How to achieve a shadow penumbra effect when running Lumen?

Development Rendering
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1

I am trying to achieve shadow penumbra effect when running Lumen. A “workaround” out-of-the-box solution is to increase a local light’s Source Radius >0. For example, setting Source Radius=250 achieves an extremely diffused penumbra. However, this is not desired for a few reasons.

  1. Increasing Source Radius, beyond convincing values for real sources, leads to massive sources that
    1. result in obviously large source reflections
    2. result in potentially ending up in scenarios where unintended lighting effects occur, ex: character jumping “into/through” the source
  2. Negatively affects performance by increasing the number of rays per light

These issues make sense but is there a better or more advisable way to achieve a penumbra effect?

Note: This effect is also achieved with a Directional Light’s Source Angle but is not currently part of my testing.

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Steps to Reproduce
Given Lumen is on:

  1. Place a floor and mesh to occlude the shadow of a light
  2. Place a light, for example shadow casting Spot Light, in the scene
  3. Aim the light at the mesh so that a result shadow is cast
  4. Adjust the light’s Source Radius to observe a change in shadow penumbra
    1. Source Radius = 0 results in a sharp shadow
    2. Source Radius > 0 results in a penumbra that grows as the radius increases
3

Hi, Source Radius and Source Angle are the intended settings for controlling the penumbra. Depending on the scene/usage, you could pair that with adjusting the light’s Specular Scale setting to reduce the size and intensity of how the light is shown in reflections. You could also use the Soft Source Radius to soften the reflection of the light. There’s also the option of changing the lighting channel of the reflective object or the light, so that it’s omitted entirely. I hope that helps, please let me know if you have follow-up questions on this topic.

4

Performance-wise, there are scalability settings/cvars, but some depend on whether you’re using VSM (virtual shadow maps) and how many lights you’re expecting to have in your scene (especially overlapping/shadow-casting lights). VSMs don’t get quite as soft as ray-traced shadows/Megalights; the latter of which is very experimental, but works with both raytraced shadows and VSMs and can help reduce the cost of overlapping lights. When using VSMs, the recommendation is to gradually increase the radius size until you hit visual diminishing returns; try not to push it much further than that, because it’ll affect performance and visual quality. The SamplesPerRay cvar will effectively limit how soft shadows can get, so this can be adjusted in conjunction with the radius size. The RayCount cvar will make the shadows noisier the lower the number is set. Other than that, it’ll come down to being strategic about light placement, overlap, and the settings mentioned in this thread to best suit your performance needs. Would definitely recommend profiling to get a sense of the tradeoff between certain settings and setups in your scenes.

5

Camille, thank you for the response. Is there any advice regarding the performance consideration for increasing Source Radius?