DivX used to be.. technically a DVD *rental* format at Circuit City. (One of the old alternatives to Best Buy, or Fry's Electronics)
After that fiasco, DivX is just a different version of an MPEG 4 codec. Here's where it gets tricky. Those MP3 files we all listen to? That's MPEG3. But that's not video.. although the underlying ideal is the same. Encode something THREE different times to remove all the unnecessary stuff. Ya know.. high frequencies which we cant hear.. possibly make it a VBR (Variable Bitrate file) so that we can put the quality where it NEEDS to be. Like.. if you're listening to a music file.. should the same amount of information pumping into your hardware be the exact same whether or not it's silence? or bass... or people singing? That's the difference between variable and constant bitrate. But.. anyway.
DVD movies.. the kind that's been out forever.. is MPEG2. DVD is specified as a 720x480 resolution with about a maximum throughput of 10 mbps although.. most scenes rarely reach 6. Superbit releases excluded.
Anyway, DivX and XviD (Divx backwards, duh) are only methods of encoding. There is no set resolution, nor is there a true quality constraint.. you can view an ugly DivX movie or a beautiful DivX movie.. it depends on who encoded it.
Filesize is the only real indicator of quality regarding Divx movies. Although it's also VERY possible to have an awful movie that takes up TONS of space. Like a camcorder version of a movie taken at the theater. Ya know the ones where people walk around, talk and scream during the pirated movie?
Also, a DivX file is only a divx file by it's method of decoding. You'll almost always see the file extension as .AVI for such things.
So while your standard MPEG4/XviD movies will have comparable quality to a DVD when they encode to the standard 2CD size.. (700 Megabytes Times 2 CD's) it's not always the same with the audio tracks. Are they encoding to full 5.1 surround? Stereo?
What about audio quality? 128 bitrate MP3?
96 kHz AC3? Is your audio set to surround? Can your computer's soundcard support that codec? Do you even have the codec package required or is it being routed through some official microsoft filter that doesnt decode/output properly?
Generally.. HD-DVD releases are painfree because the HDDVD player does all the work for you.. which is honestly very little work because movie makers have a set specification to work with. Encode to THIS resolution, use THAT audio.. and so forth.
DivX and Xvid are just codecs. Software for a PC. (Although many DVD players can read DivX files these days)
HD-DVD is just a standard dictated by the rich guys at the movie companies in Los Angeles.
