A difficult sampling case for most unbiased engines, but not for Arion.
|Mode||Lets you select the sampling algorithm to render the image|
Two options are available:
|Mutations||This value basically defines how often a failed mutation is completely dropped in favor for a fresh sample. It is more commonly known as the 'maximum consecutive rejections'||For example the default setting of 65% means that 65% of the failed mutations will be dropped and a brand new random path will be fired, even if the next mutation could have been successful. This default has been analytically determined to be the most efficient in a variety of situations. Using a too low value will result in a very low amount of consecutive mutations, making the render look almost like plain path tracing, while a very high setting will mutate every single time until the maximum number of consecutive rejection is reached (internally fixed to 1000)|
MLT should be used with care and only on problematic scenes with plain brute-force path tracing. Mutations are very likely to fail on scenes mostly based on lambertians and glossies. When a ray hits a rough surface, its bouncing direction is quasi-random, so the mutation of that path is very unlikely to stay similar to the previous one. MLT should be used on scenes with specular surfaces and caustics, where it will shine in all its glory. A nice property of MLT is that the difficult scenes more or less converges equally and clean the same everywhere.
Below is a comparison of path tracing vs. Metropolis sampling on a very hard case for a path tracer: Volume light and volume caustics. Both images were rendered for the same time:
Path tracing sampling here has a great difficulty to find the light paths efficiently.
Metropolis sampling does a much better job on those very difficult cases.
GI bounces / Exit color
Arion v2.5.0 introduces a new, non-physically correct way of improving your render times by reducing the maximum number of times a ray can bounces off a specific material quality before being killed (and returning the color black). This can immensely improve the rendertime in exchange for a controlled darkening of areas where a high number of bounces is necessary to reach full global illumination. To help controlling that undesired darkening, Arion also provides Exit colors controls to compensate for it. Instead of returning black, the engine will return the specified Exit color. This comes for free in term of rendertime since it's just replacing black by another color.
Exit color is also a great way to find out exactly how many bounces you 'need' to achieve good global illumination. By enabling a flashy color, like the default fluo green, you can see very clearly if a particular kind of materials (i.e. glossies, specular, diffuse...) requires a lot of bounces or very little. Keeping the amount of green in an acceptable range is where the best speed gain vs. quality loss lies. If the image turns out all green, there is a need for more bounces on this particular threshold, if it does not return any green at all, it means that you have set a number of bounces that's higher than necessary.
Using the Exit color to determine the best speed vs. quality ratio.
left: Full GI - Middle: too low number of bounces - right: acceptable number of bounces
|Specular||Defines how many times a ray can bounce of a specular material consecutively before returning black or the specified Exit color||Specular materials are those in the roughness range defined by the Roughness threshold parameters|
|Glossy||Defines how many times a ray can bounce of a specular material consecutively before returning black or the specified Exit color||Glossy materials are those in the roughness range defined by the Roughness threshold parameters|
|Diffuse||Defines how many times a ray can bounce of a specular material consecutively before returning black or the specified Exit color||Diffuse materials are those in the roughness range defined by the Roughness threshold parameters|
|Refraction||Defines how many times a ray can bounce of a refractive material consecutively before returning black or the specified Exit color||Refraction exit color can also be controlled per material in the material's root settings|
|Scattering||Defines how many times a ray can bounce of a Sub-surfae scattering material consecutively before returning black or the specified Exit color||Scattering exit color can also be controlled per material in the material's root settings|
Using Exit color to cut down sub-surface scattering materials
render time while maintaining good visual quality.
Since there is no such thing as specular, glossy or diffuse materials in Arion (they are all done using the same all-in-one BSDF model), it is necessary to define a range of roughness values to separate these kind of surfaces from each other. These thresholds are used by the GI bounces / Exit color settings but also by the AOVs to send specular, glossy and diffuse surfaces to their respective AOV.
|Specular||Defines the roughness level under which a surface is considered specular||Anything above this threshold and below the diffuse one is considered glossy|
|Diffuse||Defines the roughness level over which a surface is considered diffuse||Anything below this threshold and above the specular one is considered glossy|