Mosaic of contactless fingerprint images by string search in Matlab?

I am trying to enlarge the effective area of ​​a fingerprint by trying to stitch together two fingerprint images (left and center) taken by two different cameras. Images are attached below. I've tried the standard image mosaic process and it gives the wrong result. The reason I feel is that the two images contain the entire fingerprint, not part of the fingerprint.

Center image:

Left image:

Mosaic image:

The solution to this problem was explained in the following paper .

In this they tried to get a line and mosaic of images based on it.

I would appreciate it if someone could explain to me the process of getting a string in Matlab?

EDIT

Code

[x1,x2,x3,x4]=findroi(matches_geo,frames1,frames2);


   function [x1,x2,x3,x4] = findroi(matches,f1,f2)
   for i = 1:size(matches,2)
      xpos1(i) = f1(1,matches(1,i));
      xpos2(i) = f2(1,matches(2,i));
      x1 = max(xpos1);
      x2 = min(xpos1);
      x3 = max(xpos2);
      x4 = min(xpos2);
    end

      

        
        
        
      

    

Negotiation is done based on sift parameters. I am getting a rectangle by finding the maximum and minimum x-coordinate values. The next step is to find the seam line to combine the two images.