I know that in the general case, making this conversion is impossible since depth information is lost going from 3d to 2d.
However, I have a fixed camera and I know its camera matrix. I also have a planar calibration pattern of known dimensions – let’s say that in world coordinates it has corners (0,0,0) (2,0,0) (2,1,0) (0,1,0). Using opencv I can estimate the pattern’s pose, giving the translation and rotation matrices needed to project a point on the object to a pixel in the image.
Now: this 3d to image projection is easy, but how about the other way? If I pick a pixel in the image that I know is part of the calibration pattern, how can I get the corresponding 3d point?
I could iteratively choose some random 3d point on the calibration pattern, project to 2d, and refine the 3d point based on the error. But this seems pretty horrible.
Given that this unknown point has world coordinates something like (x,y,0) — since it must lie on the z=0 plane — it seems like there should be some transformation that I can apply, instead of doing the iterative nonsense. My maths isn’t very good though – can someone work out this transformation and explain how you derive it?
Yes, you can. If you have a transformation matrix that maps a point in the 3d world to the image plane, you can just use the inverse of this transformation matrix to map a image plane point to the 3d world point. If you already know that z = 0 for the 3d world point, this will result in one solution for the point. There will be no need to iteratively choose some random 3d point. I had a similar problem where I had a camera mounted on a vehicle with a known position and camera calibration matrix. I needed to know the real world location of a lane marking captured on the image place of the camera.