How to convert from world coordinates to pixel indices in Matlab

Question

I have 512x512x313 volume of dicom images and i have a point represented in world coordinates say (57.7475 63.4184 83.1515), how could I get the corresponding pixel coordinate o

Answer 1

I hate to burst your bubble, but what you are asking for is impossible. The only way that I can think of where you are able to get a correspondence between real-world co-ordinates and pixel co-ordinates is if you calibrate the camera that was used to capture the images. Once you know the intrinsic and extrinsic parameters, you then have a transformation matrix that can map real-world co-ordinates to pixel co-ordinates.

I'm assuming you don't have calibration information for your camera, and so an alternative approach would be knowing which pixels in your image map to which real-world co-ordinates. You would need to know point correspondences between those points that map between the real-world and to your image. Once you know this, you would then compute the camera transformation matrix through least-squares and then you would use this matrix to determine which points map from the real-world to your image.

Unless you have pixel correspondences to each of your real-world co-ordinates, it is impossible to do what you're asking.

FWIW, if you want to see the procedure on how to obtain the transformation matrix, check out these notes: http://www.peterhillman.org.uk/downloads/whitepapers/calibration.pdf. This was a great starting point for me when I started learning about camera calibration. Take a look at Section #5 (Page #8), as this is what I believe you are looking for.... but you will need to have correspondences between the real-world co-ordinates and your image.

Good luck!