ImPPG version 0.4 – limb alignment and FITS support

these are all of the great references that our users have shared with others.

Moderator: marktownley

Post Reply
User avatar
GreatAttractor
Almost There...
Almost There...
Posts: 584
Joined: Sat Jun 01, 2013 1:04 pm
Location: Poland

ImPPG version 0.4 – limb alignment and FITS support

Post by GreatAttractor » Sun Jun 21, 2015 6:49 pm

New version 0.4 has been released.
Download from http://stargazerslounge.com/blog/1400/e ... processor/

New features:
- Image sequence alignment via solar limb stabilization
- FITS files support (load/save)
- Zooming in/out of the view

Enhancements:
- View scrolling by dragging with the middle mouse button
- Logarithmic histogram setting is preserved

Bug fixes:
- Tone curve in gamma mode not applied during batch processing


Limb alignment is finally here - for animations that show a lot (the more, the better) of the limb. While the regular phase correlation alignment tends to keep your sunspots/proms/filaments stationary and moves the limb around, this will keep the disc steady (to show the rotation nicely).

Let me demonstrate on my last year's surge animation (Lunt 35); the first link uses phase alignment, the other – limb alignment:

Image
Image

Note that for now it will not work for eclipse shots.

The function expects input images with the disc brighter than the background, without strong vignetting or exaggerated limb darkening (e.g. during processing). So if you want to make them fully or part-inverted (or strongly darkened using low gamma), you must do it after the alignment. However, the images should be already sharpened before alignment (it will work on unsharpened stacks, but not as well).

In other news, FITS format is now supported (including alignment input/output). When saving, you can choose 8-bit, 16-bit or 32-bit floating-point.

Last but not least, the quite obvious feature of zooming the view in/out has been added. You can do it similarly to popular graphics apps: via menu, toolbar buttons or by using Ctrl+mouse wheel. The view can be also scrolled by dragging it with the middle mouse button (usually the wheel).

Screenshots:
mainwnd.png
alignment.png
alignment.png (21.45 KiB) Viewed 1570 times
My software:
Stackistry — an open-source cross-platform image stacker
ImPPG — stack post-processing and animation alignment
My images

SW Mak-Cass 127, ATM Hα scope (D=90 mm, LS50 etalon), Lunt LS50THa, ATM SSM, ATM Newt 300/1500 mm, PGR Chameleon 3 mono (ICX445)

User avatar
marktownley
Librarian
Librarian
Posts: 24375
Joined: Tue Oct 18, 2011 5:27 pm
Location: Brierley Hills, UK
Contact:

Re: ImPPG version 0.4 – limb alignment and FITS support

Post by marktownley » Sun Jun 21, 2015 7:06 pm

This is super! Thanks! I particularly like the align to limb feature, very effective!

I'll keep this here for a couple of days so it gets as much interest as possible then move it to the solar reference library.
Image
http://brierleyhillsolar.blogspot.co.uk/
Solar images, a collection of all the most up to date live solar data on the web, imaging & processing tutorials - please take a look!

User avatar
GreatAttractor
Almost There...
Almost There...
Posts: 584
Joined: Sat Jun 01, 2013 1:04 pm
Location: Poland

Re: ImPPG version 0.4 – limb alignment and FITS support

Post by GreatAttractor » Sun Jun 21, 2015 9:20 pm

You're welcome, Mark.

Some technical information on the limb alignment...

1) Locating the disc

From the start I assumed everything needs to happen automatically. I know (also from my own example) that users are put off when there's too many switches, sliders etc. Sure, at some point you would perhaps be willing to learn all the program's internals and tricks, but certainly not when in the "I've got beautiful fresh data, need to process NOW!" frame of mind. So ImPPG does a pretty good job (not perfect - note the "no vignetting" requirement) of deciding what's the threshold of brightness that separates disc from the background (identified by a quite simple analysis of the image's histogram), without resorting to a "choose background threshold" slider.

Next, it shoots a number of rays starting at the image's centroid ("center of mass") and for each ray identifies a point with the most steep brightness transition (calculated over a small interval). Not all those points are in fact on the limb; some may lie inside the disc, but on an edge of a big sunspot or a dark filament. The decision on which points to discard is based partly on the steepness of their transition, and also on the minimal fraction of neighboring background pixels (that's what we need the treshold for) - a more "topological" criterion.

It took some time to make it acceptably watertight. E.g. one of Michael Wilkinson's images with a white-hot surging AR right on the limb invalidated an older, simpler criterion.

Once we're sure we have a few actual limb points, it's easy to fit a circle to them; I use Gauss-Newton least squares approach which is quick and converges reliably – if there's enough of the limb visible. If there's just a short arc, it may diverge.


2) Aligning the discs

Since we capture the whole timelapse with the same telescope/Barlow/camera, the disc on every image should have the same diameter – correct? So alignment should be straightforward, just find the points, fit circles to them and translate the images to overlap the circles.
...wrong.

It turns out that disc radii differ slightly between images, I guess due to seeing and stacking variability. Not by much, some 2-5%, but when the disc is 1000 pixels wide, this would translate to multi-pixels jumps.

To alleviate this, all the discs are fitted to limb points again, but with the radius forced to equal the average of the radii found before. This unfortunately is still not sufficient. Below is an example path taken by a disc feature at this phase of alignment:
path.png
path.png (25.93 KiB) Viewed 1549 times
Therefore one final step is needed, basically what a human would do during manual alignment: select some high-contrast feature, track its movement across the disc and smooth it out. Note that the track in question can be an elliptical arc (if there's no field rotation) or a more complicated curve (with field rotation). I simplified this and just assumed it's always a circular arc. So I take the subsequent track's points (as in the graph above), fit a circle to them and project them onto this circle. Now the overall movement is acceptably smooth. (Tracking the contrasty feature's movement is performed using phase correlation with a small 128x128 ROI, so it's really quick).

The relevant code can be found in align_disc.cpp and align_proc.cpp (including more detailed comments).
My software:
Stackistry — an open-source cross-platform image stacker
ImPPG — stack post-processing and animation alignment
My images

SW Mak-Cass 127, ATM Hα scope (D=90 mm, LS50 etalon), Lunt LS50THa, ATM SSM, ATM Newt 300/1500 mm, PGR Chameleon 3 mono (ICX445)

User avatar
swisswalter
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 17951
Joined: Thu Feb 06, 2014 4:28 am
Location: Switzerland
Contact:

Re: ImPPG version 0.4 – limb alignment and FITS support

Post by swisswalter » Mon Jun 22, 2015 4:07 am

Hi Filip

thank you very much. I'll dive into it
Only stardust in the wind, some fine and some less fine scopes, filters and adapters as well. Switzerland 47 N, 9 E, in the heart of EUROPE :)

from 7 am - 7 pm http://www.nanosys.ch

from 7.01 pm - 6.59 am http://www.wastronomiko.com some times vice versa ;)

Post Reply

Who is online

Users browsing this forum: No registered users and 3 guests