# cross ratio - how to calculate values with it?

I have this scenario here. It is a projective image.

1. What would be the cross-ratio formula for these points?: $$v,c_{r},r_{2},r_{1}$$

2.And let be $$dst(r_{2},r_{1})=30cm$$ in the real world. How can I calculate the other values (distances) between the points given in 1.?

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