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QTtext/3GPP tx3g notes

It's interesting that with 3GPP text (tx3g) movies, as with MPEG4 generally, we now have the ability to make movies that we can't fully verify anymore after export (i.e. no more wysiwyg). The movies sometimes don't conform to their pre-export originals. Conversely, the 3GPP spec also supports some things that QT can't do. I'm basing these comments on the 3GPP 5.1.0 spec, which I believe is the version QT is supporting in 6.3 (3GPP TS 26.234 V5.1.0, 2002-06).

Specifically:

1. For QT text movies, by default, text w/o line breaks is wrapped on import. On exporting to 3GPP text is never wrapped. This makes sense, given the many different languages & terminals 3GPP has to support, but breaks with previous QTtext behavior.

2. The option to export a QT movie to 3GPP 5.1 using the current size option is not supported for 3GPP text movies. That is, the dimensions continue to be derived from the tags in the text track, not from the QT movie from which it is being exported. 3GPP 5.1 export with current dimensions does work for a QT movie with video content. But for text movies, any export size other than the ones already defined in the tags is ignored (even the two 3GPP preset dimensions).

3. The 5.1 spec states that SVG in the "tiny" profile must be supported by terminals while "basic" profile support is optional. As far as I know, QT doesn't support SVG playback.
For more info on the SVG tiny and basic profiles: http://www.w3.org/TR/SVGMobile/

4. The 3GPP spec supports a number of real-time transitions (barWipe; irisWipe; clockWipe; snakeWipe; pushWipe; slideWipe; fade). AFAIK, you can't specify these in QT via a SMIL movie. I think (I'm not sure) that if you create a realtime transition between two tracks in QT and then export as 3GPP you'll get a rendered video track of the effect. (I haven't tried to import a SMIL movie into QT and then export it as 3GPP. I bet it would work since everything is converted to QT atoms as the lowest common denominator, right?).

5. Font substitution. To me at least, the spec is a bit vague on this issue (section D.8a.4). It says that substitutes for Latin-1 characters are highly recommended but unicode support may be limited to a single font. That makes sense, but its not clear to me what QT is doing. AFAIK, the only e.g. japanese font that will render in QT or 3GPP on OS-X is Osaka. The question is, what font is chosen on a Windows system & by what mechanism? QT's unicode support allows us to create text movies (QT or 3GPP) that can't be played back on some systems (i.e. US English versions of OS9 & Win98) and with no recourse to a default font to fall back on. This is also a break with the past, where we knew that our text movies would at least play back with 'some' font regardless of the operating system.

I guess all of this comes down to the fact that QT is not a 3GPP 'soft' terminal. You will never be able predict exactly what your 3GPP movies will look like w/o more info on the playback terminal. Since (with the possible exception of SVG) QT continues to be a superset of the 3GPP spec, perhaps this opens the door for some enterprising developer to create a 3GPP soft emulator entirely in QT?

Arno Bosse
Univ. of Chicago
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