Mantra's Subdivs

[hoknamahn]

Hey guys someone knows how Mantra doing the subdiv tesselation dependent on the distance? How dense a grid will be at the distance X or at the distance Y? There will be the grid of variable density (if different parts of the object are located at different distance from the camera) or density is always constant? Can I control the the number of subdivisions from the Mantra's parameters? How the shading rate parameter influence on the number of divisions? Or it does influence only on the quality of micropolygon splits?

[stu]

As far as I know, mantra's subdivision at rendertime is absolute - the solution is always sufficient to accomodate the raster size so that you'll never see nickling. And this being the case, I don't know that you even can control the level of subdivision.

[kodiak]

Not sure if I should add this here, but the subject sounded generic enough..

I'm doing pipeline integration on Houdini and parallelly building an example project to test tools on. I'm reusing characters from a previous project that was rendered in PRman. Houdini render speed is great until I add subdivision to the characters (in fact in PRman everything was subdiv, even the environment), which apparently has a major speed hit (in fact it feels so big that I have a gut feeling that I might be doing something wrong). One character isn't that heavy, around 40k polygons or so.

Can you guys share guidelines/experience on this?

Thanks

Andras

[stu]

Subdivision surfaces, by their very nature, take longer to render and use more memory. The expectation is that the geometry can be lighter because it's being rendered as a subdivision surface.

If you render a simple polygonal sphere both "as is" and as a subdivision surface, both with the "mantra -V 1" flag in the output driver, you'll see what the time and memory difference is.

[kodiak]

Subdivision surfaces, by their very nature, take longer to render and use more memory. The expectation is that the geometry can be lighter because it's being rendered as a subdivision surface.

I understand it takes longer, but the speed hit seems extraneous (compared to other engines I got used to). I figured out that it's not that bad with direct lighting, but when there is IBL with ambient occlusion (vex gi light shader and an area keylight) involved it slows to a crawl. A 3k image that takes about 15-20 minutes as polygons took in the range of 3 hours to finish as subdiv. I expected something less..probably double the original time maximum. So you're saying that it's normal?

Edited April 7, 2007 by kodiak

[stu]

Depending on what the shading quality is set to, turning on subdivision surfaces tesselates the geometry so that there's now roughly a polygon at every pixel. That'll certainly slow down any sort of occlusion calculations where rays are being sent out to bounce off of neighbouring geometry - there are now a lot more polygons to take nto account. Kinda like taking a polygonal sphere whose divisions are set to 2 and setting it to 20.

It'll be even worse if you're doing any bump or displacement calculations.

[kodiak]

Thanks Stu,

I'm familiar with how renderman alike renderers work and what shortcomings they usually have..that's why I'm suprised on _how much_ slower it becomes in mantra if I switch them to subdiv. In PRman it does slow down a bit but doesn't make this big difference to use subdiv with occlusion (not talking about trace displacement and such). Anyways..looks like it's just something to live with

[Jason]

I'm familiar with how renderman alike renderers work and what shortcomings they usually have..that's why I'm suprised on _how much_ slower it becomes in mantra if I switch them to subdiv.

Hi Kodiak,

You're right; especially if raytracing is involved - and irradiance - mantra 8 has pretty slow sub-div code. However, since you're looking at pipeline matters and have long term goals, let me assure you that SESI (and I hope they won't object to me telling you) have addressed sub-div speed for Houdini 9/Mantra 9 in a big way.

Things are looking up for Mantra9!!

[kodiak]

Great news, thanks for sharing