So during the weekend, I decided to try writing a U3D file viewer. I had high hopes for U3D because U3D seemed to avoid many of the weaknesses of the competing X3D specification. After working with U3D for a bit though, I've concluded that the U3D specification is half-baked. It's almost as if the 33 members of the 3D Industry Forum performed almost no due diligence in reviewing the specification. Personally, I think the spec is unworkable and unimplementable. This is a real shame because Adobe went and added U3D support to Acrobat, meaning that a lot of marketing muscle and mindshare has just been put behind a losing proposition. Even if a better spec were designed in the future, since it wouldn't be supported in Acrobat like U3D, it would likely fail.
I can ignore minor annoying aspects like the fact that the spec itself wasn't well-designed, treating things like arrays in several different ways throughout the spec and going out of its way to avoid simplifying the descriptions of the fields, and such.
But the killer of the spec is its ridiculously overbearing bit encoding scheme. I just cut & paste the code from the specification and translated it to Java, and it's just awful. The code uses something like over 30 lines of code to read an uncompressed 8-bit integer. And the encoding/compression scheme has some characteristics that simply ruin the rest of the specification.
For example, the compression scheme sometimes requires you to feed in parameters of live data structures in order to read subsequent data. This means that you can't simply read the data into data structures and then process it later. You literally have to create a live mesh data structure, manipulate the mesh as data is being read in, and feed the resulting properties of the mesh back into the compression scheme in order to read the next bit of data.
And the encoding scheme has this weird property whereby previous data values read can influence future data values read. So even though the file format is supposedly tag/block-based (meaning that you should be able to process only the types of data that you are interested in and skip the rest), you still need to read, process, and understand every single bit of data in a file in order to read later bits of the file because otherwise the decoding state won't be properly configured when you get there. This even holds true for uncompressed data, which is just wonky. I wouldn't have believed it if I couldn't see it right there in the code that I cut & paste from the spec.
After having spent something like 5+ hours on trying to read in a CLOD Progressive Mesh Continuation Block from a file and failing miserably, I can only conclude that it's simply not worth the trouble to implement. I am now very suspicious about why the 3DIF still hasn't released a reference encoder/decoder three months after the release of the specification. In any case, as far as I'm concerned, U3D is a write-off. It might gain a little bit of mindshare because of its Acrobat-support, but I think it will eventually die off because the only way to create U3D files is to use the rumoured Intel SDK (since it's essentially impossible to write one's own encoder), and most 3D tools developers will decide to ignore it.