iray Low-Light Scene -- 18+ Hr render?

mmitchell_houston

Before I start, let me say that I do know I need to rework the lights in this scene. Obviously, anything that needs this long to render needs to be adjusted. So, my question is not really about best practices, but it's more academic in nature.

SETUP: I've got a large, moody render with low light and lots of shadows (I'll post a WIP in a few hours when it's done). I'm at 6 hours right now and it's still pretty grainy -- I think when it times out at 8 hours it will still be pretty grainy. I'm thinking of leaving it running over night and all day tomorrow just to see what happens. I'll be out all day, so t's not like the computer has anything else to do while I'm away. So, between sleep and work, it will have 18+ hours to just "let it cook," so to speak. I'm wondering if this will resolve the graininess, or is there some sort of threshhold beyond which iRay cannot salvage a scene, no matter how long you let it run?

Has anyone else tried a really LONG render, like two or three days, just to see what would happen?

 

 

larsmidnatt

Yeah renders can take days..I had a 4 day render once. Resolution plays a part of that. Low light will take longer too. But I don't suggest going that long obviously :) Once you post a WIP I think people can make suggestions.

 

What kind of lights are you using? How many?

mmitchell_houston

I'm using the Total iRay Lighting and the iRay Emissive shader, cranked waaaaay up high.

Here's the WIP I promised. There are three lights (red in sewer grate, green plane below zombie and a bluish light behind the camera). I obviously need a little more light, so I've added a white panel behind the camer and I'm hoping that an 18+ hour render will yield some good results. For access to a larger image, swing by my blog: http://mikemitchellonline.blogspot.com/ 

 

mjc1016

One problem...if you are using mesh lights and have double sided turned ON, you will increase render times dramatically.  Also make sure the normals are facing the right direction.  That could be one reason for having to crank emission values so high...they are pointing the wrong way, so the light is actually going 'off camera'.

mmitchell_houston

Thanks for the info about double-sided mesh lights. I did not know that. I have added another mesh light behind the camera, and when I looked at the progress this morning I was happy with what I saw. I think that extra light is giving the scene just the "pop" it needs.

mmitchell_houston

17 hours and counting. So far, I like what I see. I may let this "cook" through tomorrow to see what I get.

So far, the light is doing a nice job of casting enough shadows to give the bodies definition. I've still got some graininess in the deep shadows of the capes. Oddly enough, I've still got a little red specking in the brightest part of the image, which is the doorway with the zombie. I thought that back wall would be finished by now.

mmitchell_houston

I let the image "cook" for about 42 hours, and was definitely happier with the results. I suspect this would have taken 4 or 5 days to finish, and this just wasn't important enough to let it run that long. This gave me enough to see if it's worth looking at, and enough to do some quick post work so I'd have something I could use. This image will not be making it into the Powers Beyond RPG, but it was a lot of fun and I did learn more about iRay rendering. So, all in all, not a bad experience.

To see a larger (1,000-pixel image), visit my blog: mikemitchellonline.blogspot.com

Tobor

You could probably achieve the same look by using a somber HDRi envrionment for ambient light, then using the meshes for accent, and adjust so that the scene elements have the same lighting balance. This gives more lioght for Iray to work with. Use Tone Mapping to adust the exposure of the Iray camera. Tone Mapping has no influence on the lumens hitting the scene, and it's the lumens -- or lack of them -- that contribute to Iray's slowless in underlit renders. 

It's not just the mesh lights causing the slow render. You have a lot of irregular surfaces on the walls contributing to additional ray traces, plus there's the reflections near the floor. Since the reflections are only simplistic, try resetting the Max Path Length (under Optimization in the Render panel) from -1 to maybe 2 or 3. This controls the number of bounces of light. While in the same panel, make sure Instancing is set to Speed, not Mempory. Apparently, this is being wrongly set to Memory for some users, causing absolutely horrific slow-downs.

Know that your scene may never reach 95% convergence, or might take days and days to reach there. Convergence ratio is Iray's judgement of how finished the scene is, based on sampling the same rendered pixels, and seeing how they differ from from previous iterations. On certain scenes, the render may in fact be finished, even at 60-70 percent converged. On some others, it could hit 95%, and still be way undercooked. It's an estimation guide for controlling when a render automatically stops. Only you can really determine if the scene is deemed finished.

mmitchell_houston

Thanks. I've definitely been thinking of cutting down the Path Length, especially in this scen. 

I'll have to look more into tone mapping. 

Some of the other environments I tried were casting too much light from all sides, which was not achieving the look I wanted.

The good news is, this was just a test of the lighting. I'm going to switch to a different technique for the production work I'm doing (in other words, unless I want to play with glass and shiny metals, I think the old 3Delight render engine will deliver the results I need).

Thanks again!

larsmidnatt

how complex is the mesh that is emitting light? it should be pretty simple, otherwise that will also increase render time.

mmitchell_houston

I can check, but the short answer is:

1. I created a primative plane
2. Applied the Emissive shader
3. Adjusted the emissive properties

So, I guess whatever the default settings for a plane primative are. I definitely did not modify them (and I confirmed that it was not set to 2-sided light).