Troubleshooting Photon Effects

 
 
 

The following will help you optimize your scene's lighting effects, as well as troubleshoot common problems. Many of the tips and optimizing methods described in Troubleshooting Lights can be applied to lights transmitting photons.

NoteKeep in mind that speed optimizations usually sacrifice quality, while quality optimizations usually decrease speed.

Photon Tips

  • The first render — The first time you render photons, the render takes a little longer than usual. This is because the Photon Map is being calculated for the first time.

  • Number of receivers — The fewer objects you have defined as photon receivers, the faster the photon render.

    If not necessary, avoid defining every object in a scene as a receiver. For example, if a room's ceiling is not in the camera's frame, do not define it as a receiver.

  • Start small — When tuning a global illumination effect, start with small values and work your way up. Try using few Emitted Photons so they are clearly visible.

    A value of around 1000 should be good to get you started. Notice how the effect smoothens as you add more and more photons.

  • Use a high intensity — When tuning a photon effect, start with a high Intensity value (in the light's property editor). A value of approximately 40 000 makes your photons immediately visible.

  • When to use final gathering — Try not to use final gathering in scenes that have complex volumic effects or many speculars. Final gathering works best in scenes with diffused illumination.

Optimizing Photon Effects

Lights (Speed Optimization)

The number and placement of lights have a large impact on rendering speed:

  • If you are not creating a photorealistic scene, consider making some lights not emit photons.

  • Make sure that photon light rays can easily reach the caustic receivers or diffuse materials so that photons aren't wasted or lost unnecessarily.

Using Global Illumination (Speed Optimization)

Global illumination simulation can be very time consuming to render. Unless you require effects like color bleeding from diffuse interreflection and/or caustic effects, you should consider using ambient light or final gathering to imitate a global illumination effect.

Using Caustic Effects (Speed Optimization)

Caustics are a subset of the complete set of effects achieved by global illumination simulation. If you want to see only caustics, activate only the Caustics option for the render pass where they are used (do not activate Global Illumination as well).

Troubleshooting Photon Effects

Here are some common problems and their solutions.

My photon rendering won't launch

Make sure that you have defined a caster, receiver, and photon light source. An error message appears at render time if you have forgotten to define one of these elements.

NoteKeep in mind that if you don't specify either a caster, receiver, or photon light source, the render options reset the Enable Global Illumination and/or Caustic setting to off.

Why is my photon rendering taking forever?

  • If the render seems to be taking much too long, you may have specified too many Emitted Photons.

    Try lowering the value until you see an effect, for example, down to 1000.

  • If you are using global illumination, caustics, and final gathering all at the same time, your computer may be taxed beyond its limits.

    Try deactivating two of the effects, adjusting the settings of the active effect, and then adding the others. You may want to try adding each effect in a different render pass so your system doesn't crawl to a stop.

My render seems to abort just before or after the first frame

This problem is most likely due to an invalid path specified for the Photon Map.

Open the mental ray Render Options property editor and click the GI and Caustics tab. At the bottom of the tab is a text box with the path to the Photon Map file. You can select a new path by clicking the (...) Browse button.

My global illumination/caustic effect is too soft

Locate which light is creating the soft effect and try to increase the Number of Photons from the light shader property editor (choose Modify Shader).

My global illumination/caustic effect looks splattered

This may occur if too few photons are hitting an object. Usually the photons are hitting another object first and then the effect becomes uneven as some of the photons bounce to the desired location, but others don't.

You can fix this problem by either increasing the Accuracy parameter or the Number of Emitter Photons parameter.

My caustics don't have the sharpness I was expecting

Make sure that the caustic caster has the right shape to create sharp caustic effects. Usually, you'll want to use a curved object or an object with varying thickness.

It is also recommended that the object has an index of refraction of at least 1.33 or 1.5. The caustic receiver should also be at an appropriate distance (too far or too close results in no effect or a blasted one, respectively) and the light itself should be bright enough to create the effect.

My changes don't have any effect

Make sure that Rebuild Map is on.

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