Background
Turning single aerial photos into along-flight-path strip mosaics is an important pre-analysis step for sea ice imagery. Since the amount of overlap between each image is not well known, sticthing the images together ensures that objects in an image are not counted, or measured, more than once.
The ulitmate goal of the process is to produce a strip of orthorectified, geolocated imagery that can be treated as a fairly definitive 'map' of the flight path. However, there's an important obstacle - namely the lack of any ground control points [normally required for this process]. A possible solution is 'fake' GCPs based on camera location and orientation, pixel coordinates and some trigonometry, but we're not quite there yet.
For direct georefencing we also need aircraft orientation data. Prior to 2007 we did not have systems in place to capture airctraft orientation at image capture time, so we also need methods for just stitching images as accurately as possible.
We've tested a number of image stitching tools, each of which has its positives and negatives. Below are the processes we've tried, and the pros and cons. Our main tool-of choice is Hugin, an open-source front end for the panotools library. You'll see it at the bottom of the list here...
Image stitching programs we've tested
- ENVI
ENVI is very powerful and able to do things like direct georefencing for aerial photographs. However, as of ENVI 4.4 there is no way to feed the camera orientation of many images into the system and do 'batch rectification'. Important with the amount of imagery we collect. Further, ENVI will not really cope with older imagery where no aircraft orientation data exist. - Adobe photoshop alignment based on image content
Photoshop CS3 or later has a great image stitching tool, which works very quickly and very well for most image types. However, sea ice imagery is often almost feature-free, or quite confusing. At a glance, Photoshop does a great job, but doesn't allow adjustment if it gets the detail wrong. - Autopano pro
Another almost-awesome tool. Autopano pro is very fast, and allows some adjustment - but we could never get the control we needed to stitch a panorama as if each shot was taken from a different point of view. Autopano seemed very biased toward a single viewpoint panorama construction. - Hugin
This bit of software is currently the 'weapon of choice'. It is a free front-end to the panotools library [also used by Autopano pro], which can be tricky to install and configure. However, it offers great control over how images are aligned, and can be easily talked into stitching a panorama using a 'moving camera'. Its also multi-platform, which means we can develop control points on a local Windows machine, then send the job to a big linux box for the actual image crunching. Very neat...
Stitching
These instructions are based on Hugin v2009.4, and the following two online tutorials:
- Hugin tutorial - stitching flat scanned images; and
- the excellent case study by Joachim Fenkes: Creating linear panoramas with Hugin