REYES, Primitives & Some Philosophy
What does ‘high-level’ mean? Well, you guessed it, no polygons. If you want to render a sphere through REYES, you tell the renderer: “render a sphere, now!”, and it does. In another renderer you would tell it: “render those 10,000 polygons [forming a spherical shape]“. Both commands may result in an image that a non-biased viewer would call “a sphere” — so where’s the difference? Ok, I’ll talk a bit of a the Subdivision Mesh primitive first and then I get back to our spaceship example which will make the difference quite obvious (I hope) –
The Subdivision Mesh is a nice example of clever primitive handling in REYES. What it does is to take an input geometry and render a subdivision surface from it. “A subdivision surface?”, you might say, “My renderer can do that too and I can even tell it how many subdivision steps to apply before rendering out — ha!”. You’re right but think of the spaceship example again and read on a bit.
In theory, a subdivision surface is the shape you get when applying a sudden kind of algorithm — usually called a ’subdivision scheme’ — an infinite number of times to an input geometry, called the ‘control polyhedron’ (artists often refer to it as the [control] cage). Geeks therefore call the resulting shape a ‘limit surface’ as it results when you reach the limit of the subdivision process, that is: infinity.
Moritz- nice little article.
You may remember me from such productions as ‘Animalia’ (I’m one of the joonya’s that sat next to you).
Question after reading the article- If Subdivision is recalculated frame by frame in order to optimize micropolygon numbers, is it ever likely to encounter artifacts caused by a timestep and resulting mesh change. That is to say if the topology of the surface is changing to incorporate more or less polygons will the change every lead to problems. I wondering both about possible silhouette issues and texture problems. I suppose what I’m really asking is if I was to crank the shading rate to 50 or 100 (assuming it lets me and the control surface is suitably low poly) would the mesh pop in a random fashion either side of the ‘limit surface’ or would the subdivision remain, by some miracle, relatively uniform .
Obviously I would assume such variations would be next to impossible to detect on a moving object with a suitable shading rate but all the same I wonder.
Myles
Comment by Myles — 17. 2008f 2008 @ 9:13
Yes, it is indeed so that in REYES, the dicing of a mesh will change every frame, if that mesh is deforming/transforming and/or the camera is transforming or changing parameters, relative to the gprim in question.
This can lead to crawling artifacts on very slowly deforming geometry (slowly, as in: little change over many frames and possibly motion blur not being used to worsen things). Particulalrly displacement/bump mapping is prone to expose the problem.
Note, however, that this is all happening in alignement with the primtives’s UV parameterization which (supposedly) is the same, direction wise, over a sequence of frames. This and the fact that motion blur is almost always used in images coming out of REYES renderers, helps hiding the issue, if there is any visible one.
Consider the alternative in a ray tracer: every pixel is sampled (many times). Were a ray hits the primitive (aka: the ray-tracer’s “shading rate”) has nothing to do with the UV parameterization of the surface at all. Even with correct filtering, you can get similar popping of features but it will likely be much more pronounced than in a REYES renderer.
Comment by Moritz — 17. 2008f 2008 @ 10:33
la idea muy buena
Comment by cubrikaska — 8. 2009f 2009 @ 7:20