An analysis of 360 GPU (Xenos)

HarakoMeshi · 2006-11-26 09:02:04

This is my rant about the Xbox 360 GPU http://dpad.gotfrag.com/portal/story/35372/?cpage=2#comment_6047197 (in the comments section). If someone knows some technical details that I don't about the 360 please let me know. Based on the public information about the 360 Xenos GPU I have made my deductions. I am an experienced developer in 3D graphics on consoles and I have tried to make my calculations accurate but I may have made a mistake if there is some mystery of 360 I do not understand.The Xenos GPU has an unprecendented bandwidth to it's 10MB of EDRAM of 256MB/s. If I understand correctly this peak bandwidth is actually only available when dong 4xMSAA. The actual writing bandwidth from the GPU is 32GB/s. This number is then multiplied by 8 to account for 4XMSAA times 2 for read+write operations. Now if we consider the size of a full picture at 720p resolution (1280x720) with 8 bytes per pixel (color + Z) it almost fills the 10MB edram (over 7MB). In order to render that same picture with 4xMSAA four times the memory is required (over 28MB). This is the dilemma for the Xenos. In order to render such HD picture with AA it must do it in sections - in this case 3 sections. This slicing is where Xenos pays the price. The developer must jump through hoops to render their scenes multiple times (actually I believe the Xbox 360 APIs take care of some of the work), but all this rendering multiple times is not free. All the geometry processing is duplicated, all the GPU state changes & texture changes are duplicated and so on. The only parts that are not duplicated are the pixels. So this will eat into the Xenos' edge over the RSX. In fact if whole scenes must be rendered 3 times we find that the effectove Xenos triangle setup rate now becomes lower than the RSX. So the answer is that most developers probably won't render full 720p with 4XAA on the 360. They may drop below 720p or they may drop the AA. 720p with 2XAA still requires 2 passes. A possible resolution achievable in one pass could be 1024x576 with 2xAA. Anything higher would require multiple passes. Instantly we see that anything using 4XAA is overbooking the size of the EDRAM, while using anything less than 4XAA is underusing the EDRAM bandwidth. In fact the Xenos is not even equipped to render 480p with 4XAA in a single pass (requires 10.5MB). A slight design flaw there. The RSX can address the full 512MB of the PS3 so it has no such issues with frame buffer size. So you see there is a tradeoff with the 360 GPU. You can render true 720p at a rate perhaps comparable to the RSX or slower. Or you can render 720p without any AA, or 60% below 720p with only 2xAA - really fast. This is the contradiction of the Xenos' design: so much power but a limited space to use it in. For the Xenos design to have been perfectly suited to the 360, over 28MB of EDRAM would have been required. Taking things in context, these days consumers complain if there is no AA. And also in context I think consumers would be a bit annoyed to find out that 360 games are not truly 720p. So, developers have little choice but to please consumers with HD and AA and sacrifice their Xenos performance to do slicing & dicing. With this in mind, do not expect the 360 to beat the PS3 in graphics terms unless the 360 game is actually well below 720p. Just some food for thought, in order to render 1080p with 4X AA would require 7 passes on the Xenos, so even if the Xenos setup rate was 4 times faster than the RSX it would still be the underdog. While on the cover it seemed the Xenos would have been better at doing 1080p than the RSX, things are not what they appear when considering all factors.

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trasherhead

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#101 trasherhead

Member since 2005 • 3058 Posts

how can the RSX access 512 mb of memory when it only have 256mb of memory. the 360 has 512 of shared memory so only its GPU should possibly access 512mb Nugtoka

Because the PS3 has 512mb ram split into a CPU part and a GPU part, and the RSX can access all of it with while the cell can only access its 256mb ram.

17 years ago

Truth_Hurts_U

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#102 Truth_Hurts_U

Member since 2006 • 9703 Posts

To many things wrong with this... I could spend several hours correcting it and showing links. But I don't feel like wasting time. In another few weeks another post will be about this same subject.

Just look at the games. 360 > PS3

Look up articals on PC sites dating from around E305.

17 years ago

DaddyDC650

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#103 DaddyDC650

Member since 2007 • 1241 Posts

If the 360's GPU is so advanced and the RSX "outdated", why is it that games like FNR3, THPS, COD3 almost look exactly the same on the PS3 even though they're launch titles? Even IGN's own editor-in-chief of the Xbox 360 section has admitted that the PS3 is in fact more powerful than the 360. He must speak with developers all the time otherwise, why would he mention it? I can find the link if anyone would like.

17 years ago

bluebrad1974

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#104 bluebrad1974

Member since 2005 • 5162 Posts

If the 360's GPU is so advanced and the RSX "outdated", why is it that games like FNR3, THPS, COD3 almost look exactly the same on the PS3 even though they're launch titles? Think about this for awhile lemmings.DaddyDC650

There are 360 launch games that shame the whole PS3 line up.

17 years ago

DaddyDC650

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#105 DaddyDC650

Member since 2007 • 1241 Posts

[QUOTE="DaddyDC650"]If the 360's GPU is so advanced and the RSX "outdated", why is it that games like FNR3, THPS, COD3 almost look exactly the same on the PS3 even though they're launch titles? Think about this for awhile lemmings.bluebrad1974

There are 360 launch games that shame the whole PS3 line up.

LoL! Umm, no. For fun, I created a list of the 360's launch day titles. NONE of them match up graphically with a lil game called Resistance which was launched with the PS3 as well. Btw, I got the 360's launch day title list from Xbox.com. http://www.xbox.com/en-US/games/xbox360launchgames.htm

1. Kameo™: Elements of Power

2. Perfect Dark Zero

3. PGR 3

4. COD 2

5. NFS: Most Wanted

6. King Kong

7. Madden NFL 06

8. Gun

9. Condemned

10. NBA 2k6

11. Ridge Racer 6

12. Quake 3

13. Tiger Woods PGA Tour 06

14. Amped 3

15. Fifa 06

16. NHL 2k6

17. NBA Live 06

17 years ago

bluebrad1974

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#106 bluebrad1974

Member since 2005 • 5162 Posts

[QUOTE="bluebrad1974"][QUOTE="DaddyDC650"]If the 360's GPU is so advanced and the RSX "outdated", why is it that games like FNR3, THPS, COD3 almost look exactly the same on the PS3 even though they're launch titles? Think about this for awhile lemmings.DaddyDC650

There are 360 launch games that shame the whole PS3 line up.

LoL! Umm, no.

Lol, Umm, Yes.

17 years ago

sxs_alex

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#107 sxs_alex

Member since 2007 • 933 Posts

[QUOTE="DaddyDC650"]If the 360's GPU is so advanced and the RSX "outdated", why is it that games like FNR3, THPS, COD3 almost look exactly the same on the PS3 even though they're launch titles? Think about this for awhile lemmings.bluebrad1974

There are 360 launch games that shame the whole PS3 line up.

fanboys like this guy is embarrassing to the game community & SW. go to 360 forum and create a topic about how wonderful your 360 is that way you'll feel better about yourself than being in here.

17 years ago

DaddyDC650

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#108 DaddyDC650

Member since 2007 • 1241 Posts

Thanks for proving that Resistance looks better than all of those games. =) Btw, you posted a pic of Cod 2 for the PC. LoL! Here's da link just 4 you! http://media.pc.ign.com/media/620/620769/img_2727083.html

17 years ago

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