It's definitely a shame about mplayer's RGB colorspace conversion issues for screenshots, since it's a rather nice tool otherwise.
I've been meaning to make a BD screenshot tool for quite some time, either as a new feature of BDInfo or as a separate tool, but I just haven't had the time to dedicate to it. I don't suppose it would be too hard to come up with a patch to mplayer's screenshot module, since I use essentially the same underlying software (i.e. ffmpeg's libavcodec) for my own workflow. However, in lieue of something nice to use, here is a very, very basic command-line tool (w/ source) that I just slapped together out of some of the code that I use which dumps all of the I-frames of an M2TS to PNG with a BT.709 YUV->RGB colorspace conversion at 0-255 levels through an intermediate YUV 4:4:4 chroma upscale which seems to mostly avoid chromatic aberrations in the resulting RGB:http://media.cinemasquid.com/blu-ray...ns.v.0.0.1.zip
Basic command-line usage from the project folder:
You may or may not find this useful. The output PNG have only medium compression for timesaving considerations during the extraction process, so it's advisable to losslessly recompress the keepers again in PNG with a more aggressive setting (or make them into high-quality JPEG which can be quite satisfactory). It does not deinterlace, so should not be used with 1080i50/60 titles. Also, it does not work with encrypted files, so SlySoft AnyDVD HD or equivalent needs to be running in the background for off-the-shelf discs.
FYI, the output I-frame-only PNGs for your average 2 hour movie can consume between 20-30GB of disc space.