What is a "Progressive" rendering?
What is an "Unbiased" rendering?
What is the difference between the "Non-progressive" render methods (Easy 1-7) and the "Progressive" render methods (Easy8-11)?
What is the difference between Biased and Unbiased Rendering?
All these questions basically are trying to get the same answer: What render method should I use? What's best?
In basic terms, a light in Twilight will shoot "rays" of light out from the source. These rays will "fall" onto surfaces in the model, bounce around spaces, and become the rendered image.
The Unbiased methods, or "progressive" render methods start calculating the light bounces basically forever... they will start rendering and just keep on going until you stop them. They start grainy and progressively clear up. Depending on the scene, lighting, and materials it may or may not ever become perfectly clear. Generally after 150-300 passes, there will be little noticeable change to the image for the average person viewing.
The Biased methods will generally be much more crisp and clear, but do not calculate the light bounces until they can not be traced any longer... instead, they are generally 'faster' because they are 'biased'... that is, they don't like calculating all those light bounces, and quit after only a few bounces. The number of bounces of lights and reflections, etc, are defined in the render preset. Their second bias is called "interpolation". They interpolate the color of pixels between two known points. The lower settings interpolate the color of the pixel for your image over large distances, and the higher settings interpolate less. This interpolation is based on several calculations discussed here.
So which is best?
In simple terms,
if you want to be able to set any number of lights, with the highest quality materials, and not worry about any render settings at all, use the UNbiased "Progressive" methods. The time you save not worrying about render settings is paid for in longer render times. If you have a very good computer, even this is minimized.
If you want to be able to render an animation, or if you need a fast render of a scene, perhaps the Biased "non-progressive" method will be preferred.
Great effort has been placed into creating some render "presets" for these non-progressive methods, and they do work well in many cases. But they can not possibly work for all situations. Since the majority of people using Twilight will likely be rendering architectural or product shots, the render settings have been developed using scenes of this nature.
For exteriors stills with a lot of direct sunlight, the "Easy 08 Outdoor Progressive" is the best setting. It is progressive, but renders quite quickly in comparison to the other progressive render methods, especially when rendered with a multi-core (multi-thread) processor.
For interiors with unlimited lights, and lots of direct lighting, Easy 09 Indoor Progressive is the best.
For interiors with unlimited lights, and lots of indirect lighting or difficult caustics or other technical lighting situations, Easy 10 Indoor+ will be the best. This method is the slowest to clear.
How do I edit the Biased render method render settings?
A Future release of Twilight will allow direct user editing of the render settings. Until then, one may use the free Kerkythea to create and save render settings, then place these settings files (i.e. "mycustomsetting.xml" ) into the Custom Settings folder in Twilight's render presets folder. Or one can copy any render setting file into the custom folder and edit the .xml file using something like Windows Notepad. Looking at the different Easy 1-7 render settings files and comparing them to see what is different may give a guide as to what you may need to "tweak" for your custom setting.
See this in-depth article: Easy 1-7 Photon Mapping and Final Gather (Biased)
Frequently asked questions about Twilight Render
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