I use PNG files a lot for my work. I found that 8-bit RGBA files (non-linear, with premultiplied alpha channel) saved from the compositor have some sort of alpha contamination in the semi-transparent pixels that makes them imposible to compose correctly in other than Blender's compositor.
If the output has straight alpha (converted from premul with an alpha convert node), there are no bright artifacts. If the stored file is in linear space, there are no artifacts (both with premul and straight alpha). The artifacts become evident only in viewer* (checkers background) or taking the premulled file into a different application (GIMP, Nuke) or to Blender's VSE. Inside Blender's compositor there are no visible artifacts. Samples: http://dl.dropbox.com/u/255376/alpha-bug/Alpha-Bug-Nuke.jpg http://dl.dropbox.com/u/255376/alpha-bug/result%28VSE%20or%20GIMP%29.jpg *) phonybone did some changes in the viewer and now the bright halo is somewhat hidden. Now it's darker, but there's still alpha contamination in the image pixels Theory: Linear>log and log>linear conversions are affecting alpha channel in premultiplied alpha images. The bright artifacts are caused by the division of the corrected R*A,G*A,B*A pixels by the un-corrected alpha. I tried to re-create this incorrect behaviour by gamma-correcting the alpha channel artificially (I know it's wrong, I did it intentionally trying to confirm the bug) According to this graphic (http://www.blender.org/typo3temp/pics/ea3a513824.jpg) all 8bit images are converted to linear when used as inputs in compositor and converted from linear to log in the output stage. What happens with premultiplied alpha in that case? Is it divided before linearizing and multiplied before gamma corrected or it is not? I described the problem to phonybone in an IRC chat yesterday. He told me that seb könig recently pointed out that displaying the alpha channel (in image editor or compo backdrop) differs from color decompose result. That might be the same problem. Any ideas? _______________________________________________ Bf-committers mailing list [email protected] http://lists.blender.org/mailman/listinfo/bf-committers
