Long rendering time

[Tom]

Hi,

I'm rendering an interior. Nothing fancy, just a basic dining room with a small kitchen and a living-room.
The scene is lit by a Corona Sun, Corona Sky as environment, very basic.

Here are two screenshots: the plan, so you have an idea of what the space looks like, and the camera view so you see what the actual render will look like. These are clay viewports but all materials are done. No displacement. Few forest packs here and there, for mulch and grass mainly, few Railclone objects, for the fence mainly. Some Coronaproxies for neighborhood buildings and some small trees. Nothing extraordinary really.

When I render in 4000 pixels wide, Corona says it will last for 15 hours. I use a Threadripper 3990X: 15 hours seems a lot for this 128 cores cpu.

I tried playing with the "GI vs. AA balance" setting, but I didn't notice any particular change in the rendering time.

There are only 5 windows, so I guess what Corona is struggling with is the lack of light coming in, especially in the kitchen area.
To help brighten this area a bit, I added a simple Corona Rectangle Light in front of the kitchen island.

My question is: what can be optimised in order to reduce the rendering time?

Thanks a lot for your help,

[Aram Avetisyan]

Hi,

First, make sure the exterior windows have simple (thin) glass - this is a common user error that can significantly increase render times.
For the light sources, make sure they are easy to calculate - not inside a refractive object, not in a hard to reach path etc.

With just screenshots and not the scene itself, these are the only things that come to my mind. Full scene investigation will be the best thing.
Also make sure there are no strange environment effects in the scene - e.g. Fire Effect can kill performance very much.

[alexyork]

First, make sure the exterior windows have simple (thin) glass - this is a common user error that can significantly increase render times.

I don't think it's the best advice to say people should model things in such a fake way. Double glazing is also a thing, and makes a very visible difference when rendered correctly. Not to mention it's not practical advice depending on whether you might receive a certain level of detail in a model from a client, for example. There will be other more obvious ways to optimise the scene before doing something like that, it's just not necessary or desirable IMO.

[maru]

Please make sure you are using the newest versions of ForestPack and RailClone and Corona 12 Update 1 Hotfix 1. There were some iToo<>Corona compatibility fixes in the newest versions. Based on what render limit is this 15-hour time estimated? If you are using the noise limit, maybe it's set too low? The estimated time could also change during rendering, so maybe in reality it wouldn't take so long.
Other than that, I can recommend archiving your scene with all assets and attaching it for us here: https://support.chaos.com/hc/en-us/requests/new

[Tom]

Thanks guys.

More info:
_ Max 2023 up to date / Corona 10HF2
_ FP Pro 8.0.6
_ RC Pro 5.2.8 I can't update these as my support is now expired
_ no environment effects

I finalized a render: 3000px, noise limit 4%, render time: 9h30

_ Generally speaking I do agree with @alexyork about not modelling windows as thin planes, but in this particular case I will give it a try as I want to reduce the reflections in the windows, quite the opposite of what we're after most of the time.

_ Light sources: all optimized, except light bulbs in the ceiling. I will do that as well (including only the spot light fixtures etc ...)

[brr]

What you can also try as a basic debugging step is applying a simple override clay shader with transparency exclusion and observing the results (but only after you rayswitch the spotlights). 

If the clay geometry renders fine, then the issue is likely with the shaders—such as superfine bumps/normals, displacement, or overly complex shaders from old or poorly optimized assets. 

Another option is to scan the scene using SiniTools - Forensic. If you have CAD blocks, numerous groups, etc., these could also slow down the calculations.

+ option is to clip opacity for outside greenery and reset texture filtering to 1.0

[Aram Avetisyan]

First, make sure the exterior windows have simple (thin) glass - this is a common user error that can significantly increase render times.

