To preview the effect of light tracing, you can lower the quality threshold.
- To get a quick preview of the effect the Light Tracer will have, lower the values of Rays/Sample and Filter Size.
The result will be a grainy version of the full effect.
- Another way to get a quick preview is to make sure Adaptive Undersampling is turned on. In this group, set the Initial Sample Spacing sampling and the Subdivide Down To setting to the same value. In the General Settings group, lower the value of Rays/Sample, and set Bounces equal to 0.0. This gives a rather blotchy but fast preview of the rendering. Increase the Rays/Sample and Filter Size values to improve the image quality.
In general, you can get good, fairly quick results with a lower Filter Size value as long as Rays/Sample has a high value and Adaptive Undersampling is on.