More about stacking

In the last tutorial we stacked few frames using GIMP after manually aligning them. Since we were setting layers to screen mode it was at the end quite bright. What we could do to make it more natural is to make additional layers transparent. Opacity of bottom layer 100%, next 50%, next 25% and so on.

Automated stacking

Here Windows users have really wide choice of free or paid programs. For example Deep Sky Stacker and RegiStax. If there is dozen or more frames to stack it pays to start KVM or VBox and struggle with Windows for for few minutes in order to use Deep Sky Stacker. There is no real equivalent which will do the same task on Linux, register, align and stack frames using GUI. For smaller number of frames we can use ale - The Anti-Lamenessing Engine written by David Hilvert. Typically it is not compiled with ImageMagick support enabled, so we have to prepare images to be processed an to convert them into ppm format. We can install ImageMagick and use mogrify and convert to convert photos from terminal, but in order to make things easier we will use GUI.
If we are using DSLR camera or some of those new compact cameras we may be able store images in RAW format. It allows us to do significant amount of processing on such image. There is few really good programs for RAW processing, like UFRaw, RawTherapee or Darktable. We are going to use the last one Darktable. After installation we will right click on RAW file and select from context menu Open With Darktable. User interface is non-conventional so here is quick explanation. We want our RAW to be converted to ppm and we want to remove hot pixels.
Proper removal of hot pixels would be taking “dark frames” at the end of the session. That is place cap on lens and take picture using the same ISO value and exposure time as data frames. After that we can add dark to data frame as layer in GIMP and subtract it to remove hot pixels.
When Darktable shows-up we will see in the right pane tab which says more plugins. Clicking on it we open it and select hot pixels plugin. Again clicking on it we close it and under correct tab is hot pixels plugin, which is "switched off". We "switch it on" and it removes hot pixels.

We are happy with all default processing so far. Now we want to export image to ppm. For that we hit key “l” what brigs us to lighttable mode. On the right pane we locate export selected and make it to look like this:

Export could be achieved via export button or keyboard shortcut Ctrl+e. To go back to darkroom mode we hit “d”. We repeat process on all frames. If we added some exposure, EV or maybe two, it is likely that we have generated noise. Also if we do not want full size picture we may want to resize it, ale will finish processing much faster. In GIMP we do Filters -> Enhance ->Wavelet Denoise and here is how it looks after and before denoising:

If we want to resize that is Image -> Scale image, and we export image back to original ppm.
Now we can open terminal and cd to folder with ppms. This is what I did and what was output in terminal

ale --md 64 --dchain fine:box:1 *.ppm stacked.ppm
Output file will be'stacked.ppm'.                                 
Original Frame: 'img_0001_01.ppm'. 

Supplemental Frames: 
'img_0001_02.ppm'***** okay (88.869874% match). 
'img_0001_03.ppm'***** okay (90.897255% match). 
'img_0001.ppm'***** okay (91.984785% match). 
Iterating Irani-Peleg. 
Average match: 90.583972% 

Here is explanation --md Set element minimum dimension x. (100 is default),  --dchain fine:box:1 approximates drizzling. and two remaining params are input and output. To get more info execute ale --hA.
Result was rather cold and dark. In order to bring some warmth and dynamics I opened stacked.ppm in GIMP and did Colors -> Components -> Decompose where from drop-down LAB is desired. Now for each layer I made only one which I am currently working on visible and others invisible, clicking on eye in layers docking window. Then after duplicating layer and setting copy layer to Overlay mode, I merged visible layers accepting default option expand as necessary. That was repeated for L, A and B layer.

After that Colors -> Components -> Compose, again selecting LAB. At the end Colors -> Auto -> White Balance and Edit -> Fade Levels with default Replace mode and Opacity 33. Here is result:

Those are the same four frames from last tutorial, if you do not have own to process, download them, convert them to ppm and you can try ale stacking and post-processing with them.