Informally, the question is: What's the color at point p? Formally, how much radiance is reflected along the direction of ray w_o, which points to the camera?

Given a $w_o$, we could obtain the irradiance from the texture map (for perfect reflection here)

This is like a world map!https://en.wikipedia.org/wiki/World_map#/media/File:Mercator_projection_SW.jpg