Very long render times for Phoenix

Phoenix Issues
1

I’ve produced an simple explosion following this tutorial: Phoenix FD Detailed Tutorial - pflow Explosion Mixing RGB

I am using GI. Brute force/lightcache. If I don’t have brute up to about 180 it is noisy. I had 10 render nodes working on each frame at 1280x720 and it only completed: 105 frames in 7 hours. Each machine is i7 920-940 plus some newer i7-4770 cpus. Simulation was 7.9 million cells. Each node has 24gb ram.

https://vimeo.com/94969504

I’ve seen other tutorials and simulations where each frame took about 15 minutes on a single machine. I seem to be doing something very wrong.

2

i think the GI is the reason, it’s not a good idea to use GI when rendering volumetric objects.

3

Yes, just enable the Lights option in the Colors and Transparency->Fire section, use Grid-based self-shadowing, reduced grid placing, choose grid reduction between 1-10% and make sure you have Analytic Scattering chosen under the Smoke Color rollout :slight_smile:

4

ok. Do you think it would be better to fake GI with a few lights instead ?

5

Thanks. I’ll give this a try :slight_smile:

6

I turned GI off and checked my settings. I already had the lights set up like that in the color section. Rendering still seems to be taking ages and the lights cast from phoenix are still noisy even at high subdivs for lights etc.

7

Hm, can you send a stripped version of your scene so we can take a look at what’s causing this?

8

No problem. It’s only a simple set up so I’ll send the full scene. It will need re-simulation.

Thanks :slight_smile:
phoenix_explosion_14.zip (349 KB)

9

Alrighty, I had a look - your light has 120 subdivisions, and that’s too much for volumetric rendering to handle - you may either reduce them from the light’s settings for the whole scene, or reduce them for Phoenix only - go to Colors and Transparency → Smoke Color → Subdiv. Reduction and set it to 120 too, or more.

A couple of other things - of you need better quality in low resolution, you may change the Sampler type to Spherical from the Rendering rollout. This will be smoother but slower, so in high-res you may return to Linear to gain back the speed :slight_smile: And something else, just in case - don’t forget to return the number of threads (12 in your case) to 0 (= as many cores as you have) when you’re looking for speed :slight_smile:

10

Thanks very much. I’ll try these settings and get back to you.

11

Can you provide a bit of confirmation. Should I have direct subdivs set to 120 still or something like 20 ? and then set the subdiv reduction to 120 ?

If I try a lower value for the fire subdivs I get a lot of noise produced from them. That’s originally why I had the subdivs for the direct lights at 120 to try and reduce this.

12

You should generally decrease the light subdivisions until all geometry in the scene has as little noise as you like, and then increase the phoenix subdivision reduction while the quality of the smoke is okay and until the render times become acceptable. You may also increase the Step % parameter to say 90-100 to speed up rendering if needed.

13

seems like we have to make a tutorial dedicated to the performance questions.