Architecture wise 360s Xenos GPU is a generation ahead of PS3s RSX. But in terms of actualy performance there about equal. The most important thing to look at when comparing GPUs are there "features" in other words what graphical efects can 1 GPU do over another.
In PC terms this is very easy, compare the features of a DirextX8 level GPU to a DirextX9 level GPU. The difference betwen the 2 is staggering. DirectX8 cant do HDR, Parrallex mapping, soft shadows, it also has weaker shaders meaning the effects are low quality. Now from looking at RSX and Xenos there are NO effects that 1 can do over another, RSX cant do any magic voodoo effects that Xenos cant do and vica versa.
The only thing that both GPUs differ on is there API, RSX runs on a super tweaked PS3 version of OpenGL-ES 2.0 and 360 runs on slightly tweaked DirextX9. This is were the GPUs start to differ, OpenGL has always given developers better access to a GPUs features and functions then DirectX ( Hence why most movie studios render programs are OpenGL based ) dirextX doesnt allow developers to access ALL of a GPU features and functions as is has to be able to accomodate thousands of PC combinations. This is a big problem on the PC platform as it meens that some ultra high end GPU is'nt useing all of its feature set, in other words that GPU is being WASTED. But being closed platforms meens that developers can "trick" the API into allowing the hardware to do effects that the GPU supports that the API does'nt, thats why we seen the original X-Box do amazing things with its Geforce 3 based GPU. X-Box developers tricked the API and unlocked most of the Geforce 3s feature set. OpenGL on the other hand give acces to ALL of the features of a GPU. We all know that microsoft updated 360s API and they call it DX9.5, that update just unlocked more of the GPUs effects that normal DX9 would'nt allow the developers to access. Its no DX10 but its DX9 either.
So how can PS3 or 360 produce effects that 1 machine could'nt do? the answear? Do it in SOFTWARE on the CPU. Thats were Cell comes in, Cell can add effects to RSX's feature list that could'nt be done on 360 because
1. It has'nt got enough spare CPU cycles to do the effect
2. Its CPU just cant do it AT ALL
360 has a hardware fucntion called MEMEXPORT, its simular to PS3s set-up as it allows 360s GPU to "take" a whole core off the game and use it for extra graphics processing, but this set-up while helpful it has'nt got anywere near the flexibility as PS3s Cell+RSX combo.
So in the end it will all boil down to how much Cell can actually do?, how many extra sepcial effects features can Cell add to RSX? That is what will make the difference between the 2 consoles, but it wont happen ovenight, it wil take YEARS befor we start to see if Cell really can add that extra sparkle to PS3 games. And in my opinion, judgeing by what we have seen Cell do already in Motorstorm, resistance, Lair, Warhawk, MGS4 id say Cell and PS3 graphical will be awsome.
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