What is Max Sample Intensity (MSI) in Corona for Cinema 4D?
Maximum brightness of the secondary GI samples. This is the tradeoff between rendering performance and physical accuracy.
- Lower values suppress noise but produce a darker image with lower intensity of reflections and caustics
- Higher values produce brighter reflections and caustics, but also more noise in the image.
The default value of 20 works well in most scenes. Value 0 is a special case that enables an unbiased mode (this mode is not production-ready as it will never clear noise in some scenes).
"Max Sample Intensity" is a parameter you can find in Render Settings > Corona > Performance Settings > Speed/Accuracy balance. As its name suggests, this parameter controls the maximum allowed intensity of GI samples. Lowering it will result in the clamping of light bounced off objects, including reflective and refractive caustics, significantly reducing noise at the cost of realism. Increasing this value will reduce the amount of clamping but also increase the noise level. The default value of 20 should work well in most situations.
Setting this value to 0 enables a special unbiased mode where no clamping occurs. This mode will, however, often introduce fireflies that may never converge.
Max sample intensity and exposure settings
Max sample intensity parameter is relative to the current color mapping, so starting rendering with different exposures will result in different max sample intensity values, and in turn, a different amount of bias in the picture. For example, the Max sample intensity value of 10 should result in the same amount of bias in both dim and bright interior shots. But once you change exposure after the rendering has started, you cannot change the already rendered bias.
See: Exposure changes when I start rendering!
Examples
Below is exactly the same scene, rendered with exactly the same settings, except for MSI:
MSI = 20 (default value)
16 passes
The lighting looks realistic. There is not much noise or "fireflies".
MSI = 0 (unbiased mode)
16 passes
The lighting looks natural, but there is more noise and some bright pixels appeared. This happens because the intensity of light bouncing off objects is not clamped.
MSI = 200
16 passes
MSI value is 10 times higher than the default value. It allows more light intensity resulting in more noise, similar to when MSI is set to 0.
MSI = 0.01
16 passes
The bouncing of light is nearly disabled resulting in dark, unrealistic rendering.
MSI and caustics
MSI also affects reflective and refractive caustics:
MSI = 20 (default value)
The balance between caustics visibility and noise level.
MSI = 0 (unbiased mode)
Intensive caustics but more noise.
MSI = 2
Samples are heavily clamped so caustics are barely visible.
