Is it possible to rotate a mesh differently for shadow rendering and regular rendering?

Questions
1

I have meshes that are effectively 2D sprites, which I rotate to face the camera before each render (similar to billboarding). When shadows are rendered, I’d like these meshes to face directly towards the light so that they cast a consistent shadow.

Is there an event I could hook that fires between shadow rendering and regular rendering, such that I could give sprite meshes a different rotation for the two phases? Or is the effect I want impossible to achieve without creating “shadow impostors” (i.e. duplicate shadow-only meshes for every sprite that needs to cast a shadow)?

2

Not sure if this works but maybe a hint, use setmaterialforrendering for the depthmap, and deal with the rotation in the vertex shader of a shadermaterial.

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3

Sorry, I don’t follow. Do you mean that it’s possible to specify a separate material for the shadow render, but not anything else, and there aren’t any events that fire between the depth render and normal render?

4

You do have an event which is raised before a mesh is rendered into the shadow map: shadowGenerator.onBeforeShadowMapRenderMeshObservable. And you also have the counterpart onAfterShadowMapRenderMeshObservable.

However, the onBefore event comes too late in the process, the world matrix has already been retrieved/set.

5

I see - so is it correct to assume that the only options are to make duplicate meshes or write your own shader?

6

Yes, I think these are the only solutions at the moment.

7

Thanks!

I don’t know if I can suggest features here, but it seems like a better solution for this would serve lots of use cases. Even without the sprite/billboard angle, one might want a given mesh’s shadow to be a different scale than the base mesh, for example.

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8

I don’t think we can provide a generic solution, but you can do it with a custom shader and a shadow depth wrapper:

Note that I don’t know if it’s faster than simply having a copy of the plane for shadow only rendering, as we have to recompute the world matrix two times for each tree with the method above…

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9

Thank you for the demo Evgeni! In my case I think it will be simpler to create duplicate meshes, but the example code was very helpful.