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Thread: Step-by-step processing example (NGC 891)

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    If ever a thread deserved to be a stickie, this is it.
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  3. #22
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    Real-life example of editing in 16-bit versus 8-bit.

    From DSS, I saved the stack of M74 as 16-bit TIFF file. This is what it looks like. Since this is a JPEG version, this is obviously an 8-bit image. But you can see how most of the detail is near the dark end of the image.


    I then brought this into Photoshop and kept one copy in the 16-bit version, and saved another version in an 8-bit version.

    Then I applied my usual adjustments to the 16-bit image. Once I was done, I flattened the image, converted to 8-bit, and saved as JPEG. So even though the final image is 8-bit JPEG, all editing was done in 16-bit format.

    I then opened the 8-bit JPEG "raw" and applied the exact same adjustments (as from the 16-bit version). Then flattened the image, and saved as 8-bit JPEG.

    Here's the comparison:


    You can easily see how the version edited in 8-bit shows a lot of posterization and loss of detail. It's because it didn't have enough shades of brightness in each of the color channels to render the necessary subtle shading -- so it had to compress several shades into single shades, resulting in jumps in the gradient (i.e. posterization).

    One important note if you're editing a 16-bit file in Photoshop using adjustment layers: ALWAYS flatten all the layers BEFORE converting to 8-bit (for saving to JPEG) -- this creates the final flattened image with the detailed shading before converting to 8 bits. If you convert to 8-bit with all the separate layers present, it converts each individual layer to 8 bits first and then flattens, which is equivalent to doing an 8-bit edit.
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    Awesome, that 100% satisfies my curiosity for the difference 16 bit editing can make. When I process my M31 image in gimp I get to a point where the image will start looking the way the galaxy does in the 8 bit processing. There is decent detail but it just looks horrible. This is exactly what I have been wanting to see for a while, thanks!
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    I'm bookmarking this one in my Astronomy favs so I can find it easily again when I get the $$$ to get Photoshop! Thanks much for the tutorial!
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    Fuzz, what happens if you save the original 16-bit Tiff image not as 8bit jpeg first, but as a 24 bit RGB .bmp file, and work on that, as a comparison to the 16-bit TIFF workflow? And then only jpeg once you are happy with the .bmp result...

    That's what I do in Gimp, and I think I get better results than you've shown with the 8-bit jpeg above. Maybe.

    (You will be glad to hear that I am reprocessing some data using ISO100 flats instead of 800, even though DSS gives a "warning" message when I do. Thanks again for that experiment...)

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    Quote Originally Posted by alsetalokin View Post
    Fuzz, what happens if you save the original 16-bit Tiff image not as 8bit jpeg first, but as a 24 bit RGB .bmp file, and work on that, as a comparison to the 16-bit TIFF workflow? And then only jpeg once you are happy with the .bmp result..
    That should be fine. Most cameras capture in 12 bits (and place in a 16-bit container). So as long as you're using anything greater than or equal to 12 bits to edit in (without ever going to lower than 12 bits in between), it should be okay. I know nothing about GIMP -- so GIMP can handle 24 bit BMP, but not 16-bit TIF?

    I believe DSS saves only in either TIFF or FITS format, so BMP is not even an option? So I assume you're using some other stacking software to save in 24-bit BMP?
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    What I do is to use DSS for stacking, saving all its "autosave" TIFF file and the non-adjusted full 16-bit integer TIFF file and the adjusted image Tiff file that I've curved and maybe cropped in DSS, all three, as the 16bit TIFFs. Then I import the adjusted TIFF file into gimp and immediately "Save as" a 24 bit .bmp. This flattens the layers of the TIFF file I think; but it still seems to give me a full range of color and contrast values...I think. Then I try tweaking curves and levels but mostly just use the "auto>Stretch HSV" and "auto>Stretch contrast" functions, and layer masking to remove gradients, then cropping, resizing and so forth still as 24 bit .bmp, then finally making a jpeg for uploading or "show and tell". Saving every version along the way as 24 bit bmp.
    I also am trying CinePaint on the unadjusted TIFF files from DSS, but the learning curve is steep and I'm still not sure if I've got a fully functioning version. But it sure handles layer masks well, much faster than standard gimp, and works with the full 16 bit TIFF files, each color layer separately if you want, and even will open the DSS "autosave" files.

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    Quote Originally Posted by alsetalokin View Post
    Then I import the adjusted TIFF file into gimp and immediately "Save as" a 24 bit .bmp. This flattens the layers of the TIFF file I think; but it still seems to give me a full range of color and contrast values...I think.
    I never use BMP, so did a little reading. A 24-bit BMP file is really an 8-bit image file. The "24 bits" in a 24-bit RGB BMP file simply refer to 8 bits per channel x 3 channels = 24 bits. So really 24-bit BMP = 8-bit (per channel) image. When you're saving as 24-bit BMP in GIMP, you're really saving an 8-bit image.

    I think the key to your work-flow producing better results than my illustration is probably this:
    What I do is to use DSS for stacking, saving all its "autosave" TIFF file and the non-adjusted full 16-bit integer TIFF file and the adjusted image Tiff file that I've curved and maybe cropped in DSS, all three, as the 16bit TIFFs. Then I import the adjusted TIFF file into gimp and immediately "Save as" a 24 bit .bmp.
    DSS handles 32 bits. So when you're editing/adjusting in DSS, you're editing in 32 bits. When you open that adjusted file in GIMP (in 8 bits), you've already stretched the image somewhat to "spread" out the faint shades before down-converting in GIMP. The main problem with 8-bit editing is that stretching in 8 bits will produce posterization. But you're already stretching in 32 bits in DSS.
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    Yes, that's pretty much what I understood. The 16-bit TIFF has the three color planes each at 16-bit resolution, but converting to the 24 bit BMP flattens the planes and so you wind up with only 8 bits per color plane, so the resolution is lost at that stage.
    I'm getting pretty good at the DSS curves but they are very crude and don't give a lot of flexibility like multiple points. The CinePaint curves and levels is much more flexible and operates much like PhotoShop, I think, and works on the 16-bit TIFFs at full depth, but I'm just starting to figure it out.
    Thanks for your help, you are really making the concepts gel in my head.

    It looks like trying to stay with free opensource software may be more trouble than it's worth. Those astronomy plugins for photoshop really seem to make things go a lot smoother.

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    Quote Originally Posted by alsetalokin View Post
    It looks like trying to stay with free opensource software may be more trouble than it's worth.
    Well, you seem to be getting pretty good results out of it, so it can't that bad. Plus you can't beat the price! But yes, when it comes to complex processing (and astro-images definitely require complex processing), Photoshop is hard to beat. The other advantage is that there are hundreds, if not thousands, of websites, books, and even seminars, on every single aspect of PS to learn from.

    Anyway, if this has been helpful, I might do another thread with more advanced processing like layer masks, multi-exposure composites, etc.
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