ATI Radeon X1800 XT & XL (page 4: Perf HDR, Texturing)

HDR performances

HDR as seen in Far Cry or Splinter Cell, and which should be more widely used in the future, relies on 64 bit rendering, or 4 FP16 components to be more accurate. The scene, and some other elements, are rendered in a 64 bit buffer (RT, Render Target), and these parts are then processed (considered as textures) to assemble the final result. For all of this to work in a simple way, the GPU must support FP16 filtering and blending. This is the case of the GeForce 6 and 7 and Radeon X1000, but not with the Radeon X800. Accessing FP16 textures might be a bit costly, but it´s important to note that filtering isn’t. It requires transistors to be implemented in the GPU, but doesn’t reduce performance in practice. FP16 blending requires a great deal of memory bandwidth, twice as much as normal rendering.

There is no FP16 filtering for ATI and unlike NVIDIA, they passed this by. Developers will have to integrate filtering algorithms to a pixel shader and this will have an impact on performances although it will be reduced compared to the overall cost of HDR. ATI explains its position by the fact that in FP16, developers generally don’t want box filters (bilinear filtering, etc), but prefer better adapted ones (the Unreal 3 engine uses a specific filter with all cards, for example). If there was a demand they could include a FP16 filter support in capabilities (caps) and automatically include the filter in the shader so that it could be apparent to developers. For the moment, however, there is no real demand.

HDR implies using tone mapping, which consists in changing the HDR image into a 32 bit displayable one. The tone mapping algorithm defines how HDR data is to be interpreted and the final result. This is made in an additional path after the scene rendering. With the Radeon X1000, Avivo technology is capable of automatically processing tone mapping. Unfortunately, it´s impossible to access Avivo’s API via DirectX and ATI will avoid this problem with FourCC texture format which will be exposed in DirectX. When used, ATI’s driver automatically applies tone mapping via Avivo’s unit. NVIDIA also considered a specific unit to tone mapping in the 6800, but it didn’t work well, was deactivated and has completely disappeared with the 7800.

We measured performances with textures and surface rendered in 32 bits and in 64 bits (FP16) and without blending:

In 32 bits, NVIDIA is more efficient and more easily reaches maximum performances. The Radeon has abnormally low performances.

In 64 bits, the 6800 Ultra is clearly behind, mainly because of poor results in 64 bit texture access. It was improved with the 7800, which now features performances close to the Radeon X850 XT PE and X1800 XL. The Radeon X1800 XT does better, thanks to memory clocked at 750 MHz, which is the main factor in this test.

What about games? It is hard to tell how the X1800 XT will behave in practice as they didn´t work in HDR FP16 in our tests. The current version of Far Cry only seems to want to activate HDR if it detects a NVIDIA card and Splinter Cell uses a much simpler HDR, similar to that used with the X850. In the Serious Sam 2 demonstration, HDR activation leads to several bugs. Implementation isn’t identical for both manufacturers and although differences are minor, each game will have to arrange HDR depending on whether it´s ATI or NVIDIA.

Texture access

How do the different cards behave with different formats and texture sizes (simple access to textures without filtering)?

With standard 32 bit textures, the 2 GeForces provide similar performances. The Radeon X850 XT PE is clearly behind and doesn’t support 4096x4096. The X1800 XT has better results and is 50% more efficient than other graphic cards with 2x2 textures. This type of texture isn’t really used in practice, but it shows how the architecture´s efficiency has improved. With 4096x4096, it´s 70% more efficient than NVIDIA’s cards. Its memory clocked at 750 MHZ is once again advantageous.

With 64 bit textures (FP16), the 6800 Ultra has problems, which were corrected with the 7800 GTX, which now provides now much higher performances. It isn’t enough to compete with the X1800 XT, which is in the lead when textures size increases thanks to its memory architecture, optimised cache and higher bandwidth. Performance is three times higher and the Radeon X850 XT PE is double with 2048x2048 textures. The 6800 Ultra is far behind and is eight times less efficient than the new ATI graphic card, which is also the only one to support 4096x4096 textures in FP16.

With 128 bit textures (FP32), the situation is identical. Even if the gap between the cards is smaller with small sized textures, it grows with the texture size. The X1800 XT is almost five times more efficient than the 7800 GTX and 20 times more than the 6800 Ultra. ATI’s sub memory system clearly seems to be an advantage.