In RenderMan, you can control the opacity of an object using the Presence Parameter on the materials. Typically designed to work as a 0 or 1 "cutout" effect, values in-between can be used for semi-transparent objects or ghost-like effects.
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If using a value between 0 and 1 (or a pattern with middle gray values) you will notice noise by default since we use a stochastic sampling technique. However, integrators allow for smoother effects by specifying a depth in the Stoachastic Stochastic Opacity Depth parameter. Where each intersection counts as a depth limit. This will be explained below. If rendering thick glass, presence is ignored as we need to know when we enter and exit a volume. A cutout here could cause issues with that condition. |
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This is the traditional way to use the parameter. But you may wish to use partial presence values in rendering for something semi-opaque. By default, we use a stochastic method to determine presence. This means we randomly sample at varying levels of presence when the value is not a 0 or 1. This can generate noise and require more samples to clear up or converge. Below we have a set of overlapping planes set to a Presence value of 0.15. Notice that lower presence values will show more noise than higher values.
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- If using cached, decrease the micropolygon length to create smaller microplygons micropolygons and therefore more detail
- You may switch from cached to non-cached and instead evaluate with each shading sample, this could be expensive but it is "easiest" as a solution
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