I'm a bit confused by the following diagram (7.4) on page 207 of Real Time rendering, 3rd Edition:
The text before and after I think makes a few assumptions and doesn't quite explain what I'm looking at well enough. What I understand:
- dw is the change in steradians/solid angle w
- dA is the area irradiated by the light ray with respect to dw
- n is obviously the surface normal
- The orange line is clearly a light ray
Why am I confused? The light ray is apparently hitting a surface (hence the surface normal pointing away from the surface), and the angle depicted is the angle between the ray hitting the surface, and the surface normal. But what is the circle at dw indicating? My understanding was that the solid angle is easily visualised as a cone apexed at the light source, with the circle representing a patch surface on a sphere enclosing the light source. Why then is that circle in the diagram apparently situated at the light source? It seems to me that the cone in the diagram should be facing the other way.
p.s. I hope the authors don't mind me displaying this diagram. I will remove it and replace it with my own hand-drawn version if requested to do so.