[mgh]

Have been doing some trials. Problem occurs only with videos which contain large dark areas or large bright areas-a known problem for encoders;maybe also problem for some decoders sometimes.
Basically, I am trying it with VCDs and my initial attempts at TV captures with bad encoding/capture-to correct color balance and levels. With luma disabled, the color balance correction has worked with everything I have tried. With luma enabled it works with all vids except of the type I have mentioned.
DVDs do not have the problem-these days i just load in avi.NET and encode at default settings to get standalone compatible divx/xvid files or use DVDShrink to get the size down to fit single layer DVD-using error correction. Either way is totally painless.
Vcds tend to have blockiness and color/levels problem-blockiness because of limited bitrate and overbright or overdark to reduce bitrate requirement. DGIndex/DGDecode is excellent at deblocking and decoding. Your filter takes care of color problems with all and levels with most vids painlessly.
The problematic videos are best tackled to my eye by the LUMAYV12 filter built into DGIndex/DGDecode-the lumoff parameter is the most useful.

will try out your suggestion and let you know.
Thanx