Question about normal smoothing

Hey Franz,

I am attaching a scene with a sphere illuminated by an environment map. When rendered in shading normals diagnostic mode, the surface normals look smooth, but when rendered normally poly edges are visible, the surface does not appear smooth.

Wally

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sphere.zip878.53 KB

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Franz's picture

Re: Question about normal smoothing

Hey Wally,

This is the classical "shadow terminator" problem. It appears whenever using interpolated normals on coarse tessellations. Most renderers suffer from this problem since it has no fully satisfactory solution. "Fixes" to this problem tend to introduce other issues such as loss of details. The only proper solution is to use finer tessellations.

I believe the term "shadow terminator" was coined by Andrew Woo from Alias in the paper It's Really Not a Rendering Bug, You See. You can read more about it here and here.

Franz

Re: Question about normal smoothing

Hey Franz,

I checked out the references above. Very informative. I especially enjoyed the first one discussing artifacts that are not really bugs. I have heard of some of these issues before, but learned some new stuff from that article. I think one reason I have not seen the terminator effect before is that all the rendrers I have used previously do on the fly polygon subdivision. The sphere I used in my example image is very low poly but renders smoothly in other renderers. Is it true that appleseed is not doing poly subdivision? I had noticed this early on when I first started experimenting with appleseed. Some of my first test images used this low poly sphere and I was surprised to see the outline of the sphere showed the polygons. I meant to ask about this before and forgot about it.

Franz's picture

Re: Question about normal smoothing

Hey Wally,

Glad you found the articles interesting.

Right now appleseed doesn't support higher order surfaces such as subdivision surfaces or NURBS. These are essential features though and will be first class citizens in a future release of appleseed.

Franz