[mythtv] Improving closed captions/subtitles

faginbagin mythtv at hbuus.com
Fri Jan 31 23:45:52 UTC 2014

I'd like to improve the appearance and legibility of 608/708 captions in fixes/0.27. Some of the problems I see are:

- Default font size is smaller than should be necessary to fit 15 lines in a 90% safe area. I estimate the default FontMono pixel size could be increased from 32 to 38 for a 1280x720 display and I think I have a relatively simple algorithm to compute a good font pixel size for a variety of algorithms.

- I suspect support for subtitle zoom was added because of the first problem. Maybe it can be eliminated at some point?

- I suspect fudge factors like LINE_SPACING and PAD_HEIGHT were added for the same reason there's subtitle zoom and to work around problems created by subtitle zoom. Same goes for word wrap.

- Captions are shifted too far to the left. As a result, it is hard to tell who is speaking when two people are on screen. I believe they should be positioned to center 32 characters per line when the video is 4:3 aspect and 42 characters per line when the video is 16:9, based on what I've gleaned about the 708 standard.

I've tinkered with osd_subtitle.xml and I don't see how I can fix these problems just by tweaking the theme file. I see that the code involved has diverged in master, so I have backported a number of files in my development environment in the hopes that my work might eventually be considered for inclusion in master. I'm sure I can improve the display of 608/708 captions while simplifying the code at the same time, and I believe I know how to test support for SRT subtitles and make sure I don't break DVD subtitles (I've got both NTSC and PAL DVDs). I am worried that I could break teletext subtitles unless I know more about them and/or have test samples.

My questions are:

- Am I off base w.r.t zoom, fudge factors and word wrap? If so, please educate me.

- Would there be interest in any work I do to improve the display of 608/708 captions?

- If so, can someone direct me to some test videos for teletext, and perhaps advise me on unit test cases I should pay special attention to or haven't mentioned above?

Thanks in advance,

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