If i have a large image and i resample it to a smaller size. How can i apply the same transformation to the coordinates in the larger image. Specifically, if I resample an image to a smaller size, what are the new coordinates of the points in the larger image and how can i get them in the new coordinate system. It seems that multiple coordinates in the larger image should get mapped to the same coordinates in the smaller image. But i have no idea how to go about getting the transformed coordinates
It depends on the library you are using. ITK and SimpleITK have two sets of coordinates: physical coordinates which are independent of image details, and index coordinates which depend on image's size and buffered region. Physical coordinates will be the same in both the high-resolution ("big") and low-resolution ("small") image. To get index coordinates from a physical coordinate point p you use imageBig->TransformPhysicalPointToIndex(p) or imageSmall->TransformPhysicalPointToIndex(p).