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Energy flows amongst these waves, and our ray tracer computes radiance. This radiance is invariant along a ray, which means it doesn't change as we move along the ray. Thus, we can use 4 values (L(u, v, x, y)) as we slide up and down the ray to compute this radiance. Energy can change in distribution, however -> depending on the surface area upon which we shine light, it can appear stronger or weaker if that area is smaller or larger.