Feedback Raytracing Limitations - version to version

Hi Guys

The list of raytracing limitations does not seem to get updated from 1 version to the next. For example the HDRP 13 Raytracing docs still talk about Unity 2020.2 in the limitations:

https://docs.unity3d.com/Packages/c...4.1630323178-416189254.1557059478#limitations



So then does that mean the raytacing limitations between HDRP 10 and HDRP 13 hasnt changed at all?!?

That cant be right!!!

 

Many of these limitations, such as vertex animation and tessellation, are due to those effects being generated directly by the GPU and as such can't be represented by the BVH acceleration structure used in raytracing. HDRP 13 does however have newer "Mixed" modes for quite a few raytraced affects that get around these limitations by using hybrid of raytraced and rasterized techniques. For example, the mixed raytraced reflections uses traditional SSR and blends it in with the raytraced results to account for that missing information in the BVH.

The reason this is necessary is because getting the BVH to include the geometry generated by the GPU would require sending it to the CPU, generating the BVH, and then sending all that information back to the GPU which in more complex scenes is prohibitively expensive. However, this may eventually change as in the future it could potentially be possible to move BVH generation to the GPU via compute shaders, which would allow it to access all the information it needs to include these GPU driven effects.

 

Does HDRP 13 have a mixed mode for shadows also?

HDRP 12 does not, so its either raytraced shadows, or its not. Would be good if we could choose which objects get raytraced shadows and which dont. For example a large amount of foilage (static FBX meshes) would not require raytraced shadows in most cases, but currently there doesnt seem to be a way to exclude these from raytraced shadows and give them normal shadows, while at the same time have other objects like buildings cast raytraced shadows.

 

There is unfortunately no mixed mode for shadows. Rasterized shadows are a very complex beast which can make it difficult to combine with raytracing, at least in the way you are describing. NVIDIA actually published a paper back in 2015 on hybrid raytraced shadows however the technique they describe is used for generating realistic soft shadows using traditional rasterized shadows as a basis rather than having some objects use raytraced shadows while others using standard shadows.

While I'm not familiar with your project and its performance target, one potential work around could be to use spotlights on the specific object that you need to have traditional shadows. Another option might be to try and adapt your foliage to use the HDRP SpeedTree shader as I believe that is compatible with raytraced shadows. If I am remembering correctly and that is the case it might actually work out in your favor performance wise as raytraced shadows can actually be both faster and higher quality than traditional shadows when used in scenes with a very high number of very small objects like leaves.

 

How would I go about doing that?

For example, currently I have Nature Manufacturer Trees, and since they dont support beta versions, their shader doesnt work, so I'm just using a regular HDRP/Lit Shader instead, which looks the same as their's but of course without any wind animation (which I'm fine with). Is it simply a case of swopping the shader to Speedtree and thats it? Or its a bit more complex than that? I don't mind not having wind.

Remember, I'm doing a very large exterior render which will likely have thousands of trees, shrubs etc in view. This is for film so I'm not too worried about actual realtime performance, but more about "realtime" rendering. The whole point of all this stress and struggles is to find a solution to move our pipeline from traditional rendering to "realtime" or at least near realtime and not wait overnight for a single image and days on a renderfarm for an animation.

 

I believe it should be mostly straight forward to transition to the SpeedTree shader as it uses a fairly standard PBR approach to materials however the wind and subsurface scattering features probably won't work properly as they require data generated by the SpeedTree modeler program.

Getting access to the SpeedTree shader will require getting a SpeedTree asset which is actually pretty easy as they offer a free version of the modeler program, which you can grab here, that allows you to export a handful of example trees that come with latest version of the SpeedTree shader as well as textures so you can see how those are set up and adjust your own if need be.

If the SpeedTree shader ends up not working out I'd highly recommend taking a look at the second room in the HDRP sample scene as that has some great examples of folliage, both bamboo trees and ferns/bushes, that use the standard HDRP/Lit shader.

 

OK, I think I have a pretty solid foundational workflow and setup now in HDRP 12 and raytracing.

Now if I can only get a parallax effect of any kind or displacement of any kind going, that would be sweet and we can start moving our pipeline over to Unity (normal map isnt enough, we need displacement or parallax)

Any idea if something is in the works in that regard?

 

There is the HDRP/LitTesselation shader which does triangle based displacement. Though, like mentioned in the picture from your first post, the raytracing will fall back to the non-tessellated base mesh. The mixed modes for reflections and global illumination will still allow the tessellated geometry to represented for those effects because of their screen space fallbacks, however, the feature that will have the most glaring issues will be raytraced shadows as the displaced geometry will not be able to cast shadows and will cause inconsistent results. If you really need those shadows for a tessellated object your best bet would be to bake the displacement out as the actual mesh in your DCC of choice.

 

Hi @INedelcu ,

I've noticed that you are working on raytracing in Unity and replying to questions about similar stuff like gpu instancing/vertex animation with raytracing. I have a question related to this thread.

Do you think that it's possible that raytraced shadows will work with vertex displacement like tessellation in the future?

Any answer will be much appreciated.

Thank you.

 

I have noticed that vegetation renders quite slow with raytracing. Is that why? What can I do to make it faster?

 

I see, thank you for the reference. Is there a HDRP setting that will make the raytracer ignore cutout-opacity on foliage?

 

Hi @INedelcu

When do you estimate the work you're doing on this, to make it into an alpha/beta?

 

Hmmmmm apologies @INedelcu
I seemed to have replied in the wrong thread. The comment is for you though, and should have gone here:

https://forum.unity.com/threads/doe...instancing-in-hdrp-12-1.1194841/#post-7962564

 

@INedelcu

First of all thank you for the detailed responses in this thread. They are very helpful in understanding the systems.

I've noticed that there is a new position in the rendering roadmap related to raytracing:

https://portal.productboard.com/8ufdwj59ehtmsvxenjumxo82/c/1785-raytracing-api-out-of-experimental

Does it mean that you have some plan to support instancing APIs like Graphics.RenderMeshIndirect?

As it mentions foliage I'm wondering if you have some plan to 'translate' shader based vertex animation to the raytracing acceleration structure?

It's very interesting. Especially after reading your posts about raytracing, vertex animation and tessellation: