How can I visualize the ray in the rending picture with radiance, just like below picture?
There is a program called xshowtrace that works if you have X11 installed on your machine. Beware that setting -ad larger than 10 rays or so will create a visual mess with -ab 1.
If you have a set of ray origins and intersection points generated by rtrace with value and other information, you can turn those into glowing cylinders of various hues using rtrace and put them back into your scene as an alternate method.
-G
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I was trying to remember the name of that program! And yes, this will show you the rays, and another tool called compambpos may be used to do the intersection and ambient bounce depth visualizations Greg’s talking about. These two tools are so fascinating and valuable for understanding light transport. Radiance is more than a tool to just get the answer; it’s a great teaching tool, too!