Quote - > Quote - Hint.

The Diffuse node formula (roughly) is:

Diffuse_Color * Diffuse_Value * Cosine(alpha) * LightColor * LightIntensity

That is only for one light. When there are multiple lights, that is evaluated for each light and these are all added together.

Which is a bit different to:
diffout = clr * I

Rethinking everything. I tend to look at details and fail to abstract the core equations. Something wrong with me, there.

Well, not different. I was leaving out irrelevant details. I used the letter I to represent

Diffuse_Value * Cosine(alpha) * LightColor * LightIntensity

diffout = clr * I

or more fully described:

diffout = clr * (Diffuse_Value * Cosine(alpha) * LightColor * LightIntensity)

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