TTML: wrong attribute "imagetype", camel case expected.

[Canta]

System info

Operating System: Ubuntu 20.04
Shaka Packager Version: f0a52cb

Issue and steps to reproduce the problem

While testing some DVBSUB-to-TTML conversions with shaka packager, I could see at playback time the following message using DASH reference player:
Error : SMPTE 2052-1:2013 defines the attribute name as "imageType" and does not define "imagetype"

Checking out some text segment/chunk, I can see the attrubute (note the lowercase t):

$ cat text_5.mp4 
(...)
  <smpte:image imagetype="PNG" encoding="Base64" xml:id="img_3">
(...)

Packager Command:

packager \
'in=file4.ts,stream=0,init_segment=1080p_init.mp4,segment_template=1080p_$Number$.m4s' \
'in=file4.ts,stream=1,skip_encryption=1,init_segment=audio_init.mp4,segment_template=audio_$Number$.m4s,language=POL' \
'in=file4.ts,stream=2,skip_encryption=1,format=ttml+mp4,init_segment=text_init.mp4,segment_template=text_$Number$.mp4,language=ENG' \
--mpd_output test.mpd

What is the expected result?:
The same, but with that attribute in camel case format, as expected by the spec (which I didn't read: I'm trusting the player here).

What happens instead?
The error quoted above.

Looks like an easy thing to fix.

[kqyang]

Thanks for the bug report. Would you like to submit a PR to fix it?

@TheModMaker FYI.

[Canta]

Yes, no problem. I'm struggling since about an hour ago to have my github fork updated so I could send a pull request. As soon as I fix that, will push the quick fix for this.

[kqyang]

Great. Thanks.

[Canta]

Careful: this seems to also affect shaka player.

I was doing some tests today, and shaka player doesn't show the TTML images after the change in this ticket.
Take a look at this:
https://github.com/google/shaka-player/blob/23fe7128494d0678d6fd8e3bc951f303907ec5cf/lib/text/ttml_text_parser.js#L574

I believe the right way of handling this is changing both packager and player, as otherwise legit TTMLs would not play correctly on shaka player, and shaka packager generated files would not play correctly on other TTML-compatible players.

[Canta]

Here's the spec mentioned in the first comment: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=7291854
Section 5.7.3 (page 12) describes the smtpe:image tag with an imageType attribute (note the capital T).

[kqyang]

@joeyparrish @TheModMaker FYI.

@Canta You may want to file a bug to Shaka Player and discuss the player issue there.

[joeyparrish]

We will fix Shaka Player to accept both versions, so existing Packager-generated content with the lowercase attribute name will still function. Thanks!