How to stack images using G’MIC

After chat on Astophotography Google+ community https://plus.google.com/u/0/communities/118208937662082340807 I concluded that it may be enough interest for 16 bit image processing using G’MIC tutorial and here it is.
G'MIC stands for GREYC's Magic Image Converter and we know it as plugin for GIMP. It is less known that G'MIC framework can be used on it’s own through simple shell scripting or possible C++ or web interface. What is very important it supports 16 bits integers per channel and that is something what GIMP is lacking.
So, what we are going to do here is to take a look at G’MIC tutorial http://gmic.sourceforge.net/tutorial.shtml and load images into stack average them and save them as 16 bit TIFF.
Tutorial says if you are using G’MIC as GIMP plugin and set logging to verbose it will print what it does.

I prefer writing output to log file to suggested running GIMP from terminal (command line for Windows users). Standard path and name for log file is /tmp/gmic_log
Also from tutorial we see that 16 bit processing starts with division with 256 and ends with multiplication with 256. While that represents reduction to 8 bit it is important to note that G’MIC internally works with floating point numbers and there is no loss of precision when we convert it back to 16 bit integers. Once when we are done with processing it is important how we are going to specify TIFF output. G’MIC will pick format from extension but it by default writes 32 bit TIFF. So, to get 8 bit TIFF we need to specify output type uchar and to get 16 bit TIFF we need to specify output type ushort.
Typical operations which we are going to use are:

1.     -gimp_haar_smoothing 0.1,10,2,0,0 what is Smooth [wavelets] under Enhancements
2.     -gimp_compose_average 1,0 what is Blend [average]  under Layers
3.     -gimp_compose_screen 1,0 what is Blend [screen] under Layers

So complete workflow is we pick nicer RAWs export them as 16 bit PPMs using darktable as in http://grumpyoldprogrammer.blogspot.com/2012/11/more-about-stacking.html then we align them using align_image_stack from hugin-tools as in http://grumpyoldprogrammer.blogspot.com/2012/12/how-to-align-image-stack.html and finally we process them with G’MIC.
Averaging layers will produce at the end two layers, if we want we can save them as separate pictures, like this:

$ gmic tif0000.tif tif0001.tif tif0002.tif tif0003.tif -div 256 -gimp_compose_average 1,0 -mul 256 -c 0,65536 -type ushort -output[1] imageA.tiff -output[0] imageB.tiff

or we can stack those two layers in screen mode to boost light and stretch contrast, like this:

$ gmic tif0000.tif tif0001.tif tif0002.tif tif0003.tif -div 256 -gimp_compose_average 1,0 -gimp_compose_screen 1,0 -mul 256 -c 0,65536 -type ushort -output imageS.tiff

If we want to do resizing we can as in G’MIC tutorial use -resize2dx 1600,5 what is bi-cubic resizing to 1600 pixels width.
Instead of averaging we can try other options like -compose_median what is median or any other available filter or combination of filters, we try in GIMP, set logging to verbose and later run it in terminal without loss of data.