When we did Gouraud shading didn't we interpolate illumination values that
we found at each vertex using screen-space interpolation?
Didn't we just demonstrate that screen-space interpolation is wrong ?
Does that mean that Gouraud shading is wrong?
Yes, Yes, Yes
Gourand shading is wrong.
However, you usually will not notice because the transition in colors is very
smooth
(And we don't know what the right color should be anyway, all we care about
is a pretty picture).
There are some cases where the errors in Gourand shading become obvious.
When switching between different levels-of-detail representations