Mplayerosx

I notice that by default there's a flag for:

-lavdopts fast:skiploopfilter=all

This has a negative visual effect when playing back h.264 files making the quality look worse.

Likely this option was chosen for it's performance benefits, so at minimum I think there should be a preference checkbox regarding it and default it to the higher quality output option.

I don't know if nonref is the best option, but I currently add the following to reverse the above:

-lavdopts fast:skiploopfilter=nonref

Here's the details from the manual:

skiploopfilter=<skipvalue> (H.264 only)
Skips the loop filter (AKA deblocking) during H.264 de-coding. Since the filtered frame is supposed to be used as reference for decoding dependent frames this has a worse effect on quality than not doing deblocking on e.g. MPEG-2 video. But at least for high bitrate HDTV this provides a big speedup with no visible quality loss.

<skipvalue> can be either one of the following:
none: Never skip.
default: Skip useless processing steps (e.g. 0 size packets in AVI).
nonref: Skip frames that are not referenced (i.e. not used for decoding other frames, the error cannot "build up").
bidir: Skip B-Frames.
nonkey: Skip all frames except keyframes.
all: Skip all frames.