Compositing is done during rendering when the source pixels are combined with the destination pixels to produce the resultant pixels. The quality of compositing directly relates to the visual quality of the output and is inversely proportional to the render time. The higher the quality, the slower the render time. This is because the higher the quality level, the more surrounding pixels need to be taken into account during the composite. The linear quality setting (CompositingQuality.AssumeLinear) compromises by providing better quality than the default quality at a slightly lower speed.
Assume linear values.
Gamma correction is used.
High quality, low speed compositing.
High speed, low quality.