Has FP24 been a limitation yet?

Discussion in 'General 3D Technology' started by nelg, Dec 12, 2004.

  1. KimB

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    I could think of two possible situations where FP24 in pixel shaders would affect vertex shaders:
    1. Unified pipelines.
    2. Render to vertex buffer/texture.
     
  2. Joe DeFuria

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    Given the lack of "evidence" for the need of FP32 pixel shaders for gaming, I'm thinking it's unfortunate that FP32 is part of the SM3.0 spec.

    If what I've read is generally true, that FP32 "costs" about 30% more (die space) to support vs. FP24, then I'd have to say I'd rather see those gates go toward other features or even simply more performance. (Or hell, just a cheaper, less power consuming part).

    I get the feeling that ATI is being "forced" to support FP32 for no other real reason than SM 3.0 requires it, and it would be more marketing suicide than anything else to not support SM 3.0 next year.

    I would rather have seen SM 3.0 kept the "full precision" of at FP24....and if someone makes a FP32 or hell, FP48 part, demonstrate the quality advantage over FP24 in a game. SM 3.0 shouldn't srtificially limit shader precision, but at least for games in the forseeable future, I don't see the costs of FP32 outweighing the benefits.
     
  3. Ostsol

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    A unified shader architecture will definitely require FP32. It is already the minimum for vertex shaders, so if an IHV wants to use the same ALUs for either processing, they'll have to have full FP32 support. I can't see any way around it. Partial precision might still be supported, but that will be the only possibility for sub-FP32 precisions.
     
  4. KimB

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    I don't buy it, for the simple reason that if you compare the NV40 to the R420, the difference is about that, but the NV40 offers so much more than just FP32 support over the R420.
     
  5. Joe DeFuria

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    I would never begin to compare two different chips from two different companies and try to guess at such a thing.
     
  6. Xenus

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    And I wouldn't take ATI's word about 30% either because yhey have an agenda and didn't want the change.
     
  7. Joe DeFuria

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    Of course.

    I know for CERTAIN that it costs "more" to support FP32 than it does FP24. 10%, 30%, 50%. Pick one.

    I also have not been shown ANY convincing case where FP32 pixel shaders makes a quality difference for gaming. (At least, for the near to mid-term.)

    The only reason I can see it being "beneficial" to a gamer, is when it's a natural consequence to the the API specs "in effect" requiring a "unfied shader". That doesn't look to happen until probably WGF 2.0.

    Just because one company may not have wanted to make the change...doesn't mean their interests / agenda doesn't overlap with my own agenda! :) (My agenda being, better and better gaming cards...more value for my dollar for gaming).
     
  8. _xxx_

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    I'll be sure to do it for the marketing reasons. New unique features etc. helps sell more HW. Remember the first GeForce and T&L (well, almost T&L)? It was also not really good, but it did set the path.
     
  9. KimB

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    And I feel that video cards should be forward-looking. As texture sizes continue to increase, for instance, it would be rather disappointing to suddenly realize that artifacts become visible in dependent texture reads that weren't there for normal texture reads.
     
  10. Joe DeFuria

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    So do I. There is a difference between "forward looking" and "ahead of its time." Agree?

    Sure, it would also be equally disappointing to find out that artifact-free dependent texture reads are so slow to be prohibitive with any meaningful implementation.
     
  11. Bob

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    You're assuming all transformations on those vertices are 'perfect' (ie: there is no precision loss). This is not usually the case. If you assume that you lose 0.5 LSB per basic operation (MUL or ADD), that you do your modelview-projection transform in a single matrix multiply, and that your original 5 km estimate was correct, then your world can only be 70m wide for a hypothetical GPU with FP24 vertex shaders.
     
