View Full Version : measuring curvature in PrMan

 mjs108 August 2005, 04:20 AMI have a high amplitude bump map that looks good everywhere except in areas of high curvature (tight concave and convex corners). In these areas the normals are pushed too far in relation to each other and result in some unwanted artifacts. One possible solution is reducing the bump amplitude in areas of high curvature, or where the normal directions are changing rapidly. What's the best way to do this in PrMan? Thanks
rendermaniac
08 August 2005, 11:55 AM
This may be useful to you http://www.affine.org/curvature.html

Simon

mjs1
08 August 2005, 04:30 PM
Thanks! I will take a look at this. I should have mentioned also that this is a PointsGeneralPolygons object.

rendermaniac
08 August 2005, 04:55 PM
dPdu and dPdv can be tricky with polygons - by default they use the local object space axes - which is usually exactly not what you want. In this case you'd need to make sure you have very good UV's and calculate it yourself using s and t - using du(P,s) and du(P,t) (I think...).

Simon

mjs1
08 August 2005, 08:43 PM
Agreed. Polygons make the problem more fun. :) In PrMan u an v parameters are defined as the x and y coords in object space. Since the model is a complicated terrain model I don't think the u and v parameter space will be that useful. Any polygon perpendicular to the xy plane will have strange parameterization.

I will try the shader you mentioned using the derivatives of P with respect to s and t instead of u and v and see what happens.

A lower-tech option I was thinking of was baking out an ambient occlusion brickmap and using the dark-colored nooks and crannies as a multiplier for the bump magnitude. Unfortunately I need to reduce the bump in convex high curvature areas as well.

If I just knew the derivative of the surface normal with respect to a unit change in P, that would give me a measure for curvature, right? Given that du, dv, Du(), and Dv() might be unreliable on a PointsGeneralPolygons object, does anyone know how you would find the surface normal change per unit change of P in SL?

rendermaniac
08 August 2005, 10:16 PM
To get any neighboorhood information in SL I think you'll need to use derivatives or raytracing. Otherwise some pointcloud DSO type thing might work.

Simon

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08 August 2005, 10:16 PM
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