Stitch multiple images using OpenCV (Python)
computer-vision, image-stitching, opencv
Solution
Step by step, assuming you want to stitch four images `I0`, `I1`, `I2`, `I3`, your goal is to compute homographies `H_0`, `H_1`, `H_2`, `H_3`;
- Compute all pairwise homographies `H_01`, `H_02`, `H_03`, `H_12`, `H_13`, `H_23` where homography `H_01` warps image `I0` into `I1`, etc...
- Select one anchor image e.g. `I1` which position will remain fixed i.e `H_1` = Identity
- Find image that better align with `I1` based on maximum number of consistent matches e.g. `I3`
- Update `H_3` = `H_1 * inv(H_13)` = `inv(H_13)` = `H_31`
- Find image that better matches `I1` or `I3` e.g `I2` matching `I3`
- Update `H_2` = `H_3` * `H_23`
- Same as above for image `I0`
- Do bundle adjustment to globally optimize alignment
See section 4 of this seminal paper Automatic Panoramic Image Stitching using Invariant Features for an in depth explanation.
Problem
Hi have seen a lot of tutorials how to do simple image stitching using two photos and that is no problem. But what to do when I want to make a panorama from 4-6 images or more? I have code that takes in list of image files(the images are in order from the first image in the sequence to the last). Then for each image I compute the SIFT feature descriptors . But then I am stuck, for two images I would set up a matcher using FLANN kd-tree and find matches between the images and calculate the Homography. Similar to this tutorial http://docs.opencv.org/trunk/doc/py_tutorials/py_feature2d/py_feature_homography/py_feature_homography.html#py-feature-homography But in stead of showing the lines between feature points at the end I have used this https://stackoverflow.com/a/20355545/622194 function to make a panorama from 2 images. But I am not sure what to do when I want to add the third and the fourth image to the panorama. EDIT: From the answers I have tried to implement my image stitching script to calculate a homography matrix between images that are next to each other in the image sequence. So if I have I1 I2 I3 and I4 I now have H_12, H_23 and H_34. Then I start by stitching I1 and I2 using H_12. Then I want to find cumulative homography to stitch I3 to the current panorama. I fing H_13 = H_12*H_23 and stitch the image 3 to the current panorama but here I get very apparent gap in my panorama image and when next image is stitched it is even bigger gap and the images is very stretched. Can anyone tell me if I am using right approach for this or can someone spot the error or see what I am doing wrong.