Rays per sample too high

There are several measures of rendering performance other than “how long do I need to wait for the noise to go away”, that can be used for performance debugging, (i.e. determining why a scene is taking too long to render). One of them is "rays per sample" displayed in the renderstamp and in the “Stats” panel of the Corona VFB.

Rays per sample

Each image sample involves casting a single primary ray from the camera into the scene. This ray can then split/generate many secondary rays. The number of generated rays depends on the scene and influences performance - more rays/sample means lower number of passes can be rendered in given time. The number is also directly dependent from the GIvsAA balance and LSM - higher values will result in higher rays/sample. This is normal behaviour, and not a bug. Usual value of rays/sample is about 5-50 for GI vs AA samples set to 16 and light samples multiplier 1. Using UHD Cache results in lower rays/sample and better performance over time.

Very high rays/sample for given GIvsAA balance means the rendering is doing too much work for a single image sample, usually because of an incorrectly set up scene, decreasing the overall performance. Common reasons for high rays/sample include:

  • Too high albedo of walls in an interior scene. Albedo is the percentage of energy a material reflects, and in Corona is the sum of diffuse, reflective, refractive, and translucent components. In reality almost no materials have near-white albedos, and using white albedo (for example 255-white diffuse color) in rendering gives very unrealistic and slow results and cannot be correctly rendered with an unbiased renderer. Try keeping albedos of all your main scene objects under RGB 180, and get whiter walls by brightening light sources.
    See: What is albedo?

  • Excessive use of transparent/refractive materials

  • Complex blend/layered material setuops

To solve the problem, try:

  • Using the UHD Cache

  • Investigating your blend/layered materials - try simplifying them

  • Replacing transparency maps with geometry for foliage

  • Decreasing albedos of your scene objects to at most 70%, or RGB 180

  • Lowering GIvsAA balance and light samples multiplier to the default values