> could you please elaborate? Do you know the corresponding point pairs (known_i,target_i)?
Each known_i and target_i point represents an unique named feature of the original image, and their locations (thus, the vectors between any two known_i and target_j point) are fixed on the reference image.
> There are many "transformations", I remember seeing a projection model based on a 4x4 matrix in uni.
The list of possible transformations include translation, rotation, skewing (so all affine transformations) plus barrel distortion, lens distortion etc. Think of a reference version of a poster in an image authoring software vs. the same poster photographed with a potato camera, not necessarily from a head-on orientation.