With Advanced Render’s powerful caustics engine, you can create realistic patterns of focused light such as the bright patches that a whiskey tumbler casts onto a wooden floor. For a general introduction to caustics, including commonly-used terms, look up What are caustics? in the index.
You use three dialogs to set up caustics. On the Caustics page of the render settings, set the basic properties of the caustics effect such as its strength. On the Caustics page of each light source (Attribute Manager), refine the effect for light sources. On the Illumination page of the Material Editor, fine-tune the caustics effect for any material. Note that you can also access the caustics settings for materials via the Attribute Manager, provided that you have selected the desired material.
What are caustics?
Caustics are patterns of focused light created by curved surfaces that are reflective or refractive.
Advanced Render enables you to create both surface caustics and volume caustics.
The surface caustics algorithm
For surface caustics, you need:
Caustics are calculated in a very different way to global illumination and raytracing. Global illumination and raytracing send rays from the camera into the scene. With caustics, however, photons are sent from the light source into the scene. Each photon is given a starting energy. A photon’s energy is reduced as it passes through the scene and is reflected or refracted by object surfaces (Figure 4).
When a photon hits a reflective or transparent surface, the photon is reflected or refracted. The photon loses some energy in the process and the material’s properties are taken into account.
For example, if the surface reflects or refracts blue light only, the photon’s color changes to blue. If the photon still has some energy left at this point, it continues to fly through the scene. When the photon hits a surface that is neither reflective nor transparent, the photon ends its journey there and its values are stored in a data structure called the photon tree — later, the values in the photon tree will be interpolated to calculate the caustics effect.
Once all photons have ended their journey in one way or another, the values in the photon tree are interpolated to calculate the surface caustics effect for the image.
Double-caustics are possible. This happens when an object focuses caustics onto another object that generates caustics. See Figure 5.
The volume caustics algorithm
Volume Photon caustics are calculated in a similar way to surface caustics. First you need the following:
As with surface caustics, when a photon hits a reflective or transparent surface, the photon is reflected or refracted. The photon loses some energy in the process and the material’s properties are taken into account. For example, if the surface reflects or refracts green light only, the photon’s color changes to green. Provided the photon still has energy, it then continues to fly through the scene.
In contrast to surface caustics, the photons of volume caustics are saved in the photon tree at regular intervals. When the photon hits a surface that is neither reflective nor transparent, the photon ends its journey there. Once all photons have completed their journey, the values in the photon tree are interpolated to calculate the volume caustics effect for the image.
Troubleshooting Surface Caustics
The best Photon caustics settings depend on factors specific to the scene such as the distance between the light source and the object that is generating caustics. However, the following steps should help you to achieve a good result.
Troubleshooting Volume Caustics
If you notice artifacts of the type illustrated below when rendering volume caustics, on the light’s Visibility tab, try reducing the Sample Distance value. This will, however, add to the render time.