  12. dksuiko

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    I feel the same. But when consideirng the costs mentioned in this thread and the lack of evidence for FP32's benefits, why spend resources on it when you can make a reasonably good compromise with FP24? The sole purpose of it being foward-looking isn't good enough, in my opinion. It's similar to the whole native PCI-Express vs. AGP Bridge situation. Sure, ATI's native PCI-Express may be better in the longrun, but nVidia's AGP-to-PCI-Express bridge is just as good for now, not to mention cheaper. Had they went on to make a forward-looking native interface for the sake of making it forward-looking, more costs would have been required for no practical performance benefit.
     
  13. OpenGL guy

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    What? FP32 is required for vertex shader operations. And this doesn't apply to any current product. I thought we were talking about current products? Hypothetical (and stupid) products have no interest to me.
    Render to VB is not supported by the API. I don't see how render to texture is affected and differently than normal rendering.
     
  14. KimB

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    Here I'm talking about a FP32 texture that would be used in the vertex shader. This, then, would be an argument for those stating that the FP32 requirement for PS3 is ridiculous.
     
  15. OpenGL guy

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    You really think that you need the full range of FP32 for vertex textures?
    Not at all. What it argues against is the need for FP32 vertex textures.
     
  16. sireric

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    That's actually pretty trivial to do -- Moving an R300 to FP32 would of simply required upping the 24b DP units to 32b (it's really just a substitution -- we have FP32 units all over as well) and increasing GPR space to 32b. It's really simple 30%+ increase in area (GPRs have 30% more space reqs, adders are pretty much 30% larger, while multipliers are larger than 30% more).

    The problem is not getting good FP32 performance in R300 (reasonably easy), it's making it cost effective. That's hard. 30% more for pixel shaders is quite a bit of extra area, which, for all pratical and useful purposes, is a simple waste. You can reduce something else, or increase the cost. I think it's fair to say that if we reduced "something" else, then performance would of been affected and that's not acceptable. Increasing the cost, well, that's something that one has to decide upon. We were given a target cost, and we sort of went past it; another 30% growth would of meant less parts out (9700 at 499 back then, would not of been cool) or smaller margins -- These are not acceptable.

    Bigger worlds really has no effect on PS precision requirements. You can construct a world so that it is divided into regions of interests, so that within those regions, limits of PS (I can't even think of many cases where PS precision matters; possibly for some large repeating textures) and Z aren't important. In all those cases, geometry is computed in 32b SPFP, and doesn't involve 24b.

    I hope it's good beer. All in all, I still believe that for current applications FP24 is more than ample, and very few things would of required, if any. In fact, feedback from ISVs has been great on R300 and now R420 -- Easy to program, very predictable performance, clean orthogonal feature set (wrt to performance), etc... -- Compared to the competition (at least last gen), it's been night and day. I don't remember one complaint about FP24. BTW, FP24 was not an ATI only decision. MS specified that FP24 was the high precision format for DX9 (SM2) -- It's really not an ATI only decision. You at least need to blame everyone involved (including potentially all IHVs that agreed to it), if you're so against it.
     
  17. sireric

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    I'm sorry, but that's not an example of FP24 limitations. Comparisons between completely different rasterizers (refras and ATI's), will lead to tons of differences. That's why DCT/WHQL specifies tolerances. There's no way the two can be the same, unless the rasterization algorithm (including setup, Scan conversion, iteration, etc...) is identical in both. I hope this never comes to be, though who knows in the future.
     
  18. sireric

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    As well, I did not mean 30% die increase, but 30% shader increase. That's simple enough math that it's hard to not believe (24 -> 32 is 30% more storage, approx ~30% more adder, and more than 30% more multipliers). The die size increase is not something I'll specify.

    As for NV40 vs. R420, that's ridiculous to compare die size. You're better off just trying to find out the company's per chip revenue and margins -- That's the best metric that I could think of to compare the two products, but you will not be able to get that information from either company.
     
  19. sireric

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    I agree with that. That's my personal opinion, and nothing to do with ATI.

     
  20. sireric

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    Well, I never said 30% more die -- but I'm dumbfounded to think that you would not believe that a FP32 shader would be at least 30% larger than an equivalent featured FP24 shader.
     
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