[mythtv] bobdeint observation

[Andy McMullan]

First of all, thanks to those responsible for getting the bobdeint code 
into myth, as it's made a huge difference to the playback quality on my 
system (bt8x8/PAL/Nvidia Tv-out). Finally, recordings really look like 
the original broadcasts.

There's just one minor issue - the picture seems to 'bob' up and down 
slightly. It's not always noticeable - just depends on the program. It's 
particularly noticeable where there are graphics with horizontal 
straight lines.

 From what I've read, this is a well-established side-effect of bob 
deinterlacing. For example, it's discussed here - 
http://www.100fps.com/why_bobbing.htm.

Based on what I've read, I tried some crude experiments with the code, 
and I got the best results by making the following change to 
videoout_xv.cpp:

XvShmPutImage(data->XJ_disp, xv_port, data->XJ_curwin, data->XJ_gc,
               image, src_x, src_y, src_w, src_h, dispxoff,
===>          (t == kScan_Intr2ndField) ? dispyoff+1 : dispyoff,
               dispwoff, disphoff, False);

Basically all this does is draw the second field one pixel lower than 
the first field. The effect is pretty subtle, but I'm pretty sure it 
gives a better result.

I'm not suggesting this change be made in cvs - merely that if other 
people notice the bob/dance effect, they try this change and see if it 
helps.

Andy