At least one direct light source (a point light or spot light) must emit photons for global illumination or caustics to work.
For more information on direct light sources, see Sources of direct light.
To turn photon tracing
- Select the light source you want to emit photons.
For example, spot light.
- Open the (+a).
- Select the light’s shape tab.
- Set to.
This causes light levels in the scene to decrease in intensity based on the inverse square law (naturally).
- Set the photon emission attributes.
See mental ray attributes for lights (point spot directional area).
- Repeat this procedure for every light source you want to emit photons.
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