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mrheisenberg

OpenGL
Is Clustered Forward Shading worth implementing?

46 posts in this topic

Which is a reason why I've viewed the use of multiple BRDFs with skepticism. The less the artist has to learn, and thus the more time spent actually making things, the better. And while certainly better can be done than Blinn-Phong, I'd simply rather not even give most artists the opportunity to sit there and switch between Beckmann to GGX to Cook-Torrance to etc. just to see what each did to "Get it right."

 

This isn't the problem you think it is; the BRDF/shaders are set up by tech artists (in association with rendering programmers) which are not the same guys doing the models/animation/rigging which take the time. Those guys are handed shaders and told 'use these' so they won't be swapping from one function to another to 'get it right'.

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Which is a reason why I've viewed the use of multiple BRDFs with skepticism. The less the artist has to learn, and thus the more time spent actually making things, the better. And while certainly better can be done than Blinn-Phong, I'd simply rather not even give most artists the opportunity to sit there and switch between Beckmann to GGX to Cook-Torrance to etc. just to see what each did to "Get it right."

 

This isn't the problem you think it is; the BRDF/shaders are set up by tech artists (in association with rendering programmers) which are not the same guys doing the models/animation/rigging which take the time. Those guys are handed shaders and told 'use these' so they won't be swapping from one function to another to 'get it right'.

Yes, and setting up a thousand BDRF shaders doesn't take up their time? Of course it does, otherwise Disney wouldn't have gone to all that trouble to reduce their rendering down to a single BDRF.

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ah yes, the old reductio ad absurdum argument....

 

A game is NOT going to have thousands of BDRF setup for it, a few will be selected from a small batch and those will be used (and result in many INSTANCES of the materials with different parameters) but the BDRFs themselves will be quite a low count. Over time more BDRF might get added but you aren't going to say "we are going to make a game, quick make ALL the BDRF shaders!" - that would be so many shades of dumb even thinking it is possible is.. well.. dumb.

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ah yes, the old reductio ad absurdum argument....

 

A game is NOT going to have thousands of BDRF setup for it, a few will be selected from a small batch and those will be used (and result in many INSTANCES of the materials with different parameters) but the BDRFs themselves will be quite a low count. Over time more BDRF might get added but you aren't going to say "we are going to make a game, quick make ALL the BDRF shaders!" - that would be so many shades of dumb even thinking it is possible is.. well.. dumb.

 

Tri-Ace was developing several already, for a game that was supposed to be this generation. Next gen there won't be nearly as much restriction, if Hollywood sees it as a problem to solve why would games, unburdened by much in the way of hardware constraints (as far as BDRF goes) be any different? Budgets can certainly be similar, and games often have far, far more assets.

 

With a single generalized BDRF, such as the one Disney has developed and I linked too, you would need a fat G-Buffer to store enough parameters sure. But you can go deferred and avoid dynamic branching, and you do save time. I'm not saying it's going to be as huge a benefit as a movie CG might get. Obviously artists wouldn't have time to sit there and fiddle with materials ad nauseum. But if it saves time it saves time. Point is I don't see much reason NOT to have a single, highly artists friendly BDRF as opposed to multiple kinds.

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For realtime, you're not going to want to compute sub-surface-scattering, or anisotropic reflections, for every surface if you can avoid it, etc, etc...

 

Awwee... But I want anistropic-specularity on my cream cheese, the bagels upon which 'tis spread, and my hot cup O' goodness!

 

There's an obvious engineering solution to this... isn't there? Ha... I'm sorry but I think it's just too simple to explicitly indicate. It's a very elementary premise of tool making... You don't need to internally implement such an "artist friendly BDRF" ... Too many hints ...

 

So yeah.

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Haha, whoops. I don't know why I wrote BDRF. Here's a reminder (saw this on Twitter smile.png):

 

7zZgSIk.png

BeaRDed F!

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There is a thing I don't understand.
It appears there's this thing still going on which plain forward can only do 8-10 lights per pass. How? In the past I've had quite some success encoding light data in textures and looping them on entry-level SM3 hardware. Perhaps I'm not seeing the whole picture but in SM4 with the much higher resource limits and the unlimited dynamic instruction count... shouldn't we go easily in the thousand range? Of course we'll neeed a z-only pass first.
So I guess there are additional practical reasons to stay in the 8-10 range.

 

At the top of page 2, I read about extra pressure and lower execution efficiency. I understand.

But, as much as I love lighting modularity coming from deferred, as a DDR3 card owner I still don't understand how the improved processing makes up for the bandwidth increase required. The trend on bandwidth is set. It looks to me we want to trash compute in the future.

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Hi Krohm,

 

In shader model 4.0 you can have up to 4096 entries in a constant buffer (which would limit lights to ~256 if they have position, direction diffuse, specular). Or you can use texture buffers and have near limitless lights.

Let's say you use the latter, so no worries about the light count. And today with SM 5.0 we really don't need to worry about loop count limits either. So we're good on that front

 

Indeed you can loop through a 1000 lights in SM4+ hardware. But let's say I'm running at 1920x1080 resolution and the whole screen is covered.

1920x1080 x 1000 lights = 2.073.600.000 light evaluations per frame.

Not to mention some BRDFs are expensive (i.e. Cook Torrance). Framerate would be sloooooooooow. So slow in fact, that it could trigger the Windows watchdog for believing the GPU is stalled and restart the driver.

 

The secret behind Deferred shaders (or Forward+) is that even though there are thousands of lights, they're not covering the whole screen at the same time.

In other words: many small lights = few big lights.

It's typical that a single region of the screen isn't lit by more than 4-20 lights, may be 5 on average. Let's be pessimistic and say 10.

1920x1080 x 10 lights = 20.736.000 light evaluations per frame

That's a lot more reasonable for a GPU to perform in real time. In such scenario every region of pixels (called tiles) would only have to loop through 10 lights (on average), not a 1000 and waste gpu time on 990 lights that aren't needed.

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But what if you would test whether a light actually should be covering the individual pixels inside this loop and skip all the lights, that shouldn't? The only difference to a tile-based deferred renderer would be, that the light culling is performed per pixel instead of per tile. But you wouldn't have all the BRDF, transparency, bandwidth and Anti-Aliasing issues. It would basically be a worse version of a light indexed deferred renderer, because the list of lights is not precomputed. Edited by CryZe
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Also with more traditional forward rendering you would typically have stage (performed either offline or online) where you determine which lights will affect a given mesh, so that you only apply those lights when rendering it. Once again the only major difference is your granularity, and when/where you cull your lights at your given level of granularity. Doing everything on the GPU lets you achieve very fine granularity (per-tile or per-pixel) with relatively simple code, which is the primary draw of deferred techniques.

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