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mrheisenberg

OpenGL
Is Clustered Forward Shading worth implementing?

46 posts in this topic

One thing I like about the tiled Clustered is that it become "cheaper" to handle transparent object. In the case of tiled deferred you have to build 2 lists, one that used the depth buffer for the light culling and one without. So yo can have a massive overhead on the transparent pass. With clustered 1 culling is necessary.

 

But again that depend of the light count (also clustered is a heavier in term of memory size if I'm correct) and the scene.

 

Also at the Siggraph Asia , they were a presentation about a 2.5D culling techinque that you can find here : https://sites.google.com/site/takahiroharada/

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the Z-prepass worries me,does that mean I have to do the tessellation twice as well?(tessellation already hits my FPS big time)

Edited by mrheisenberg
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deferred shading is really unhandy when it comes to anti aliasing and lighting transparent objects is not solved in this approach.

forward shading is the way to go, I expect in the next generation consoles to go back to it. I use a similar approach on my phone engines, I've a view space aligned 3d grid (texture) that has a 'count' and 'offset' value per voxel, that I use to index into a texture containing the light sources that affect that voxel. the grid creation is done every frame on CPU, I don't have 30k of lights, but I run with antialiasing, I use the same shader for solid and transparent objects, very convenient to use, I can even assign this texture on the vertexshader for lighting particles in a cheap way.

 

one problem you still have is to apply shadows/projectors, it's solveable by having an atlas and store more data per lightsource (projection matrix, offsets,extends etc), but it makes quite a lot of overhead.

 

Many have solved transparency with deferred, Epic and Avalanche among them. Anti Aliasing is also doable. Multiple BRDF's are handled straightforward in deferred. You also have direct access to all those buffers should you need anything, and don't have to worry about processing and pixels you can't see it. And most modern hardware, including the 4th Gen Ipad and Tegra 4 from what I've heard, have enough bandwidth and memory to get some sort of deferred done, though if you're doing thousand and thousands of lights mobile probably isn't your target platform anyway.

 

I'd rather make sure there's not any unnecessary shading going on. Of course you can't do 8xMSAA with deferred, at least not cheaply, but you can do something like SMAA, which looks just as good and is cheaper in any case. I suppose it's all based on what you'd like to be doing. If you've got the time for it, and are on the right platform (new consoles, high end pc stuff) then I don't see any reason not to go deferred. If you don't have the time to solve all those problems, or somethings I'm probably not even thinking of, then forward might be your solution. But calling out all the old problems with deferred isn't relevant, as they've been solved for most part.

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Many have solved transparency with deferred, Epic and Avalanche among them. Anti Aliasing is also doable. Multiple BRDF's are handled straightforward in deferred. You also have direct access to all those buffers should you need anything, and don't have to worry about processing and pixels you can't see it. And most modern hardware, including the 4th Gen Ipad and Tegra 4 from what I've heard, have enough bandwidth and memory to get some sort of deferred done, though if you're doing thousand and thousands of lights mobile probably isn't your target platform anyway.

I don't remember Avalanche using Deferred Shading in it's titles. Which titles do use it?

 

Handling transparency... nice way of saying "solved". Switching to forward is not a "solution", neither is using lighting accumulative aproaches. It's a workaround. Anti aliasing is doable, but at a gigantic cost. I'm talking about MSAA and CSAA (SSAA is always expensive). Not about "FXAA" & Co. which is a cheap trick.

As for multiple BRDFs, it's not straightforward in deferred. It needs an extra cost in the MRT to store material ID, and you either use branching in your code and pray for high branch coherency (low frequency image) to get the best BRDFs (Cook Torrance, Oren Nayar, Phong, Blinn Phong, Strauss, etc) at decent speed, or resort to texture array approaches (which produce very interesting/creative results that I love, but aren't optimal for those seeking photorealism).

 

So, no, I wouldn't call the old deferred problems as "solved".

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the reason a lot of games went deferred is that it's not possible on current consoles to go forward. dynamic branching etc. would just kill you, and you don't really have benefits of it as most games are not rendering insane AA resolutions. that might change on future gen, they'll probably be very alike to PCs and there you don't worry about branching, but you want to support high AA resolutions without paying the cost of shading every sub sample.

