[ttml2] textCombine clarifications

cconcolato has just created a new issue for https://github.com/w3c/ttml2:

== textCombine clarifications ==
1) The spec says: 
"For the purpose of determining applicability of this style property, each character child of a p element is considered to be enclosed in an anonymous span." 
I know this is common text used plenty of times, but I find it nevertheless confusing and possibly wrong in this case. I checked how the following paragraphs would render in TTPE and the results are different.
<p xml:id="subtitle1" begin="0t" end="231600000t" region="region1" tts:extent="80.000% 30.000%" tts:position="center bottom 10vh">セミ<span tts:textCombine='all'>2</span><span tts:textCombine='all'>5</span>の鳴き声</p>

<p xml:id="subtitle1" begin="0t" end="231600000t" region="region1" tts:extent="80.000% 30.000%" tts:position="center bottom 10vh">セミ<span tts:textCombine='all'>25</span>の鳴き声</p>

I think we should change the text (throughout the spec) to say:
"For the purpose of determining applicability of this style property, the [construct anonymous spans] procedure applies." 
because the anonymous span construction procedure is clearer, creates on span per text node and removes unnecessary ones (nested spans).
This relates to #784. 

2) The spec says"
"If the specified value of this attribute is digits, then all affected characters should be combined if they are a sequence of a digits which length is equal to or less than a specified count, or two (2) if no count is specified."

This seems to be contradictory to the example:
<span tts:textCombine="digits">AB34</span>
Here all the affected characters do not form a sequence of digits. 
(Note: I tested in TTPE and it does not seem to be supported)

Also it is not clear what would happen if multiple sequences of digits would be present like in:
<span tts:textCombine="digits">AB34AB34</span>
or
<span tts:textCombine="digits 3">AB3434AB343434</span>

I think it should say:
"If the specified value of this attribute is digits, then among all affected characters, the longest sequences of consecutive digits of length L or less (where L is the specified count or two (2) if no count is specified) are formed; and within each such sequence the characters are combined."

3) CSS limits the range: "Integers outside the range 2-4 are invalid."
We should do the same.

4) The spec says:
"Combination applies after all affected characters have been processed as if a horizontal writing mode applied."
Are we clear that the horizontal mode only applies for combination and not for other features such as shear (See #784). 


Please view or discuss this issue at https://github.com/w3c/ttml2/issues/797 using your GitHub account

Received on Friday, 25 May 2018 17:05:16 UTC