Caustics

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Contents 1 Introduction 2 Recipe for caustics 3 Additional notes 4 External links

Introduction

Caustics are the bright spots of light caused by the refraction or reflection of light from surfaces such as glass, water, metal etc. For this, the software needs to work from the light source into the scene, which is opposite from what traditional raytracers do.

In mental ray, caustics are calculated using a special case of photon mapping. This needs to be enabled in the 'Indirect mental ray render dialog. Also, the scene has to have suitable light sources as well as suitable materials. Finally, at least some objects have to be configured to generate/receive caustics.

Recipe for caustics

So, here are some basic steps to get a basic caustics setup:

1. Model your scene to scale (i.e. set up and use your System and Display Units properly)
2. Use photometric lights if possible
3. In the render dialog, turn on caustics and activate 'all objects generate caustics'
4. Apply a DGS material to the object that should cast caustics and use a non-zero 'Specular' color. If you want an energy conserving material for physical correctness, make sure that the diffuse level and the specular level add up to 100%

Additional notes

• While it is not strictly necessary to work in physical dimensions (scale and photometric lights), it helps a lot to set things up and ensures that you do not get some wild results
• Instead of the DGS material, you can also use the new A&D material, which has the added benefit of being energy-conserving (i.e. you cannot reflect more light than there was incoming light).
• Other materials that support caustics are the 'Glass (lume)' shader or the 'Glass (physics_phen)' material.
• The detail, local intensity and overall look of the caustics (as well as render times!) are heavily influenced by the number of photons, which you can adjust individually for each light or globally in the render settings ('Average Caustic Photons per Light'). Also, the 'maximum number of photons per sample' in the render settings influences how the caustics will look.
• Also see the cookbook section on glass for information about refractive caustics

External links

• Mental Ray Caustics - 101 by Jeff Patton
• Creating Light Caustics Using Mental Ray in 3D Studio Max by Stealth Snake
• Creating Caustics In 3dsmax 6 Tutorial
• Caustics through Mental Ray by Mario Malagrino
• HDRI and Caustics for the Beginner by Rick Timmons
• Max9/mental ray caustic test scene using various A&D materials (simple scene file for beginners) by Jeff Patton
• Caustics by refraction by Mastering Mental Ray

--MartinB 13:03, 19 August 2007 (PDT)