Hi Junhwan, > One more confirm. Is the "t" in ray["t"] the built-in field (like Density) > or the derived field that I need to make in my own? To be pedantic, "t" is a derived field, but it's already defined for you for ray objects, so you can just use it. -- Stephen Skory s at skory.us http://stephenskory.com/ 510.621.3687 (google voice)