What is the photography theorem?

There is no photography theorem, only projective theorem. The projective theorem means:

Projection Theorem, also known as Euclid Theorem: In a right triangle, the height on the hypotenuse is the median term of the ratio of the projection of two right-angled sides on the hypotenuse, and each right-angled side is the median term of the ratio of the projection of this right-angled side on the hypotenuse to the hypotenuse. Projective theorem is an important theorem in mathematical graphic calculation. In Rt△ABC, ∠ ABC = 90, and BD is the height on the hypotenuse AC.

Prove the idea:

Because the projection is to scale the length of the original figure (called the height in the triangle), the width is unchanged, and because the area ratio of the plane polygon = the product ratio of the side length. So it is the ratio of the length of the figure (called the height in the triangle).

Then this ratio should be the cosine of the angle formed by the plane. Make a right-angled triangle on two planes so that the hypotenuse and right-angled edge are perpendicular to the edge (that is, the intersection of the plane where the original polygon is located and the projection plane), then the hypotenuse and the other right-angled edge of the triangle are the length ratio of its polygon, that is, the area ratio of the plane polygon. This ratio can be proved by calculating it in a triangle on the plane.