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> Nurbs or any smooth geometry needs to be subdivided too

Not true; lots of smooth surfaces, including NURBS, can be and are ray traced without subdividing.

> Polygonal geometry, even subdivided, is typically not a big part of memory or time in rendering in all but the most pathological cases.

I don’t buy this either, speaking from experience using multiple commercial renderers. It is true that texture is larger, but not true that polygonal geometry is not a big part of memory consumption. RenderMan, for example, does adaptive tessellation of displacement mapped surfaces because they will run out of memory with a uniform displacement.

The balance of geometry vs texture usages is also changing right now with GPU ray tracers, and geometry is taking up a larger portion because it has to be resident for intersection, while textures can be paged.

> I don’t buy this either, speaking from experience using multiple commercial renderers.

It of course depends on exactly what is being rendered, but typically texture maps of assets for high quality cg are done at roughly the expected resolution of the final renders (rounded to a square power of 2). Typical assets will have three or four maps applied to each group of geometry, with higher quality hero assets having more groups.

> RenderMan, for example, does adaptive tessellation of displacement mapped surfaces because they will run out of memory with a uniform displacement.

It is specifically screen space displacement and this has been effective, but was originally crucial in the days where 8MB of memory cost the same as someone's yearly salary. In PRman actually polygons are even less of a burden on memory because of this with micropolygon and texture caches for efficiency, even with raytracing.

The real point here though is that nurbs don't really have much of an advantage, even in memory, because polygons are already lightweight and can be smoothed. Subdividing of polygons is typically not going to be too different from burbs and heavy polygonal meshes are likely to be extremely difficult to replicate with nurbs.

Don't get too caught up in exactly what is technically possible, this is about why nurbs are not an ideal form of geometry that anyone is trying to use again. Their disadvantages outweigh their advantages by a huge margin.

berkut 2244 days ago

> Polygonal geometry, even subdivided, is typically not a big part of memory or time in rendering in all but the most pathological cases. 4k would still mean that one polygon per pixel would be 8 million polygons, which is going to pale in comparison to texture data typically.

Erm, at VFX level at least, that's not really true: once you have hero geometry that needs displacement (not just bump/normal mapping), you effectively have to dice down to micropoly level for everything in the camera frustum. And with path tracing (what everyone's using these days, at least in VFX), geometry caching/paging is too expensive to be used in practice with incoherent rays bouncing everywhere. Disney's Hyperion renderer does do that, but it spends a considerable amount of time sorting ray batches, and it was built to do exactly that.

Image textures, on the other hand, can be paged fairly well, and generally in shade-on-hit pathtracers (all of the commercial ones), this works reasonably well with a fairly limited texture cache size (~8 -> 16 GB). Mipmapped textures are used, so for most non-camera rays that haven't hit tight specular BSDFs not much texture data is actually needed.

Once things like hair/fur curves come into the picture, generally geometry takes up even more memory.

The original thread was about nurbs not being used anymore over polygons. Displacement doesn't change the equation between polygons and nurbs of course because they both go in as high level primitives. Hair is a its own thing of course. I know you know this, but I think a lot of people missed the original point.

You keep saying “polygons”. Are you talking about subdivision surfaces? Films don’t model in “polygons”. Subdivision surfaces are a curved surface representation, not “polygons”. Some people still use NURBS too.

berkut 2244 days ago

Subdivision surfaces end up as curved surfaces when rendered, i.e. an approximation of the limit surface, but the modellers most definitely do model them as polygons in the DCC apps.

Some of the studios still don't even bother with crease weights and still "double-stop" ends with extra vertices/faces to create hard edges.

My original point was that it is possible to render subdivision surfaces without dicing down to micropolygon (i.e. you approximate the limit surface with Gregory Patches or something), but only if you don't have displacement: as soon as you need displacement, you pretty much need to dice down to micropolygons, and in this scenario, the geometry representation can be extremely expensive in memory with large scenes.

> but the modellers most definitely do model them as polygons in the DCC apps.

Yes, right, I know. I phrased that poorly, so I guess I should give BubRoss a break. The point I’m trying to make is that starting with the idea of polygon modeling, and starting with the idea of subdiv modeling, are two different things. If we’re talking about subdiv modeling, they it should be called subdiv modeling. Modeling “polygons” doesn’t just automatically produce decent looking smooth models and good connectivity and UVs, you have to use subdiv tools while you work.

That you can render subdivs without subdividing is related to what I was trying to say, that these surfaces are higher order, have an analytic definition, etc... they’re not just polygons. I guess it’s a good thing that subdivs are so easy to work with that they’re equated with polygons.

> Films don’t model in “polygons”

I can promise you they do. They are treated as the same thing. Everyone uses polygons knowing they will be smoothed/subdivided/declared as subdivision surfaces. Sharp edges, cusps and bevels are typically made by creating more subdivisions in the actual model instead of using extra subdiv data on the geometry, though pixar might be the exception.

> Some people still use NURBS too.

I think this is very rare. Maybe blue sky never transitioned away.

Yeah, so you’re talking about subdivs, not just any polygons. Yes you create subdiv geometry using polygon modeling tools, but modeling pure polygon models, e.g., for games, is a different activity. Subdivs are easier than NURBS, it’s true, but they do come with their own whole set of connectivity, workflow, texturing, pipeline, etc. Just saying “polygons” is misleading.

BubRoss 2243 days ago

> modeling pure polygon models, e.g., for games, is a different activity

It really isn't. I think you want to drive home some distinction, but the vast majority of work flows model straight polygons and the only difference is that they know they are going to be subdivided and smoothed later by the renderer.

You say that there are all sorts of different issues with subdiv surfaces, but it just isn't true. Modelers and texture artists might look at everything smoothed to make sure there aren't any surprises in the interpolation and distortion in the UVs, but everyone deals with the raw polygons.

michal_f 2244 days ago

3delight also traces SDS and nurbs analitycally. In offline renderers geometry data constitutes to most of memory usage. Texture RAM usage is kept under a few GB by use of page-based cache.

Multiple gigabytes of geometry is a lot. That ends up working out to potentially dozens of hundreds of polygons per pixel. Even so the person I was replying to seemed to wonder why everything converted to polygons, which is because of a more holistic pragmatism.

berkut 2244 days ago

Now try it with displacement :)

Agree in general about geometry memory though: in high-end VFX, displacement is pretty much always used, so you have to dice down to micropoly for hero assets (unless they're far away or out of frustum).