I don't think it's the best advice to say people should model things in such a fake way. Double glazing is also a thing, and makes a very visible difference when rendered correctly. Not to mention it's not practical advice depending on whether you might receive a certain level of detail in a model from a client, for example. There will be other more obvious ways to optimise the scene before doing something like that, it's just not necessary or desirable IMO.

It has nothing to do with double glazing ;) it is about glass having no refraction (thin mode) for outside light to be calculated easier. Of course it is good to keep things as they should be, but when render times are a concern, checking this is one of the first things. I, in fact, have been using double glazed windows everywhere for a very long time. As mentioned, full scene check is the best way.
Hope it's clear now.

[Tom]

So I did some tests, given what you said, but the problem remained, except when I turned off caustics.
I turned On Caustics so that the glass on the table casts nice light patterns on the table. And this is what is causing this massive slow down: when I turn caustics Off, the image renders 6.5x faster!

I hardly understand why, as the only object in the scene set to receive caustics is the table. How can a small object like this glass being responsible for the render time being 6x slower?!

[maru]

Do you mean enabling caustics for that single glass object in its material settings, or enabling the caustics solver in Render Setup > Performance?
If it's the latter, then the caustics solver then becomes active for pretty much all reflective surfaces. You can add a caustics render element to see what exactly is handled by it.

[Aram Avetisyan]

So I did some tests, given what you said, but the problem remained, except when I turned off caustics.
I turned On Caustics so that the glass on the table casts nice light patterns on the table. And this is what is causing this massive slow down: when I turn caustics Off, the image renders 6.5x faster!

I hardly understand why, as the only object in the scene set to receive caustics is the table. How can a small object like this glass being responsible for the render time being 6x slower?!

Now you get what the text in parentheses next to caustics mean ;)

Enabling caustics without caustics solver may only improve the look of the glass (shadows/colors nearby) but the photon calculation (light shapes, ray concentration) won't happen.
Enabling caustics solver automatically enables reflective caustics, but you can still control the refractive caustics per material.

Remember there is caustics include/exclude list too.

[Tom]

Do you mean enabling caustics for that single glass object in its material settings, or enabling the caustics solver in Render Setup > Performance?
If it's the latter, then the caustics solver then becomes active for pretty much all reflective surfaces. You can add a caustics render element to see what exactly is handled by it.

Both. I enabled caustics in the glass material settings and I enabled the caustics solver in Render Setup > Performance.
The only refractive material in the scene with caustics enabled is the glass on the table.

I understand that enabling the caustics solver makes photon calculation available for the whole scene, and this is why I used the caustics include/exclude list: the include list contains only the table, nothing else.

And by inspecting the Caustics render element, I can see caustics being rendered on the table only. So I still don't understand why the render takes 2x longer when only such a small portion of the whole image is concerned by the photon calculation.

[Aram Avetisyan]

Hm hm, the include exclude doesn't stop the calculations from actually happening. It just removes them from the image so that you don't need to worry about caustics looking noisy in a part of a scene you're not really concerned with - hence you can stop rendering sooner. The calculations do still happen though.

Yes, it is just like internal masking.

I suggest you render the caustic-y part separately (region, selected mask etc.) and all the rest in another render. That should bring reasonable render times.

DEV INSIGHT EDIT:
Photons are sent for the entire scene but gradually Corona learns that photons should not be sent in the direction of the excluded objects. The actual computations of caustic illumination from the photons are NEVER done on the excluded objects. So we could say that the caustics are NOT computed for these objects.

[maru]

@Tom still, such a performance hit may not be expected. Perhaps there is something we could improve in Corona or an issue we could find in your scene. Could you please share the scene archived with all assets with us? You can attach it to a support ticket here: https://support.chaos.com/hc/en-us/requests/new

[maru]

Ok, I was able to reproduce it and logged as (Report ID=CMAX-1505).

[Tom]

@maru: Thanks, so you confirm there's no need to upload the scene and it's investigated by your team to be fixed in the future?
That'd be great.

In the meantime I'll do some region render only for the glass area.