Mental Ray: Caustics



"Caustics" is the term for the light patterns caused by light refracting through a glass object.  For example, take a clear glass of water and set it out in the sunlight.  The bright streaks and patterns you see are caustics.

3ds max 6 now comes with Mental Ray.  Mental Ray is a renderer that can calculate photons, creating Global Illumination and Caustics lighting, which will be explained in this tutorial.

This tutorial mainly focuses on refractive caustics caused by light going through an object.  There is also reflective caustics, such as the light bouncing off of shiny things.

For this tutorial, you will need:

    *

      3ds max
    *

      Mental Ray (comes with 3ds max 6)

You'll notice the glass is the same one from my Lathe tutorial.  The plane is just a normal plane with a Perlin marble texture for effect.

When you open the scene, you'll notice a wineglass and a plane. Render, and you'll get something like this:



First of all, the wineglass isn't even glass.  Mental Ray's photon effects can only work on objects that are Raytrace material, have a Raytrace Map or Flat Mirror applied, or are one of the special Mental Ray materials (e.g. Glass Physics_Phen).

So we'll make the material, which should be pretty basic.



For other scenes, you should probably adjust the Reflect box for reflections, and play around with the Specular highlight, but we'll just make it 100% transparent.



Well, it's glass, but not much else.  Now we'll make the caustic generators.

First, we need to change the renderer to Mental Ray (of course). The default Scanline renderer can't do photons.



Now we need to make special lights that can emit photons.

Go to Create > Light > MR Area Spot and drag in the Left viewport to make a light.  It'll render something like this:



There's another property we need to set before we can have caustics.  We need to tell Mental Ray what lights and objects we want to make caustics with.  Select the wineglass and the light and right-click.  Go into Properties and go to the mental ray tab.  Turn on Generate Caustics.



We also need to turn on caustics for the scenes itself.  Go into the render menu (F10) > Indirect Illumination tab and check Enable for Caustics.



We should now have caustics.  Render, and you'll get something like this:




Notice that I changed the position of the light a little to show the shadow better.

Well, we made caustics, but it could be better.

For example, the light is kind of dim.  It's probably due to my bad units setup, but we can make it brighter.  Go into the Indirect Illumination tab of the Render menu again.  Scroll to the bottom and find Global Light Properties.



We can simply crank up the Energy field.  I raised it all the way to 300,000, and got this:



Hmm.  Well, it's bright, but the doesn't look to good because it's spotty in places.  You can also increase the number of Caustic Photons, but this will increase the rendering time.

I raised it to 50,000 and got this:



Cool.  Now, you can add a couple aesthetic touches.

Real glass isn't so perfectly round and smooth as my NURBS wineglass.  We can make a subtle bump map.  I just added the default Noise map into the Bump Map slot, and lowered it to 4.

I got some nice results:



Alternatively, you could try MR's Glass (physics_phen) material.  It can produce some good effects, although it's different from the usual Raytrace material

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