Support of fontSize cellResolution units in TTML subtitles

[glhvta]

Have you read the FAQ and checked for duplicate open issues?
Yes

What version of Shaka Player are you using?
v2.5.9

Please ask your question
Hi,

According to the TTML spec fontSize
https://www.w3.org/TR/ttml1/#style-attribute-fontSize
fontSize value can be percentage or scalar in the next units
https://www.w3.org/TR/ttml1/#style-value-length

• "px"
• "em"
• "c"

We have ttml subs with the next styles

<styling>
   <style xml:id="s0" tts:color="rgba(255,255,255,255)" tts:fontSize="0.80c"  />
</styling>

tts:fontSize="0.80c" is in cellResolution units

We are using the default shaka.text.TtmlTextParser
And seems like fontSize will not be provided to the text displayer in this case, as now default TtmlTextParser does not support cellResolution units for the font size.

https://github.com/google/shaka-player/blob/697a6ff785046dff3b7dea1d44fc8fc87cee925b/lib/text/ttml_text_parser.js#L531-L535

https://github.com/google/shaka-player/blob/697a6ff785046dff3b7dea1d44fc8fc87cee925b/lib/text/ttml_text_parser.js#L971-L977

Is it possible to add the support of fontSize cellResolution units to TtmlTextParser?

Thanks!

[joeyparrish]

Sounds reasonable. Do you have any sample TTML we could use for testing and development?

[glhvta]

yes, in this archive, you can find samples TTML with fontSize attribute in percentages and in cellResolution unit.

ttml-captions.zip

[joeyparrish]

Perfect. Thanks!