Fix #3421 - accept non-compliant imagetype attribute name in SMPTE-TT image

[davemevans]

Turns out #3332 broke one of the test assets (https://livesim.dashif.org/dash/vod/testpic_2s/img_subs.mpd) so was easy to verify the fix.

[nigelmegitt]

As a reference player should dash.js be supporting invalid input data like this? SMPTE 2052-1:2013 defines the attribute name as imageType and does not define imagetype at all. XML attribute names are case sensitive. Seems like the correct fix for this breaking a test asset is to fix the test asset, not make the code support something that's not in the specification.

Of course for use not as a reference player, this kind of forgiving behaviour is probably what folk generally want, so the right option here depends on the target requirements for the player.

[dsilhavy]

I think this is a general question, and it depends on the situation. dash.js is used in production by various companies so we should design the player as robust as possible. Throughout the code we already have some fixes for non standard compliant content.

In this case the fix was simple and has no negative influence on any other part of the code. In addition, it does not add complexity. So I think it is fine to add this. We might consider giving a warning in the console so developers know that their content is not standard compliant.

[davemevans]

This isn't just a test asset issue: there is production content in the wild that uses the incorrect case for this attribute - see the original ticket (#3421). FWIW, the player erroneously supported only imagetype until August, and no one noticed for about five years ...

A warning would be a sensible addition, as would fixing the test asset 😉

[TobbeEdgeware]

I've updated the test asset https://livesim.dashif.org/dash/vod/testpic_2s/img_subs.mpd with the correct spelling, as well as some other variants on the same site.