Zerene settings question

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Chris S.
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Post by Chris S. »

Rik, thanks for your very well-thought-out and useful response. I've taken quite a bit of time to say anything about it because I've returned to your response several times, with thinking breaks in between, in order to thoroughly digest what you wrote. I find it very, very useful. Thanks again!

--Chris

rjlittlefield
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Post by rjlittlefield »

Chris, thanks for the note. I'm glad to hear this helped. It's always a challenge to figure out what to document and how to document it so as to neither overwhelm people with information they don't need, nor omit information that they do. At some point I'll figure out how to get this stuff into the main ZS documentation. Until then, I suspect this thread will become one of my standard references.

I notice also that realjax posted out the tiger beetle he was working on. At least I presume it's the same one. Came out well, in any case!

--Rik

dhmiller
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Post by dhmiller »

"Alignment in ZS is done by choosing one image as master"

Curious how the master image is selected - I occasionally start my stack list with a frame that may not have an area entirely in focus - sometimes it's hard to tell... Is the master frame the first frame in the list, by any chance?

rjlittlefield
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Re: Zerene settings question

Post by rjlittlefield »

The master frame is the first one in the list at the time that the frame-by-frame stacking process begins.

Before that, the entire order of the list may be reversed, if both Scale and "Automatic order" are selected and ZS decides that reversal is needed to start at the narrow end. This decision is made by comparing just the first and last images. With deep high magnification stacks, those two images may be so much different from each other that the choice becomes essentially random. But then Scale should be turned off anyway in those cases, for the reasons discussed at viewtopic.php?p=79878#p79878 .

--Rik

Lou Jost
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Re: Zerene settings question

Post by Lou Jost »

Rik, since the earliest frames and latest frames are usually almost featureless and out of focus, they seem like the hardest to match properly. Could using them as the master introduce distortions or other errors?

rjlittlefield
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Re: Zerene settings question

Post by rjlittlefield »

Starting at one end versus the other does not have much effect on distortion. This is because with focus stacks, each image is registered by comparing only against its immediate predecessor in the stack, so starting at different ends still results in comparing the same pairs of images.

What will be affected is framing. This typically shows up as degraded edges, either streaked or blurred, where parts of the output frame are outside one or more source frames, after registration. The usual solution in those cases is to crop off the degraded edges, which is what starting at the other end would have done automatically in most cases.

I think it's true that a more robust decision of which end is narrow could be made by comparing two frames near the middle of the stack, rather than always looking at the extreme ends. Along with a more robust decision, that method should also allow to robustly compute an average frame-to-frame scale adjustment. On the list of things I think about doing, that change to the comparison strategy is bundled with allowing a single frame-to-frame scale adjustment value to be used for the whole stack, as opposed to computing scale adjustment independently for each pair of images. In typical use (as I imagine it!) that constant scale value would come from the near-center-of-stack comparison. It seems like that whole scenario would provide a clean handling for a stack where scale adjustment is too important to be turned off, but the stack has so much OOF at the ends that automatically computing scale changes frame-by-frame gives distortions. But all this is hypothetical. I cannot recall ever encountering such a stack in a real application. In all the ones I recall seeing, if frame-by-frame scale changes were problematic, then turning off scale adjustment altogether worked well.

--Rik

dhmiller
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Re: Zerene settings question

Post by dhmiller »

Well as Lou noted, sometimes the first and often the last (I usually throw in a few extra on front and both due to backlash concerns) are often OOF. So that may be part of the issues I have been running into. I will have too rethink how I choose the frames to export to ZS... Thanks for the clarification. Might be nice to be able to choose the "master, " which much of my astrophoto stacking/integration software, which is very much in the business of alignment, allows. (just a thought)

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