Drag the tips of A and B. Every mode is the same two arrows — only the question changes.
What to notice. The dot is one number that peaks when the arrows agree and crosses zero at 90° — that is the whole of diffuse lighting. The cross is the area they span, zero when they are parallel, and its sign is "left or right". In Basis the two arrows are the matrix: flatten them together and the determinant hits zero and so does the shape.
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