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After the displacement has been done, any smooth analytical normals that the original surface may have had (for example a smooth subdivision surface or NURBS patch) are no longer valid – they do not correspond to the displaced surface. Each micropolygon has a geometric normal Ngn. In addition, for some surface types a smooth shading normal will be automatically computed for each ray hit – this is done by considering the orientation of not only the micropolygon that the the ray hit, but also the adjacent micropolygons.
Displaced polygon meshes deserve a special mention here. Polygon meshes do not by themselves have smooth normals, but are often defined with a normal at each polygon mesh vertex point. When a ray hits a face of an undisplaced polygon mesh, the shading normal Nn can be computed by interpolation ... interpolating the normals of the face vertices, leading to a smooth appearence.
RixDisp() ?
appearance. But for a ray hit on a displaced polygon mesh, those vertex normals are no longer valid after the displacement. A classic and very useful trick is to add back the difference between ... at ray hits on the displaced polygon mesh. WAIT A MINUTE – this trick is for bump mapping, not displacement, right??
The PxrBump shading node and RixBump() function both employ this trick.Talk about common trick for adding back smoothness? Also,
Displacement bounds
BVH bbox tightening. Not too large: important for ttfp. Not too small: holes. Balance. Will give warning after (rendering completed) if too small or more than 10 times too large.
TO DO: Figure with holes due to too small displacement bound.
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Attribute "displacementbound" "float sphere" [0.21] Pattern "dispstar" "sin5theta" "float scale" 0.2 Displace "PxrDisplace" "pxrdisp" "reference float dispScalar" "sin5theta:resultF"
Sphere 1 -1 1 360
The dispstar shader is a very simple shader written in Open Shading Language (OSL). It computes a displacement amount depending on radial angle in the x-y plane. When applied to a spherical shape, it produces a round star with five spikes. When applied to a teapot (with higher frequency), it produces a pumpkin-like shape.
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// Compute displacement amount float angle = atan2(Nobj[1], Nobj[0]); float disp = scale * sin(freq*angle);
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In this example, the displacement amount computed by dispstar gets passed on to the PxrDisplace displacement shader, which does the actual displacement. Here is an image of a sphere and the same sphere displaced with this combination of shaders:
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The seasoned RenderMan user will notice this is all very similar to how displacement was done in the classic RenderMan Shading Language (RSL) – see for example Apodaca and Gritz, 2000 section 8.2.