Action-201 and Issue-225

In the absence of automated change notifications from the action and issue tracker, here is a manual notification that I've updated Issue-225<http://www.w3.org/AudioVideo/TT/tracker/issues/225> as per my Action-201 from yesterday. The update to Issue-225 is:


As per action-201<http://www.w3.org/AudioVideo/TT/tracker/actions/201> [1], I've looked to see if pixel aspect ratio would or should affect the meaning of vmin and vmax.

[1] https://www.w3.org/AudioVideo/TT/tracker/actions/201


It is possible to construct an example where taking pixel aspect ratio in to account to calculate actual dimensions would give a different result from omitting it. However the root definition of vmin and vmax, in [2], is unclear or at best non-specific about the intended behaviour. It appears to depend on how the viewport size itself was defined, i.e. if in physical units then vmin and vmax should relate to those physical sizes but if in pixels then vmin and vmax should relate to the pixel dimensions.

[2] http://www.w3.org/TR/css3-values/#viewport-relative-lengths


In TTML we only define a logical coordinate plane but we do allow ttp:pixelAspectRatio to be defined. I think there is very likely a use case for referring to the minimum or maximum size after pixelAspectRatio conversion to relate these terms to 'real' sizes. Certainly this should be considered and made explicit in the definition of the attributes.

For reference, here's one contrived example in which vmin and vmax have different values depending on whether the extent is multiplied by the pixelAspectRatio before the min/max decision or after it. Take an extent of 400w x 300h and a pixel aspect ratio of "1 1.5". Before conversion vmin=300/100 and vmax=400/100. Taking pixelAspectRatio into account, the min/max choice is between 400x1=400 and 300x1.5=450, and the decision is made the other way, so, after returning to our extent coordinate system, vmin=400/100 whereas vmax=300/100.

The question comes down to whether vmin and vmax apply numerically to our extent or is intended to compare actual sizes.

Nigel Megitt, 6 Sep 2013, 09:44:24


I've also updated Action-201 and set it to Pending Review.

Kind regards,

Nigel

--

Nigel Megitt
Lead Technologist, BBC Technology, Distribution & Archives
Telephone: +44 (0)208 0082360
BC4 A3 Broadcast Centre, Media Village, 201 Wood Lane, London W12 7TP




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