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Concept
Unbiased rendering
Summary
NOTOC
Within the field of computer graphics, unbiased rendering refers to any rendering technique that does not introduce systematic error, or bias, into the radiance approximation. The term refers to statistical bias, not the broader meaning of subjective bias. Because of this, an unbiased rendering technique can produce a reference image to compare against renders that use other techniques. In simple terms, unbiased rendering tries to mimic the real world as closely as possible without taking short cuts. Path tracing and its derivatives can be unbiased, whereas ray tracing was originally biased.
Mathematically speaking, the expected value (E) of an unbiased estimator is the population mean, regardless of the number of observations. The error found in a render produced by an unbiased rendering technique is due to random statistical variance, which manifests as high-frequency . Variance is reduced by (standard deviation by ) for data, meaning that four times as many data are needed to halve the standard deviation of the error; this makes unbiased rendering techniques less attractive for realtime or interactive applications. This means that an image produced by an unbiased renderer that appears noiseless and smooth is probabilistically correct.
A biased rendering method is not necessarily wrong, and can still produce images close to those given by the rendering equation if the estimator is consistent. These methods, however, introduce a certain bias error (usually in the form of a blur) in efforts to reduce the variance (high-frequency noise). Often biased rendering is optimized to compute faster at the cost of accuracy.
It is important to note that an unbiased technique cannot consider all possible paths (because there is an infinite number of them), and may not select the ideal paths for a given render (because to select certain paths over others introduces bias). Path tracing, an unbiased approach at its core, cannot consistently handle caustics generated from a point light source, as it is highly unlikely to randomly generate the singular path that directly reflects into the point.
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