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There may be rounding errors in the color conversions, but the chroma up- and downscaling can be lossless depending on the algorithm. If I use pointresize(), it's lossless. So it does not have to result in any blurriness.
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Are you using ConvertToRGB(matrix="Rec601", interlaced=false, chromaresample="point") , (and back to YV12 using same thing) ?
or using some other chroma resampling method?
If you have time, here is a test video from a DVD menu for you . (The goal was to redo the text animating on, but the reason is not important). It's really an almost worst case scenario, because of thin red text.
Using "normal" ConvertToRGB.ConvertToYV12 routine (and then ConvertToRGB for the screenshot) . It's similar result in other programs which use slightly different chroma sampling methods
Comparisons:
Original YV12 (and RGB for screenshot)
http://i40.tinypic.com/1zofcjp.png
YV12=>RGB=>YV12 (and RGB for screenshot)
http://i40.tinypic.com/348ivbo.png
original .m2v
http://www.mediafire.com/?52xp6d7p5881fvo