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Thread: What is debayering?

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    Default What is debayering?



    Not exactly sure what this means when one shot color cameras are used. Is that turning the image into monochrome? Why?
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    Quote Originally Posted by Nickwatcher View Post
    Not exactly sure what this means when one shot color cameras are used. Is that turning the image into monochrome? Why?
    Debayering, or demosaicing is the process by which the encoded Bayer matrix in a OSC camera creates a color image.

    It does not turn the image into monochrome - it is what creates the color image from the Bayer matrix encoded data, which is essentially greyscale.
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    The sensor array in an OSC (e.g. a DSLR) is covered by a matrix of colour filters called a Bayer matrix. Each pixel only receives light from one of the coloured filters. In a true Bayer matrix there are 2 green filters for each red/blue filter - you can think of them as an array RGGB or RGBG etc...
    When the sensor captures an image it actually captures 4 sub-images, one per filter - one per pixel actually. So there is one lot of red and blue data and 2 lots of green data. The process of taking the individually "colour" data from each set of pixels and combining it to create a composite full colour image is called "debayering".


    The key thing is that a colour camera is simply a monochrome camera with a bayer matrix on its sensor. What the sensor pixels capture in both cases is purely light levels.... i.e. a grayscale image. To make a human visible colour image from that data, it has to be "debayered" somewhere...


    In normal photography you don't need to worry about this as it happens "behind the scenes" and all you ever really see are the full colour composite images...

    However, in AP... it is possible to work on the RAW (undebayered) data which is all held in the RAW files from your camera. The advantage is that different debayering algorithms are available and some work better than others on AP data (compared to terrestrial photography). The most sophisticated algorithm for debayering at the moment is VNG (I think).

    Note that you can't correctly debayer a raw file if you have done any editing to it... for the obvious reason that the pixels may have moved from their original relative position to each other!


    The reason for caring about debayering is mostly to do with stacking.

    You want to do ALL of the calibration of darks, lights etc with the RAW (undebayered) data to ensure every pixel "lines up" in both position AND colour (RGGB) properly. Once the images are stacked you can debayer...
    Does this help?
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    Yes Phil. Than you very much for the explanation. I appreciate it. So the debayering is done in the post processing software, not the camera. Programs like DSS, photoshop, etc?
    -Nick
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    Here's a question. I may be oversimplifying. Why don't they make a one shot color CCD with a removable Bayer filter so it can be both mono or osc?
    -Nick
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    Debayering is done in the post-processing step - not in camera. Photoshop doesn't do debayering directly, although there are some plug-ins for this.

    There is usually a "raw processor" specific to the OSC camera in question that serves to handle debayering, and there are some external processors that fit into the workflow for a given framework (like CCDsoft, etc.) that support multiple debayering algorithms (i.e. user selectable).
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    Debayering is done both places..
    If you shoot JPGs from the camera you are telling the camera to process the RAW based on its firmware that is tuned for daylight photography. I almost never shoot JPG. It is a digital Polaroid where most of you image date is never used.
    When you shoot RAW as has been said you can not only process images in the software of your choice including some excellent software designed for AP. but you can process it again next year when the tools get better.
    RAW is the negative.. but with a lot more data then any film negative ever held. Photoshop comes with the plug in to do debayering.. there are tools that just do that and hybrids like Lightroom and ACDSEE Pro etc that can so all the normal processing most normal photographers need. I actually PP totally in lightroom the challenge TIF in the AP forum.

    The Wikipedia page on Bayer matrix is excellent:
    Bayer filter - Wikipedia, the free encyclopedia

    When advanced AP users shoot CCDs that cost hundreds or thousands more than a DSLR with filters.. they are picking up the ability to record pure luminosity data for each pixel then add real color data for each pixel vs borrowing data from several pixels to create a full color data. But it takes more specialized tools to process that data..tools like lightroom can't handle seperate color data.
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    Indeed, I use PixInsight for AP and that lets you control the when and how of the debayering algorithm... whereas for normal photography I just open my RAW files in Photoshop and let its RAW converter do it all for me automatically.

    I don't think it would be technically possible/easy to have a removeable bayer matrix... it is presumably a tiny thin delicate thing...probably actually part of the sensor chip itself...
    Not sure why you would want one either, aside from having 4x the pixel resolution but in monchrome... :-)

    Anyway, AFAIK it can't be/isn't done today.

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    The bayer matrix is mechanically integrated into the device - it can't be removed...
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