Indirect Contribution Slider

[mckenzie2VER2]

According to the Arnold manual:

Indirect Contribution for Lights

"The relative energy loss (or gain) at each bounce. This should be left at its default value of 1.0 for physically meaningful results. Values bigger than 1 make it impossible for GI algorithms to converge to a stable solution."

What is this referring to in terms of a "stable solution"? 

I'm comparing interior scenes using the Physical Sky to the same scene in VRay. Even with say 8 ray bounces and portals, the Arnold version is a lot darker unless I increase the indirect contribution of the Sky/Sun to say 4 instead of 1. At that point its similar to VRay.

I could add Quad lights to help out but that's more lights to trace and more work to setup.

So, what's the downside to increasing the indirect value? Artifacts? 

[CiroCardoso3v]

You need to increase the Ray Depth on Diffuse for get more light inside

This is going to increase the rendering times. Don't forget that Vray by default uses GI cache (or something like that) to produce faster results, but as soon you switch to brute force it does behave the same way as Arnold. Are you using a Skydome with the Portal mode Set to Interior Only or Interior\Exterior?

[mckenzie2VER2]

Thanks for the response.

To give a little context I've been comparing the results of VRay, Redshift and Arnold on some interior scenes. I can get all of them pretty close but it's hard to beat VRay sun/sky model for interiors, seems to give much better colors at twilight and sunset.

Thing is, I like Arnold, so here we are.

I've tested physical sky/portals vs distant light and the physical sky is just to noisy by comparison. The scene already had a diffuse ray depth of 8 with an increased ray depth total. Using ACES on C4D with light gray shader on everything.

In the end I used a distant light for the sun and a quad for a little fill. I used the skydome with portals for the exterior view through the glass but turned the specular, sss and indirect contributions off.

That's a lot more work that just putting a skydome in the scene.

Lastly, in VRay and Redshift you can change the overall exposure via the cameras aperture, shutter etc. How should this be done in Arnold? Via the camera tag or the exposure imager or are they one and the same?

[CiroCardoso3v]

Interior scenes need some extra work in Arnold due to the brute force nature. Usually, I use the Skydome with an HDRI map, set to Interior\Exterior and some portals. To give an extra hand, I had some quad lights to help bounce light around, similar to what a Director of Photography would do in a real set.

I personally control the camera exposure with the Exposure Imager. If you use the Exposure Control in the Environment and Effects window, you are automatically adding a Reinhard Tonnemapping to the render and you have no control over it.

[lee_griggs]

Could you post some renders with settings to get a better idea of what you are seeing?

[mckenzie2VER2]

Thanks for the info. I photograph architecture and interiors for a day job and yup, quite familiar with having to using fill light unless I get lucky with tall ceilings and skylights 🙂 The scene I'm trying out would be a nightmare for real and I absolutely would use extra light to fill it out.

I guess I'm trying to find a balance between look and render time.

I've attached a few screen shots.

Pic 1: Vray with sun/sky, sRGB and 80% grey.

Pic 2: Arnold with sun/sky, portal, sRGB, 80 grey, 4 diffuse bounce. Exposure imager +1.5 stops

Pic 3: As above with, portal, ACES, Exposure imager +2 stops

Pic 4: Histogram of Pic 3.

Looks to me like Arnold is compressing the highlights, certainly looks like that in the histogram. Why would it be doing that? I'm using C4D. No tone mapping imager added.

[mckenzie2VER2]

Last pic.

[lee_griggs]

Not sure which images are which but a tone map imager (using a LUT) would definitely help here. Has ACES color management been setup correctly?

https://help.autodesk.com/view/ARNOL/ENU/?guid=arnold_user_guide_ac_post_processing_ac_imager_tonema...

https://help.autodesk.com/view/ARNOL/ENU/?guid=arnold_for_3ds_max_ax_arnold_renderer_ax_imagers_html

[CiroCardoso3v]

I think Vray does apply some tonemapping automatically. If you use Filmic Tonemapping, you are going to see a massive improvement in the image. If you need some LUT, I can share with you some that are free, but the Filmic option will improve drastically the render.

[mckenzie2VER2]

@lee_griggs Using the default ACES config in C4D 2023 and Arnold is set to use the Project settings for color management.

@CiroCardoso3v I have been using both, i.e. Kim Amland photographic Luts in addition to a bit of the filmic imager. My point was that out of the box that they are different. 

That being said, if you have LUTS I could try that would be great. Do you have any examples I can take a look at? The web is very sparse when it comes to ArcViz and Arnold, even on Autodesk's website the article they use to promote Arnold is from 2019. 

[CiroCardoso3v]

Yes, Kim's LUT is the one I use constantly.

Is the 4.55.53 Vray and the 4.55.59 Arnold? I can see a difference between them in terms of colour, but I guess that is related to how the Sun and Sky is implemented.

Unfortunately, there aren't many examples from Autodesk with Arnold in Archviz. When I worked at HayesDavisdon, I used Arnold quite a lot. Here are a couple of examples

https://www.instagram.com/p/CakgMrCv8t9/

https://www.instagram.com/p/CaaHIcis_oY/

https://www.instagram.com/p/CLZiMG7sSIX/

https://www.instagram.com/p/CIyTOQps3ze/

Interior scenes are always tricky to work with, but a good mix of extra bouncing light, LUT and OIDN denoiser produce better and faster renders than some other renderers.

[mckenzie2VER2]

Yes. On my screen the difference is clearer obviously. The Vray (4.55.53) has more contrast and slightly different colors. Arnold is more washed out and the highlights seem clamped.

Those examples are incredible!

From the testing I've done it seems ODIN does a better job of de-noising interiors than Noice with the same amount of samples, particularly on high frequency detail. Has that been your experience?

[CiroCardoso3v]

Ok, I can see it now. Well, it is hard to compare to that detail because we don't know what kind of tonemapping changes Vray is doing in the background. Yeah, I prefer to work with ODIN in general and Noice when rendering animations. ODIN sometimes can work well in animations that don't have drastic changes.

The thing with denoisers is that you need to find the balance between rendering time, samples and detail you are getting with the denosier. For example, small details can become murky if the samples are too low.