What is A-vector photography in the direction of C-vector?

Refers to the projection length of a vector on the included angle of b vector.

Projection is the projection length of one vector on the included angle of another vector. Given non-zero vectors A and B, the included angle is θ, then the projection length of vector A on vector B = | vector a|*cosθ, where | vector a| is the modulus (size) of the pointing quantity A.

Projection theorem, also known as Euclid theorem: in a right triangle, the height on the hypotenuse is the proportional average of the projections of two right-angled sides on the hypotenuse, and each right-angled side is the proportional average of the projections of this right-angled side on the hypotenuse. Projective theorem is an important theorem in mathematical graphic calculation.