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Thread: Processing steps, what order?

  1. #11
    Phil Leigh's Avatar
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    Default Re: Processing steps, what order?



    Quote Originally Posted by dcbrown73 View Post
    I don't know, I process in PixInisight which supports 32bit color depths. Downgrading the color depth IMO isn't something you want to do unless you have too as you lose data when you do it. If there are tools that you want to use that don't support higher color depths, then you don't have much of a choice.
    If using DSS to stack and Photoshop to post-process you need to convert the Autosave.tif file produced by DSS to a true 16-bit file. Please refer to this thread: How to use the DSS Autosave.tif for best possible image quality

    The data that comes from CCD cameras is 16-bit (DSLR's are 12 or 14-bit). When 12/14/16 bit data is stacked some of the maths involved (division) gives rise to results that is more accurately represented in 32 bits (hence why the DSS autosave.tif file is 32-bits). PixInsight can handle this natively but PS can't (at least not all of its tools can yet).

    PS assumes that if you have a 32-bit file it must be an HDR (high dynamic range image) and has its own way of dealing with that. This is fine for terrestrial photography but not for AP. Follow the tutorial above to correctly convert the 32-bit DSS autosave.tif file for use in Photoshop.
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    Default Re: Processing steps, what order?

    Quote Originally Posted by MartinF View Post
    But can I postpone the gradient removal as long as I only do linear stretching? Or does the stretching increase the noise too much?
    The noise is baked into the data from the camera. "linear" Stretching will not increase it. non-linear stretching can make it impossible to treat properly.
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    Default Re: Processing steps, what order?

    INteresting Thread thanks. IS this all the same for Modified DSLR?

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    Default Re: Processing steps, what order?

    Quote Originally Posted by Phil Leigh View Post
    The noise is baked into the data from the camera. "linear" Stretching will not increase it. non-linear stretching can make it impossible to treat properly.
    And just to make sure I understand - linear stretching is using levels, non-linear would be curves?
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    Default Re: Processing steps, what order?

    There is also no such thing as 'linear stretching', by its nature stretching a histogram will turn linear data into non-linear. Linear data is where the value associated with a pixel directly corresponds to the count of photons received by that pixel during exposure. In deep sky photography where our targets are so dim these values are very small, i.e. A histogram mostly to the left. 'Stretching' is a mathematical process whereby we modify the values associated with each pixel so that our histogram covers more of the available range, it becomes 'stretched' literally across the histogram display and therefore more visible to the eye. The data is then non-linear, in that the values associated with each pixel no longer directly correlate with a count of photons received by the sensor at that pixel site.

    Any noise present in the linear data will also be stretched, and made worse/more visible. You can also see why some operations are better performed on linear data, such as colour calibration, where we are working with data exactly as received by the sensor, in the same proportions.

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    Default Re: Processing steps, what order?

    Quote Originally Posted by Christo View Post
    INteresting Thread thanks. IS this all the same for Modified DSLR?
    Yes absolutely the same.
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    Default Re: Processing steps, what order?

    Quote Originally Posted by frant View Post
    There is also no such thing as 'linear stretching', by its nature stretching a histogram will turn linear data into non-linear. Linear data is where the value associated with a pixel directly corresponds to the count of photons received by that pixel during exposure. In deep sky photography where our targets are so dim these values are very small, i.e. A histogram mostly to the left. 'Stretching' is a mathematical process whereby we modify the values associated with each pixel so that our histogram covers more of the available range, it becomes 'stretched' literally across the histogram display and therefore more visible to the eye. The data is then non-linear, in that the values associated with each pixel no longer directly correlate with a count of photons received by the sensor at that pixel site.

    Any noise present in the linear data will also be stretched, and made worse/more visible. You can also see why some operations are better performed on linear data, such as colour calibration, where we are working with data exactly as received by the sensor, in the same proportions.

    Richard
    PS can do (what they call) linear stretching by shifting the black or white point. This shifts all the data up or down in value but the relative proportions stay the same. Almost exactly like boosting or cutting the ISO :-) Provided you don't shift ANY data past the clipping points of course!
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    Default Re: Processing steps, what order?

    Quote Originally Posted by frant View Post
    There is also no such thing as 'linear stretching', by its nature stretching a histogram will turn linear data into non-linear. Linear data is where the value associated with a pixel directly corresponds to the count of photons received by that pixel during exposure. In deep sky photography where our targets are so dim these values are very small, i.e. A histogram mostly to the left. 'Stretching' is a mathematical process whereby we modify the values associated with each pixel so that our histogram covers more of the available range, it becomes 'stretched' literally across the histogram display and therefore more visible to the eye. The data is then non-linear, in that the values associated with each pixel no longer directly correlate with a count of photons received by the sensor at that pixel site.

    Any noise present in the linear data will also be stretched, and made worse/more visible. You can also see why some operations are better performed on linear data, such as colour calibration, where we are working with data exactly as received by the sensor, in the same proportions.

    Richard
    The way I understand linear stretching is that the bins are redefined and the numbers are redistributed across the bins. So suppose we have a histogram stretch where 2 bins become 1, then the numbers are added so there is still a linear relationship between the numbers and the photons just with a lower resolution. The "linear" part apparently refers to how the bins are redefined. Am I misunderstanding something? Here's the link that I base my response on: Image Processing, Linear stretch and OpenCV
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    Default Re: Processing steps, what order?

    Linear stretching involves adding (or subtracting) a constant value to every pixel value. There's no resampling involved.

    Lets say you had 3 8-bit pixels, values 1, 5 and 10
    A linear stretch might shift those to 21, 25 and 30. As a result the image will appear brighter overall (because the screen gamma correlates to the pixel values) but the contrast/dynamic range will remain unchanged.
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    Default Re: Processing steps, what order?

    Very interesting Henk.

    It intuitively feels more like that operation is just a re-quantisation of the data, or perhaps more akin to binning. But looking at the resulting histogram, it does indeed appear to be stretched, such that data is moved from a smaller to a larger range. In order to maintain the linearity though, by spreading the bins throughout the range, there has to be a consequent drop in resolution, perhaps that's why I've never thought about it like that. Given that hit, I don't think I'd want to apply it to my data.

    I did wonder when I made the statement "There is no such thing as linear stretching..." whether I'd have to eat my words.
    Thanks for your post, I always enjoy learning something new

    Richard
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