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Stacking of raw images

 
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nielsgeode



Joined: 31 Jul 2012
Posts: 232
Location: Groningen, Netherlands

PostPosted: Mon Mar 23, 2015 2:42 am    Post subject: Stacking of raw images Reply with quote

The topic of stacking raw images directly has been discussed before. It is not supported by zerene stacker and I remember that I was technically not possible to do this.

To my big surprise, I came across a new version of Helicon Focus which does support stacking raw files directly! Very Happy

Here is a description how it works according to the authors of the software: http://www.heliconsoft.com/raw-in-dng-out/

What are your thoughts and would it be possible to also integrate this option in zerene stacker?

Niels
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rjlittlefield
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Joined: 01 Aug 2006
Posts: 18095
Location: Richland, Washington State, USA

PostPosted: Mon Mar 23, 2015 11:38 am    Post subject: Reply with quote

Neils, thanks for finding this reference. It confirms what I had thought after I first saw this new feature a couple of weeks ago.

Quote:
Helicon Focus which does support stacking raw files directly!

Well, sort of, but that's for a definition of "directly" that I suspect is different from what you're imagining.

If you look carefully at their diagram, you'll find a couple of small blocks labeled "Demosaicing". It's the first operation in both workflows, right up at the top.

Demosaicing is the step where some fancy computational process takes the raw sensor data, one color per photosite, and interpolates like crazy to produce an RGB image that has values for all three bands at every pixel position. Depending on the quality of the raw converter, this process may also integrate defect removal, sharpening, noise reduction, removal of lateral CA, and so on, since the best place to do some of those operations is while there's still a clear separation between photosites.

If you go back to my long explanation from four years ago, you'll find a closing paragraph that says:
Quote:
On the other hand, since staying in RAW is attractive, maybe it's worth thinking carefully about why that's attractive and what might be done to enable the same benefits but in a different representation that's still compatible with stacking. Linear color spaces come to mind, but given the length of this post, I think I'll leave that for another day.

As far as I can tell, what Helicon has done is exactly what's suggested in that paragraph. They still do most of the raw processing up front, producing output in a 16-bit RGB linear color space that is chosen to match what was used in the raw file. Then they stack as usual, doing on-the-fly color space conversion so that the screen displays look normal. Finally, they save the result image as 16-bit RGB pixels in a DNG wrapper so that you can do further processing using your favorite "raw" converter, despite the fact that you don't really have raw data any more.

The feature strikes me as being a very nice piece of design, engineering, and marketing. I expect it to to be reviewed quite well, independent of whatever benefit it actually provides by moving all the white balancing and levels/curves adjustments to a later stage of the workflow.

Quote:
would it be possible to also integrate this option in zerene stacker?

It's on the list someplace, and has been for quite some time. But it's not near the top of my priority list. I'd still rather defer to a specialist raw converter to do that critical demosaicing step under full user control of the process.

--Rik
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