You're completely right to question that assumption. It's very much possible to quantify the amount of error, or even just ballpark it. But numerical math is ha

You're completely right to question that assumption. It's very much possible to quantify the amount of error, or even just ballpark it. But numerical math is hard! :) And .. even a raytracer doesn't sum that many numbers, maybe a few thousand[0]? So even then it's still a very reasonable question to ask. [0] assuming a Monte Carlo pathtracer type of thing with 5-10 bounces or so. BTW, do you think you'd ever need more than five? Unless you're rendering close-to-parallel reflective surfaces, but the appearance of those is often as confusing in real life as it is when rendered correctly.