Oh yah, I forgot about that. +1 for matching nuke's screen math (which boils down to max(a,b) if either a or b is outside of 0,1, and screen otherwise. It's less surprising to artists than clamping.
-- Jeremy On Fri, Jul 13, 2012 at 10:52 AM, Jonathan Egstad <jegs...@earthlink.net> wrote: > fwiw, > Some of the issues with merging HDR pixels have been hashed out in Nuke for a > while, so a peek at its Merge node may be helpful: > > For example 'screen' in Nuke is implemented to avoid the 1-A negative problem: > if (A >0 && A<1 && B>0 && B<1) C = A+B - A*B > else if (A > B) C = A > else C = B > > It also has the operation named 'hypot'(hypotenuse) or 'diagonal' which works > pretty well for HDR pixels: > C = sqrt(A*A + B*B) > > And lastly 'geometric': > if (A>0 && B>0) C = 2*A*B / (A+B) > else C = 0 > > -jonathan > > On Jul 13, 2012, at 9:37 AM, Jeremy Selan wrote: > >> Yup. >> >> screen = A+B-AB, where AB term represents the if intersection of the >> subpixel geometry. >> >> But in the case where you have two mattes where you know the >> correlation == -1, (which denotes no overlap in the subpixel >> geometry), adding mattes reduces to a simple A+B. An example of no >> overlap in the subpixel geometry would be the alpha rasterization of >> two adjacent triangles in a mesh. >> >> So to summarize, if the subpixel geometry has a correlation of 0, the >> math for combining alphas is A+B-AB. If the correlation is -1, the >> math is A+B. And for perfectly correlated subpixel geometry? Sounds >> like a good white-board project for this weekend... Surely there's >> some paper from 1985 that has worked this all out? :) >> >> -- Jeremy >> >> On Thu, Jul 12, 2012 at 9:05 PM, Troy Sobotka <troy.sobo...@gmail.com> wrote: >>> And the inevitable question: What is the canonical compositing formula >>> for correlated then? Simple add? >>> >> _______________________________________________ >> Oiio-dev mailing list >> Oiio-dev@lists.openimageio.org >> http://lists.openimageio.org/listinfo.cgi/oiio-dev-openimageio.org > > _______________________________________________ > Oiio-dev mailing list > Oiio-dev@lists.openimageio.org > http://lists.openimageio.org/listinfo.cgi/oiio-dev-openimageio.org _______________________________________________ Oiio-dev mailing list Oiio-dev@lists.openimageio.org http://lists.openimageio.org/listinfo.cgi/oiio-dev-openimageio.org
