Monday, December 10, 2012

How to align image stack

Aligning images in GIMP is not very difficult if we do not have rotation to attend to, if rotation is present we rather leave that task to computer.
Aligning stack of images is not only used in astrophotography, it is common task in creation of HDR images. So, we can use align_image_stack from hugin-tools to align images. Installation procedure on Mint or Ubuntu is simple, we find it:

$ apt-cache search hugin
enblend - image blending tool
enfuse - image exposure blending tool
hugin - panorama photo stitcher - GUI tools
hugin-data - panorama photo stitcher - common data files
hugin-tools - panorama photo stitcher - commandline tools

and after that we install it:

$ sudo apt-get install hugin

It is available from repositories and there is no need to compile it from source. People using different operating system from Linux should visit Hugin website and download installer for their operating system.
Once Hugin is installed align_image_stack should be available and we can align image stack. If you do not have own images in this tutorial you will find download links.
We place in some empty directory JPEGs, if you have RAWs convert them, cd to that directory and execute:

$ align_image_stack -a tif *.JPG

Option -a tif means we want tif indexed output, so after a while we will see tif0000.tif, tif0001.tif and so on. Parameter *.JPG is input list, align all JPEGs. Alternatively we can specify file by file. Once alignment is done we can start GIMP and open all images as layers.

In order to check alignment we can change mode for top layer from Normal to Difference in Layers - Brushes floating window. To check further we make top layer invisible and repeat the same procedure for next layer.

Once we are happy we can stack images as described in previous tutorials or we can use GMI’C plugin. GMI’C is located under Filters and it opens as separate window. We expand Layers, select Average and set Input layers to All and Output mode to New image.

Clicking Apply or OK button will create new image. If you have new version of GIMP Median is also interesting. Average produces two layers and Median only one, we can stack them again. For final processing you may like to do some contrast stretching, maybe some curves as well.

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