# Rank of image intensity matrix

I've been reading a paper about using Matrix Completion for Photometric Stereo but I am having some troubles in section 2.2 trying to understand why irrespective of the number of pixels and the number of images $$rank(\mathbf{O}) = 3$$ where $\mathbf{O}$ is the matrix with the images linearized and stacked as column vectors.

Can someone give me a hint to understand this?