Greyscale shading

Hello, I'm trying to make a shader that gradually spreads greyscale across the scene from a single point. I tried putting the shader material on every object, and it has the result shown on the picture below.

Can anyone help me make it so instead of greyscaling every seperate object its bottom half, it greyscales whatever part of any object is on the bottom half of the screen?

So in that case the floor would completely greyscaled, the lamp would be completely coloured, and the curtain would be about 50/50.