sd_lavc: support rendering bitmap subtitles with libaribcaption

[vroad]

libaribcaption is a library for decoding / rendering ARIB STD-B24 based TV broadcast captions (subtitles). The library supports ouputting decoded subtitles in two ways: ASS subtitles or bitmap subtitles. Building recent version of ffmpeg with libaribcaption also enables ARIB subtitles rendering on mpv.

Currently mpv only supports rendering ARIB subtitles with ASS subtitles driver when libaribcaption codec is in use, which renders poorly compared to bitmap subtitles driver. ASS subtitles lack black transparent background that makes subtitles more readable. Subtitles decoded with libaribcaption render best when rendered with bitmap subtitles driver.

New mpv commandline option: sd-lavc-o

On ffplay you can use -sub_type=bitmap option to render ARIB subtitles as bitmap.
There is currenly no way to set the same option to subtitle decoders on mpv unfortunately, so I added one. This would also allow configuring other libaribcaption options like font from mpv.

libaribcaption support

mpv only uses bitmap subtitles driver only for codecs listed here:

mpv/sub/sd_lavc.c

Lines 79 to 89 in 6234a70

	// Supported codecs must be known to decode to paletted bitmaps
	switch (cid) {
	case AV_CODEC_ID_DVB_SUBTITLE:
	case AV_CODEC_ID_DVB_TELETEXT:
	case AV_CODEC_ID_HDMV_PGS_SUBTITLE:
	case AV_CODEC_ID_XSUB:
	case AV_CODEC_ID_DVD_SUBTITLE:
	break;
	default:
	return -1;
	}

To render subtitles correctly when sub_type is set to bitmap, I added some code that choose subtitles driver based on the value of this option.
Since ffmpeg's libaribcaption codec defaults to ASS subtitles, just adding AV_CODEC_ID_ARIB_CAPTION to the list breaks rendering, because mpv expects bitmap subtitles, and the codec still outputs ASS subtitles.

Building

• ffmpeg must be built with --enable-libaribcaption. I used this revision: https://github.com/FFmpeg/FFmpeg/tree/2f8690c5d450b984640bd27dd7fd3a96d77ecc6b
• mpv must be built with -Ddvbin=enabled if you want to test ASS subtitles support too.
• I built ffmpeg with forked libaribcaption becuase it does not install correctly on NixOS

Running

Tested on NixOS 22.11. Without --cache=no, subtitles does not show up until I seek (Probably because of this issue): #9646

libaribb24 probably has the same problem as libaribcaption and this PR may fix it, I just haven't tested with libaribb24 yet.

Do I need to send m2ts files to mpv developers for testing?

Rendering subtitles with bitmap subtitles driver:

mpv --deinterlace \
--sd-lavc-o-add=sub_type=bitmap \
--sd-lavc-o-add=font='Rounded M+ 1m for ARIB' \
--cache=no \
--sid=1 \
/path/to/video.m2ts

Rendering subtitles with ASS subtitles driver (Requires mpv to be built with -Ddvbin=enabled):

mpv --deinterlace \
--sd-lavc-o=sub_type=ass \
--cache=no \
--sid=1 \
/path/to/video.m2ts

mpv defaults to ASS subtitles if sub_type is not set:

mpv --deinterlace \
--cache=no \
--sid=1 \
/path/to/video.m2ts

[vroad]

I followed @dyphire 's suggestion and updated PR branch, but his comment is gone? 🤔

[dyphire]

I followed @dyphire 's suggestion and updated PR branch, but his comment is gone? 🤔

Ah, that's just my personal opinion, and later I thought it would be up to the maintainers to review whether this behavior change was reasonable.

[vroad]

@dyphire
I changed my mind, mpv should follow libraries' default as you said.

VLC exposes libaribcaption (and libaribb24?)'s option in easy-to-understand GUI, mpv's GUI/CLI interface does not.
Instead, mpv's CLI interface exposes libaribcaption options directly via sd-lavc-o option. Following ffmpeg's default probably make it easy to understand mpv's behavior for users, I guess.

I removed the code for changing default value for sub_type to bitmap from PR. If everyone thinks that changing default value for sub_type is a good idea, we can do it later with another PR.

[Dudemanguy]

@vroad: Not sure if you're still interested in this but coincidentally 02a20b4 just added --sub-lavc-o so if you want to pick this up from there that would be appreciated.

[kasper93]

Similar change would be needed for AV_CODEC_ID_DVB_TELETEXT to fix #13471

[kasper93]

Superseded by #13752

I've made it work also for teletext and checked actual context defaults too, not only our options.