Rotation

General discussion about StarTools.
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dx_ron
Posts: 25
Joined: Fri Apr 16, 2021 3:55 pm

Rotation

Post by dx_ron »

Does rotating an image during processing have any possible detrimental impact?

I don't normally rotated anything, but I recently imaged the Needle Galaxy and my framing had the galaxy at a 45° angle across the frame (which I think is a more dramatic framing choice, but that's neither here nor there).

In order to set a reasonable ROI during Autostretch I rotated the image 45°, then rotated another 315° after completing Autostretch - then cropped back to the original size (the image canvas grew with each rotation). Is there any loss of information fidelity doing that? (if so, could there be more degrees of freedom for the process of defining the ROI?)
hixx
Posts: 167
Joined: Mon Sep 02, 2019 3:36 pm

Re: Rotation

Post by hixx »

Hi Ron,
as rotating in angles different from 90/180/270°does not match pixels 1:1, there might be a detrimental effect.
But I do not see why you would rotate in the first place for Autodev. To define the ROI, you only need to include the Dynamic Range of interest, not the whole galaxy. So You should be fine with just selecting a small bar which includes the core, parts of the spiral arms with H-II regions and dust lanes. To fine tune, opertate +/- buttons - even single pixel changes will make a difference.
Remember, Autodev shall just provide a content-optimized starting point for a global stretch that will allow Contrast, HDR and Sharp modules add stretching locally to work out details.
Clear skies,
Jochen
Mike in Rancho
Posts: 491
Joined: Sun Jun 20, 2021 10:05 pm
Location: Alta Loma, CA

Re: Rotation

Post by Mike in Rancho »

I agree with Jochen on both. If the rotations don't maintain pixel integrity (by being square up), I imagine interpolation is required, and you'd want to do that only once I have to think.

While not ST, in Gimp I've tried to align layers by rotating, moving, and scaling repeatedly, and eventually the layer's detail becomes mushy.

And yeah no need really for a rectangle to tightly fit the entire target. All sorts of creative ways to draw a ROI and grab the full dynamic range that you want to optimize. And because of that you'll often want to include some amount of background sky anyway. And occasionally bright stars too, if you can get them in, as otherwise they can blow up a bit too much. Other settings like outside ROI % may help with tweaking also.
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