Image alignment with imgalt for stop motion astronomical animations
When photographing Sun with H-alpha solar telescope we can easily see prominences, but showing their movement with time may be hard as it's hard to align all images for a smooth animation. There is however an application for that called imgalt.
The application was published on astropolis.pl where you can download sources (for Linux) or MS Windows binary ready to run.
To compile it under Linux you will nee the boost library. For Debian/Ubuntu based distributions you will need libboost-dev, libboost-chrono-dev, libboost-system-dev and libboost-filesystem-dev. When you have all dependencies installed you can compile the source code with:
After which you will get ./imgalt binary ready to use.
In the case of MS Windows you can put images in the application folder and run it. It will generate new files under new names (BMP and TIFF files only).
As it's a command line application (no GUI) you can control it using a terminal. On Linux that's not a problem, while under Windows I use Total Commander to open the Windows terminal within given folder.
How it works
The algorithm is based on phase correlation and can align images quite efficiently so that animations won't wobble. It can operate on BMP and TIFF files and aside of executing it as is for aligning files withing the application folder we can use command line to control how the application works (input/output folders, cropping, subpixel alignment etc.). Just check ./imgalt --help for options.
When the frames are aligned we can make an animated GIF. I use convert application from ImageMagick software stack:
Parameters are as follow:
I often make the animation with additional reversed order frames so that it has a fluid continuous animation effect (frame list like 1, 2, 3, 4, 3, 2).