Making Good with Ray Tracing Part 2 – Biased or Unbiased…

by Cedar Thokme

Visual software engineers have two methods of computing ray tracing: Biased / Unbiased. Deciding speed/efficiency versus accurate effects in the render is one of the critical issues. So, what is Biased Rendering and what is Unbiased Rendering?

Exterior Visuals and Interior Visuals

From an artist point of view, minus all the computing jargon, if you are rendering lights in a scene that are ‘out there in nature’ , you might choose to have light rays act as close to nature as possible so the latest Unbiased path tracing is the best option, since as the name implies, there is no ‘biased’ selection of say, number of rays, direction or frequency. Countless rays are shooting randomly in all directions and bouncing and reflected or refracted and re-bounced or re-transmitted, like in nature’s light. Think …’Light Cache’ for ‘Global Illumination’ – the unbiased method creates a general light dispersal effect.

However, for interior visualization, the artists may now wish to play with the lights and the mood of the scene so Biased path tracing now works best as it gives you some control over the frequency of rays and the direction they are shooting e.g. rays are shot toward the light source (and rays can be restricted from shooting anywhere else). You could say then that the light acts quite specifically, and skillful artists can use ‘shot cuts’ to achieve photo-realism or add moods to the scene.

finalRender is a biasHybridTM that allows for both Biased and Unbiased ray tracing controls. Difference between Path and Ray tracing? That will be another blog article.

biasHybrid TM and Caustic Effects in Rendering

Caustic is a great way to simulate reflection and refraction properties bouncing from object / object(s) at random when illuminated by lights. In rendering visuals where there are one or more light sources and object(s) of varying translucence, caustic effects are critical in making the scene photo-realistic. In finalRender biasHybrid, in the case of caustic effect the unbiased path tracing is used which makes it trickier but speed-accuracy had to be optimize. Hence, the enhanced fR-AreaLight helps create well-defined caustic effects.

In the simulation above an area light was used to create the well defined caustic effects. The image above actually represents indirect illumination (GI), only caustic effects.

Light rays are followed along their paths and eventually they will start to 'built up' or focus in some areas more than in others. Such an effect will create those recognizable light patterns we know as caustics.

Without / With Caustic Effects (finalRender Unbiased path tracing)

Normally known as 'Surface Caustics' in CGI. Surface caustic are a natural phenomenon produced by one or more focal points from multiple light rays. Mother nature doesn't differentiate between caustics created by reflection or refraction; the cause is always the same. Light rays that collide with a reflective surface bounce off in a specific way and will be directed to a focal point somewhere near to the object. Likewise, transparent objects will bend the light rays and some of them will be directed into one focal point creating a caustic light effect on other surfaces.

finalRender supports all possible types of caustic effects: one from light rays reflecting off a surface like metal, or refracting through surfaces like glass, and another that is generated through the use of Volume Lights (called Volume Caustics). And to enhance even further in terms of capturing realism, functions are sampled many times over known as the Monte Carlo technique.

See ' Making Good with Ray Tracing' Part 1 https://www.cebas.org/blog/

Learn more @ https://vimeopro.com/cebasvt/finalrender-quick-tutorials