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Gaps in displacement

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SSGGMM
Gaps in displacement
on Sep 15, 2006 at 11:32:33 pm

Interesting problem, and quite maddening. My displacement map appears to be tearing apart my (polygonal) object with MR. The gaps appear between faces. MR does this so matter-of-factly I simply can't imagine I'm the only out there who's had this problem. Weird how it hasen't turned up till now.

Please help. Merci - Serge


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Jon W.
Re: Gaps in displacement
on Sep 16, 2006 at 10:29:44 am

Without seeing a picture of the actual tear and poly object my guess would be to increase the Feature Displacement Samples for a more accurate displacement. Click on your PolyShape node in Attribute Editor/Displacement Map/increase initial sample rate and extra sample rate. maybe also play w/ the normal threshold.

If that doesn't work have you tried other renderers like Maya Software and Renderman for Maya (there's a free demo download).


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SSGGMM
Re: Gaps in displacement
on Sep 16, 2006 at 12:52:04 pm

Mr. W.,

Thanks for the reply. I tried Maya Software and I tried tweaking the sliders you mentioned, but the gaps still appear.

I can't post an image, but I can describe it rather easily. It's a poly staircase, as simple as simple gets, I haven't even beveled the corners yet. The render breaks the faces apart, creating small gaps between each. It's just like what happens when you right-click your object in scene view and pick "vertex faces". Now picture that in your render window. It's probably worth noting that, other than these gaps, the detail from my displacement map comes out just fine.

I'm able to have some kind of effect on these gaps when I play with alpha offset in the displacement file's attributes. But all this does is it shifts the same problem around: One corner gets fixed, but an adjacent one worsens.

I don't know. Any other ideas?


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Steve Sayer
Re: Gaps in displacement
on Sep 16, 2006 at 3:39:09 pm

Hello again.

I did a couple of quick tests, and may have the solution for you.

It seems to me that the Mental Ray renderer takes edge hardness into account when deciding whether or not a displacement map is 'allowed' to separate faces like this.

Try softening the edges, and see if the problem persists.

-Steve


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SSGGMM
Re: Gaps in displacement
on Sep 16, 2006 at 4:46:54 pm

Hi Mr. Sayer,

Softening the normals certainly helps, but it makes the faces look a little inflated. If I can manage to negotiate this with the map's alpha gain it should be ok. There's a happy middle somewhere.

Thank again - Serge


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Jon W.
Re: Gaps in displacement
on Sep 16, 2006 at 6:00:12 pm

Are you using a projection node for the displacement? Maybe it's the vertical faces that are the problem (since it is a staircase, the vertial ones connected to the horizontal faces are creating the gaps and ripping the poly object apart?). Also could you just get away with bump mapping?


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SSGGMM
Re: Gaps in displacement
on Sep 17, 2006 at 9:31:06 pm

Hello Mr. W.,

The displ. map isn't a projection. I think the problem has something to do with unshared vertices but I can't be sure because my "heads up" edge count says there's only a single edge there. In any case, softening a few normals seems to have done the trick. As far as bump mapping goes: the staircase is made of stone and cement. I sorta want the corners to be jagged, and I've never figured out how bump map around corners. Whenever I render bump maps, I seem to get straight and smooth edges around objects.

Regards - Serge


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Steve Sayer
Re: Gaps in displacement
on Sep 17, 2006 at 5:57:27 pm

If you add a tiny bevel to the edges, thus leaving two or three edges (all soft) right near each other, the 'inflated' problem might go away.

Happy to help.

-Steve


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SSGGMM
Re: Gaps in displacement
on Sep 17, 2006 at 7:50:10 pm

Way ahead of you... :) That's exactly what I did. Thanks again - S.


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