At least according to this developer:
http://www.avsforum.com/avs-vb/showthread.php?s=&threadid=93797

Here's why XBOX games don't support 1080i
The biggest reason isn't performance, but memory footprint. The XBOX has 64 MB of unified memory. Any leading-edge game will fill all the memory with sounds, textures, frame buffer, etc. There isn't enough memory left for the high-def modes.

For example:

Each pixel displayed requires (at the minimum):

2 bytes for the front buffer (16-bit color buffer)
2 bytes for the back buffer
2 bytes for the z buffer (16-bit zbuffer)
= 6 bytes per pixel
(BTW, for 32-bit color and zbuffer, that 12 bytes per pixel!)

With a 480p, this means the screens use

640x480x6 = ~1.8 megabytes

for 1080i, this means the screens use

1920x1080x6 = ~12.4 megabytes

That's a _huge_ difference. I tested a game that could run at 1080i. It looked unbelievable on the XBOX, but that version of the game wouldn't run on retail xboxes.

BTW, it is theoretically possible to cut the footprint in half by using field mode instead of frame mode. In practice, it is incredibly difficult to program a game to use field mode since that means each field must be completed in less than 1/30th a second to keep from seeing artifacts. Doing this is difficult even at 480i, let alone 1080i.

It was NASCAR Heat. I was the leader of the team that worked on it. When we were testing the HDTV pack, we made special versions of the game for fun. One ran at 720p and the other ran at 1080i. They both looked beautiful, even though we had to turn off antialiasing to get a reasonable frame-rate. I hooked it up to my LT150 projector system. It was stunning @ 16:9/1080i. I was really impressed with the quality of the signal from the XBOX. Rock solid and great color fidelity. It really is a shame that the DVD player doesn't support the progressive modes. It really was spectacular, even on my system that doesn't have enough resolution for true 1080i.

Richard G

This is really not a surprise. For all the HDTV pack hoopla I didn't expect to see very many games actually running at that resolution. There just aren't enough HDTV sets out there for developers to bother trying to make the same game in two seperate resolutions. Like you said, if it's going to run at 1080i you're going to have to remove some art assets from memory. So the 1080i version would have less textures or fewer objects on screen or lower fidelity sound effects than the 480p version. It'd be a pain in the ass. And you'd have to playtest the game at both resolutions to make sure it looks right both ways, and there are no performance issues...

Brad Grenz

Hmm, okay, but doesn't DOA3 run at 1080i? Microsoft says that it does. If that's what you mean by "less textures and fewer objects" well, I'm not terribly worried about the future of the resolution. =)

-Vederman

DOA3 runs at 480p. If MS says otherwise they're full of crap. But that's pretty much par for the course with them.

By "runs at 1080i" they might mean "is compatible with 1080i." Marketing speak for "the video encoding chip scales it up." =)

HDTV is still kind of a big deal for Xbox. Most HDTVs I've seen don't support 720p, but 720i should look a LOT better than a regular NTSC TV, even a really good one. And since most console games are locked at either 30 or 60 frames per second, using a field mode for 720i shouldn't be too big a deal (hell, nearly every PS2 game is in a field mode so the frame buffer fits in the embedded DRAM) - and the memory footprint would be pretty small. Smaller than 480p, even.

I wonder if most games would look better running at a native 480p, or native 720i? Hmm...