CGTalk - Shadow artifact using bump maps

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FhdeVilliers

10-27-2009, 02:12 PM

Hi all.

I've come accross a strange artifact using bump maps and raytrace shadows in xsi.
I'm using linear workflow so a gamma of 2.2 in the render view.

The problem is that if an object is in shadow and has a bump map on it there is light bleed that happens right at the terminator where the self shading and the shadow meet.

Here is a simple illustration of what happens.
You can see on the shadow pass the strange shadow jaggies as well.

Has anyone seen this before and is there a way around it. I used a mia shader and it solves it but that means that I have to use the mia for all the objects.

Thanks.

kanooshka

10-27-2009, 04:59 PM

In the future you may want to specify the renderer you're using, I'm assuming it's mental ray because you mentioned the mia material. I haven't run into this issue myself but could you post some images/specifics on your lighting setup and shading network? Also, do you have roughness on the material. I find this can create issues sometimes.

FhdeVilliers

10-28-2009, 07:18 AM

Thanks.

Yes I am using mental ray. Im just using a distant light with raytrace shadows, Blinn shader with bump generator. The issue happens on blinn's, phong's and all others accept the mental ray shaders, carpaint and architectural.

tharrell

10-28-2009, 01:33 PM

The reason you're seeing that issue is that the bump map is tweaking your mesh's normals... it isn't actually changing the geometry.... and that's a pretty darned deep bump map. Bumps are just cheats best used for minor surface movement of detail at range... not stuff as deep as you're trying to do. Particularly as big as it is in-frame.

I'd suggest changing your bump to a displacement. It should clear up your issue, and look nicer to boot.

--T

FhdeVilliers

10-28-2009, 03:38 PM

Thanks for reply.

I do understrand the diff between bump and displacement but this doesnt only happen on large bumps. I first saw this on a scene with a cliff. The cliff had fine bump detail and light was just leaking through all over the mesh. I put the same bump on the mia shader and it works, why I ask you does the mia shader know that the bump is in shadow and the xsi shaders not?

Werner

11-12-2009, 08:01 AM

Hi Francois,

Werner from Luma up in JHB here...

Have you tried it with the Bumpam node instead of the bumpmap generator? I found the later to be very problematic.

I could not recreate your problem with the bumpmap node.

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