 

so the question whether you go deferred or forward is also very much dependent on what your hardware has to offer (beside the question of what you're trying to achive).

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the reason a lot of games went deferred is that it's not possible on current consoles to go forward.

Many current-gen console games are forward, and forward has stuck around because it's very hard to go deferred on current-gen consoles... The amount of bandwidth required kills you. Even 16-bit HDR (64bpp) is a huge burden on these consoles.

the more advanced games are, the more likely they become deferred, the reason is that it's not possible to get the amount of light-surface interactions with forward rendering in a fast way. as you said, it would seem deferred is more demanding, yet it's the only way to go if you want flexibility.

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A little off topic but still on topic, does anyone have any links to good tutorials on deferred vs forward rendering? I've read a fair bit about the detail on deferred but would rather get a good grounding on it before look into it further - couldn't find any decent sites with 'why deferred' other than 'you can have more lights'.

Apologies for borrowing this thread quickly...
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Not really; deferred might have solved some problems with regards to lights but it brought with it a whole host of others with regards to memory bandwidth, AA issues, problems integrating different BRDFs, transparency and other issues which required various hoops to be jumped through.

exactly, one would think, having no MSAA (for shading), no solution for alphablend, problems with getting different BRDFs running, high memory storage and bandwidth cost, why on earth would anyone do that.

simply because the current gen console hardware does not offer another solution to create worlds that player, designer and artist expect, where you have tons of dynamic lights, where even particles light the close-by geometry.

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A little off topic but still on topic, does anyone have any links to good tutorials on deferred vs forward rendering? I've read a fair bit about the detail on deferred but would rather get a good grounding on it before look into it further - couldn't find any decent sites with 'why deferred' other than 'you can have more lights'.

Apologies for borrowing this thread quickly...

I think that's a good start:

http://http.developer.nvidia.com/GPUGems2/gpugems2_chapter09.html

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A little off topic but still on topic, does anyone have any links to good tutorials on deferred vs forward rendering? I've read a fair bit about the detail on deferred but would rather get a good grounding on it before look into it further - couldn't find any decent sites with 'why deferred' other than 'you can have more lights'.

Apologies for borrowing this thread quickly...

I think that's a good start:

http://http.developer.nvidia.com/GPUGems2/gpugems2_chapter09.html

 

That link just reinforces his belief that 'why deferred' is just 'you can have more lights'.

Effectively, that's the main reason it appeared, and that's the main reason it's still strong.

 

There are other side effects that are good:

  1. The GBuffer data can be very useful for screen space effects (i.e. Normals can be used for AO, refraction mapping, and local reflections, depth can be used for Godrays, fog, and DOF). Even if you do you forward rendering, you'll probably end up spitting a sort of GBuffer for those FXs. Of course, you don't have to do magic to compress a lot of parameters into the MRT that you won't be needing in the postprocessing passes (like specular colour term).
  2. Shading complexity becomes screen-dependant. This benefit/disadvantage (depending on the application) is shared with Forward+. Assuming just one directional light is used, every pixel is shaded once. In a forward renderer, if you render everything back to front, every pixel covered by a triangle will be shaded multiple times. Hence deferred shader's time will be fixed and depends on screen resolution (hence lower screen res. is an instant win for low end users). A deferred shader/Forward+ cannot shade more than (num_lights * width * height) pixels even if there are an infinite amount of triangles, whereas the Forward renderer may shade the same pixel an infinite number of times for an infinite amount of triangles, overwriting it's previous value. Of course if you're very good at sorting your triangles (chances are the game cannot be that good) Forward renderer may perform faster; but in a Deferred Shader you're on more stable grounds.

Edit: As for the "more lights" argument, take in mind that a deferred shader can easily take 5000 lights (as long as they're small) while a forward renderer can max at 8-16 lights per object.

Edited by Matias Goldberg
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Very insightful guys, thanks. My renderer is nicely abstracted so I might give it a go. My game only requires one directional light at the moment but I still see the plus with effects like AO, etc

Anyone know which method the call of duty engines use?
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