 PM, Mikael Sundell <[email protected]> >>>> wrote: >>>> >>>> I seems as if Nuke writes data with associated alpha in PNG files, >>>> according to the spec it's not valid: >>>> >>>> http://www.libpng.org/pub/png/spec/1.2/PNG-DataRep.html >>>> >>>> "The color values stored for a pixel are not affected by the alpha >>>> value assigned to the pixel. This rule is sometimes called "unassociated" >>>> or "non-premultiplied" alpha. (Another common technique is to store sample >>>> values premultiplied by the alpha fraction; in effect, such an image is >>>> already composited against a black background. PNG does not use >>>> premultiplied alpha.)" >>>> >>>> This causes the PNGReader to associateAlpha(…) on already alpha >>>> multiplied data. Did anyone else experience this problem? >>>> >>>> Mikael >>>> >>>> >>>> -- >>>> Larry Gritz >>>> [email protected] >>>> >>>> > > _______________________________________________ > Oiio-dev mailing list > [email protected] > http://lists.openimageio.org/listinfo.cgi/oiio-dev-openimageio.org > >
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