GNOME Bugzilla – Bug 677114
videoscale: lanczos downscaling is not pretty
Last modified: 2014-11-27 13:23:06 UTC
Created attachment 215267 [details] comparing VideoScale bilinear, VideoScale Lanczos and gimp GIMP Lanczos3 Pitivi uses VideoScale's method #3 (GST_VIDEO_SCALE_LANCZOS) for thumbnailing (height = 50 pixels). It seems there is a slight improvement over the default method (bilinear), but the difference is very minor and I can't help but to compare with gimp's bicubic or lanczos3 scaling that looks much better (because it's smoothed, somehow). Attached is a screenshot comparing - pitivi 0.15 (videoscale bilinear) - pitivi GES (videoscale lanczos) - the same images (from a 2K source) resized by GIMP's Lanczos Perhaps there is something suboptimal going on with VideoScale when downscaling to small sizes? Or is it something that is done incorrectly in Pitivi's code? I tried playing with various values of the "sharpen", "shapness" and "envelope" properties, no luck. Code: http://git.gnome.org/browse/pitivi/tree/pitivi/timeline/thumbnailer.py#n457 (the bus-handler sets the element's props.method = 3)
Just in case I didn't mention it on IRC: this scaling method is known to be buggy/crashy at least in our unit tests, so chances are it's buggy/crashy in general too, so it might not be the best idea to use this in pitivi at this point.
videoscale has seen massive changes and improvements recently. Please test again with git master, and if you still feel the downscaling is worse than expected, re-open this bug and provide a test case (e.g. png/jpeg/video file to scale, plus target size and scaling method).