brightness constancy constraint


![1 equation in 2 unknowns](https://www.notion.so/image/https%3A%2F%2Fs3-us-west-2.amazonaws.com%2Fsecure.notion-static.com%2Fc85cc5fa-f02b-471f-88d7-88d6c6df798e%2FUntitled.png?table=block&id=dc6d69f3-686e-4d37-94f8-7a0b5bf42bf6&cache=v2)
$$I_xu + I_y + I_t = 0$$











