[imsc] Maximum number of regions

All,

I remain concerned about the limit of maximum number of simultaneously
active regions in IMSC 1.

A colleague has scanned the BBC's archive of subtitles and discovered one
file that would require 4 regions, and about 0.01% of files require 3
regions.

I attach a screenshot from one of the 3 region files. It is admittedly
rather old and not representative of present day practices however when we
repeat the programme we reuse those original subtitles.

An additional use case for more regions is when the subtitle file contains
an accessible version of on-screen text ("object based broadcasting"), for
example a combination of the hard of hearing or translation subtitles,
clock, news ticker, "Breaking News" caption, contributor name and job
title etc., as currently visible on rolling news channels e.g. BBC News.
In that scenario it would be easy to exceed the 4 region limit - I got to
5 or 6 just then without any special cases. Moving to this method of
presentation is in my opinion both an area of research and a direction of
travel.

My conclusion from looking at this is that although the current incidence
of subtitles needing 4 regions is extremely low, at least in UK English
language BBC content, the "safety margin" present with a 4 region limit is
too small and it is likely that a 4 region limit on IMSC is too
constraining of future possible uses and presentation styles.

My preferred proposal for addressing this is simply to remove the
simultaneous region limit altogether, since I do not believe there's a
strong implementation case for having it, especially when rendering
complexity is already tackled by the HRM.

My less preferred proposal, if there is a genuine reason for needing such
a limit, is to increase the value from 4 to 10, to allow for some
additional safety margin.

Kind regards,

Nigel











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