Iray, Diminishing returns?

Rendoo

Hi everyone, I've been playing with Iray quite a lot and getting renders that I enjoy and have a quality that I am not unhappy with.  I am writing to ask some advice in an effort to streamline my technique. 

This has to do with render times and sample rates etc. I have used settings in which the render has only progressed 15%-20% on the progress bar but processed for 2 hours, I cancel the render i save the image, it looks great.  I have also had images that have completed render processing in 30-60 minutes with midline settings and have looked great as well. 

My questions is what is your sweetspot? At what point does raising the sample rate etc cause diminishing returns or quite frankly adds a time sink that isn't commensurate with the quality produced?  Any tips on this?  How important to you is it to complete the render progress in DAZ? Do you set your render samples sky high and just kill it and save at say an hour? two? three? or do you proceed with a plan and factor in a time and sample estimate accurately and if you do, can you share your formula with me? :) 

 

Thank you! 

PerttiA

The one taking 2 hours and being at 15-20% has dropped to CPU, that's why it's taking so long.

If you want to keep the scene from dropping to CPU you must keep the memory usage of the scene low enough for it to fit the VRAM on the GPU, but there is no accurate way to determine the projected memory usage of a scene. Textures and maps assigned to materials are the biggest memory hogs, but if you go crazy with SubD you can have geometry eat up the memory too.

Robinson

There are quite a few variables to consider.  Lighting is one (too low lights and it'll take ages to converge).  Then scene complexity (number and type of lights, amount of geometry).  Finally unless you're doing close ups or a 4K render you probably don't need 4K textures, so get one of those scene optimisation scripts.  It'll drop to CPU if it runs out of memory (or worse, freeze your entire rig if you're using post denoiser - at least on my PC it sometimes does).  I've found the most problematic scenes in terms of convergence time are scenes that use a lot of emissive lighting.  You know, nightclubs and stuff like that. 

 

 

cajhin

I frequently set render quality to sky high and finish it whenever I think it's good enough.

Simple bright scenes with little subtlety can look nice after a couple of minutes. Dark complex scenes with lots of translucency etc pp I sometimes run overnight.

onix

At this time when we have Ai denoiser, the situation changed considerably.

some time ago the main reason for longer render times was to get rid of the noise. now the noise is no longer a problem and you will get a clean image in just a few iterations.

the reason for long rendering times now is to get more picture detail. Even if you will get a pretty good image almost instantly it will take some extra time until weak things like reflections and shadows will be finalized.

so you should choose rendering time depending on your picture complexity and on how many reflective/shiny/translucent surfaces it has

I noticed that 10 minutes is practically the most you need for a Full HD size image with good illumination

and generally, you decide it on a case by case basis just by looking at the result and deciding if it looks good or not. if you are leaving computer for the time you can just leave it rendering forever, it will not hurt. just take whatever you got when you come back

 

 

 

davidjones8418

I am no sort of expert, but some basic tips I can offer on render times from personal experience (ans forum help!) are..

1. Hide all body parts hidden by clothing.

2. Hide off camera props, structures etc unless it messes with your lighting. Try to give the light somewhere to escape to. The more it bounces around, the longer it takes.

3. Scene optimize people if they are not up close. I generally don't optimize the environment, props etc.

4. Only use hd/subd on people who are up close

5. Instance anything you can.

6. For your final render, set update interval high. 600, say. Less resources spent on refreshing.

7. Set instancing optimization to speed.

8. Altern8 supposedly reduces render times for affected skin, but I don't know if it does. I use it a lot anyway.

9. When in doubt, add more light. As long as you are happy with the balance of the lights, you can change the overall exposure with a graphics package afterwards.

10. Run a test render to get a feel for the duration and alter convergence on that basis. I will accept 95% convergence for a slow scene, 99% otherwise and 100% for a quick one. Totally depends on your preference, but for the scenes I make I definitely see detail improvements by going the whole hog. People have very different views on this. That's art!

11. Unless you have glass, mirrors etc in your scene you can save resources without obvious impact by turning caustic sampler off.

12. Be sensible about what youy are putting in. 3-4 different figures is probably about the most you are going to get away with (without instancing them).

13. Obviously, iRay GPU and the more vRam the better. One thing I didn't expect to help was ordinary Ram. Upgrading to 32gb was huge for me. Not because it made the renders quicker (it didn't!), but it meant I minded less because now I can surf the internet, write in open office, answer emails etc while the rendering is happening. Before, the PC was basically a brick while rendering.

I generally make the image 50-100% bigger than it needs to be, then export it from affinity photo at the needed size.

An 8-9 hour render used to be normal for me. They are more like an hour now. Partly tips like this, partly a better GPU and partly planning scenes to be efficient (more clothing allows more hiding, indoor scenes with roofs that come off, etc).

margrave

A Youtube video once advised me to crank the samples and render time to maximum, and let it render until it looks like it's done. I've been doing that ever since.

As others said, the most important concern (besides having enough memory) is light. If you're doing an indoor scene, or an outdoor scene with lots of shadows and indirect light, throw up some point lights for ambience. Set the geometry to "Sphere", the width to 100cm, and fiddle with the lumens until it's appropriate for the mood (start at 100,000 to 250,000 for very dark scenes and keep going up). That way you get nice, even lighting throughout your scene. Even if it's supposed to be dark, do it with the tonemapper or in post, rather than by taking light away. Always increase the light, until the point your meshes turn into ugly swarms of fireflies. That means there's too much light and/or your surface is too shiny. Then, it'll look ugly no matter how many samples you go for. At that point, you can dial your lights back.

Also, the Scene Optimizer is a lifesaver. Smaller maps mean scenes render quicker. Use depth of field to hide the lower-quality environment textures.

margrave

davidjones8418 said:

7. Set instancing optimization to speed.

Do not do this.

What "Speed" actually does is turn instancing off. It gives every instance its own copy of the geometry.

You can verify this by watching the geometry memory when rendering a scene to Iray. If you have a 15MB object and an instance and render with "Memory", Iray will tell you 15MB were sent to the GPU. But if you set it to "Speed", the geometry memory will double.

The names are very misleading. "Memory" means "It saves memory by using instances", whereas "Speed" means "It's quicker because it doesn't use instances".