Mirror light reflection to ground

[Sam_Jay]

Hi all,

I would like to create a mirror that reflects light. Every mirror reflects light, so you can imagine how difficult it is to google that. What I mean is, light hits the mirror, gets reflected and then hits the ground. Please see the attached image of what I would like to achieve. I have no working idea about which material goes well with which renderer. I think it somehow worked back then with Mental Ray and setting an object to be a caustics emitter, but as of Max version 2024 it looks like I have to start from scratch.

It doesn't have to look super realistic, it just has to work. I would like to play around with the shadow and the reflection. If it worked in a live preview instead of just in rendering, it would be awesome.

So my Max 2024 has Quicksilver Hardware Renderer, ART Renderer, Scanline Renderer and Arnold - no Octane, just in case you would suggest this one. There are dozens of ways to create a reflective material, but I'm afraid the material alone doesn't do the trick.

So, how can I achieve this?

Cheers,
Sam 

[wernienst]

Unfortunately, this is currently impossible with Arnold. When using the Standard Surface shader, you can check Enable Caustics which will create some diffuse reflections.

However, to get an effect like in early Mental Ray times you have to do some dirty tricks.

Create a second material, increase IOR to 10.0 and push the Specular value beyond 1.0 by wiring a range map to it. Notice that the material preview will turn to the complementary color.

Now create a Ray Switch material and assign the original material to the Camera and Specular Reflection channels and the second material to all others.

Now you will get something like this:

[wernienst]

This also works with (simple) textures. Wire the original material's texture map to the range input and to the Base and Specular map slot of the second material.

[Sam_Jay]

Wow you're a source of tricks! I played with the values, but I cannot manage to create a sharp reflection.  Increasing the contrast just adds darkness around the reflected light, not sharpening its borders. I also changed the light to sharpen the shadow in hope that would sharpen the reflection, but it didn't help.

In the meantime I tried to fake the reflection by playing with normals (don't help) and another light that creates a white shadow and that is constraint to the position of the actual light and the rotation of the reflective surface. White shadows are invisible, so red has to do it for now until I find a better cheating method. That said, that's misguiding from the post's title and forum category and that solution would only work for a flat surface - not really helpful for the next person with a different geometry.

Anyway, back to the topic. Interesting that Max lost this functionality. What renderers can do this just like that? I suspect Octane can, as I found a great guide on Behance - for C4D, but probably that's not an issue. Maybe there is a 30-day trial... What other renderes could trace photons and do caustics?

Well, definitely thanks for digging into the topic and also making pictures!

[wernienst]

No problem!

I know that both V-Ray and Corona renderers can do photon caustics, but I don't have further experience.

[Sam_Jay]

Thanks anyway. I will do more research if the urgency arises and might do a 30-day trial.

For the moment, I followed the plan of using a second light casting a "bright" shadow which, when the light position is cleverly connected to the object's rotation, works as a fake reflection. During this workaround, I realized that parameter wiring can only be done one-to-one. So a second object needed a second fake reflection light. With some including / excluding, multiple objects and multiple lights give me the look I wanted to achieve. Big workaround.

Just to close the circle with the topic, I am linking the caustics guide I mentioned, and I'm sharing the current state of what I work on, without caustics. So this only works "correctly" with planes, nothing else.

Shadows can't be white, so I went with colors - multiple ones to help me organize the lights.

So, thanks again 🙂

[MartinBeh]

Yes, the rendering term for this is "caustic reflections". 

For this to be effective (reasonably fast and noise-free) the renderer normally needs to work backwards, i.e. not as usual from the camera into the scene, but from the lightsource into the scene.

To quote from the online help:

"Arnold is a unidirectional path tracer and therefore realistic caustics can be expensive to render. This is because caustic effects create a lot of noise and require very high sampling values, which can lead to long rendering times."

I believe the built-in ART renderer can do it in the "Advanced Path Tracing" mode (if you do not want to buy a 3rd party renderer such as  Vray or Corona...). 

Here is a related post: https://forums.autodesk.com/t5/arnold-for-3ds-max-forum/is-caustics-possible-in-3ds-max/td-p/1105198...

Edit: After re-reading, I realize you seem to be more interested in a (realtime?) visualization of reflections rather than a full rendering, is that correct?

Anyway, I found this Arnold documentation (note the introduction bit "At Solid Angle we dislike photon mapping...") and did some test, and this is what I came up with (without denoising):

It is quite slow/noisy, and really hard reflective (noise-free) caustics seem to be difficult to achieve without waiting for hours.