== Potential conflict with respect to pixel aspect ratio. ==
IMSC1 permits the simultaneous specification of both ittp:aspectRatio
(as an expression of DAR) and tts:extent (as an expression of SAR). If
the ratios of these two parameters are different, then the effective
pixel aspect ratio (PAR) is not 1:1. However, IMSC1 prohibits the
presence of ttp:pixelAspectRatio, and, further, TTML1 requires that in
the absence of an ttp:pixelAspectRatio attribute, PAR is 1:1. As a
consequence, IMSC1 allows a PAR not equal to 1:1 while making use of
TTML1s default PAR of 1:1. Since both cannot be true at once, we have
a semantic conflict in the IMSC1 specification.
For example, see EXAMPLE 2 at , where we have:
ittp:aspectRatio="4 3" and tts:extent="400px 300px"
Now in this case, both ratios are the same, so PAR is indeed 1:1.
However, nothing prevents the author from specifying tts:extent="300px
400px", in which case we have PAR (= DAR/SAR = 4/3 ÷ 3/4) of 16:9
(1.777..). But since we can't specify ttp:pixelAspectRatio='16 9', we
have a defaulted PAR (from TTML1) of 1:1.