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Thread: Processing steps, what order?

  1. #41
    Phil Leigh's Avatar
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    Default Re: Processing steps, what order?



    Quote Originally Posted by kszakonyi View Post
    What you see with STF on, is not what will be seen in a printed photo. You have to get that stretch onto your photo if you like it and then turn STF off. If you don't see the stretch in your photo saved on disk, kinkos won't see it either.

    If you like what you see in STF, then you can copy it to the histogram stretch window, by dragging the blue triangle from the STF window to the corresponding area on the histogram tool window and then apply the changes from your photo from the histogram window. I think the Jason Brown tutorials go over this. You want to do your DBE functions and the other previously mentioned functions first though. Also you have to turn STF off at some point or your file will never reflect what you see on the screen and this will drive you crazy.

    It is actually kind of confusing at first. I use STF while doing DBE and canon banding and that sort of thing, so that I can see the results. Once I start doing histogram stretches I turn STF off. Sometimes I like the STF result, so I drag it onto the histogram tool and then apply it to my photo and then turn STF off and close it. Don't close it before resetting it though.
    The new release of Pi (1.8 official release - came out this week) has an improved STF tool and also when STF is active on any image the top left of the image window turns green... very helpful.
    Telescope: SWEquinox ED80 Pro (Schott/Ohara Fluorite)+Baader Steeltrack, C8 XLT Mount: Skywatcher HEQ5 Pro, Rowan belt drive on Pillar mount+EQDIR+ADM saddle/dovetails Camera: JTW1100D mono , CentralDS 600, ASI120MM. ASI120MC, IDAS & Baader NB 2-inch filters, TS Filter Drawer System Accessories: TV Nagler T4 12 mm, Baader 8-24 MkIII, Ortho 5mm, Aspheric 31mm, Meade 12mm reticle, TV Powermate 2x & 2.5x, JMI+BAST Motorfocus+FCUSB, TV 0.8x FR/FF, Celestron 0.63FR, Orion RACI & Mag Mini, Canon IS 15x50 Bins
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  3. #42
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    Default Re: Processing steps, what order?

    This is all very good to know though a lot of it is tool specific.

    Getting back to the "linear stretch" question, reading back I see that PS, Maxim and OpenCV define linear stretch as the histogram transformations. Calling that a "linear stretch" is incorrect IMHO because re-binning and clipping is not quite linear in terms of pixel values. The re-binning has to do with the display resolution of course but aside from that, clipping the data at the black and white points is definitely not linear. It is more fair to call it a "screen stretch" like PI does.

    And because screen stretches are nonlinear it makes sense to separate them from operations on the actual just like PI does. If both DBE and screen stretching were linear I suppose it would not matter but since this is not the case it makes sense to do the DBE first while looking at the resulting data using the STF, and apply the STF changes later, either by dragging STF onto the histogram or by using the histogram itself from scratch.

    For now I have not run into anything that DSS and Photo Gallery could not handle (well if I feed it decent data). But I will keep this in mind, if I intend to spend money on some editor it will be PI I think.
    ... Henk. Telescopes: 6" Mak-Newt (Comet Hunter), ES ED127CF, ES ED80, Zhumell Z12, Coulter Odyssey 10, AT6RC, Venture RX-7, Celestron Skymaster 20x80, Mounts and tripod: Losmandy G11S, AVX, LXD55, Tiltall, Cameras: Fuji X-a1, Canon SX40, Xt, XSi, T6, ELPH 100HS, DIY: Dob and camera barndoor trackers, afocal adapter, Dob with foldable base and Az/Alt setting circles, Accessories: SSAG, Plossls, Barlows, Telrad, laser collimators (Seben LK1, Z12, Howie Glatter), Cheshire, 2 Orion RACIs 8x50, Software: DSS, ImageMagick, PHD, Nebulosity, Photo Gallery, Gimp, CHDK

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    Default Re: Processing steps, what order?

    Hi,
    In post #25 Mike explains exactly what Linear Stretch is. (Thanks Mike, perfect done)
    Linear Stretch happens when you use a linear function. It is mathematics and has nothing to do with the tool i use.

    So, when you move in the histogramm the left (dark point) triangle to the right, as long as nothing is clipped -
    in any tool - this is a Linear Stretch. If you multiply the image with a constant in PixelMath (according to Mikes example)
    only the angle of the "line" changes and the image is stretched linear and still stays linear!

    Gerald

  5. #44
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    Default Re: Processing steps, what order?

    Gerald, first you say that a linear stretch is a rescaling of the pixel values according to y=a*x+b like Mike said. Then you say that a linear stretch is the same as moving the slider in the histogram. But that is different - moving the slider is a re-assignment of pixel values to bins, not the direct application of a linear function to pixel values. If there is no clipping, the net effect may be close to the application of some linear function - because the re-assignment is based on a linear function - but there are discretization effects in the re-binning that make it not quite the same even in that case. You may want to take a look at the Maxim article that I referenced earlier.

    The main point of the discussion is not what the definition of a linear function is but concern about the effects of discretization during the various subsequent stages in the workflow. Linear stretching by the STF (re-binning for visualization) is a way of visualizing operations on data with low signal content without actually changing the pixel values. This is perceived as being beneficial because it allows users to keep the discretization effects related to visualization outside the first stages of data processing.
    ... Henk. Telescopes: 6" Mak-Newt (Comet Hunter), ES ED127CF, ES ED80, Zhumell Z12, Coulter Odyssey 10, AT6RC, Venture RX-7, Celestron Skymaster 20x80, Mounts and tripod: Losmandy G11S, AVX, LXD55, Tiltall, Cameras: Fuji X-a1, Canon SX40, Xt, XSi, T6, ELPH 100HS, DIY: Dob and camera barndoor trackers, afocal adapter, Dob with foldable base and Az/Alt setting circles, Accessories: SSAG, Plossls, Barlows, Telrad, laser collimators (Seben LK1, Z12, Howie Glatter), Cheshire, 2 Orion RACIs 8x50, Software: DSS, ImageMagick, PHD, Nebulosity, Photo Gallery, Gimp, CHDK

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    Default Re: Processing steps, what order?

    Quote Originally Posted by CamelHat View Post
    Gerald, first you say that a linear stretch is a rescaling of the pixel values according to y=a*x+b like Mike said. Then you say that a linear stretch is the same as moving the slider in the histogram. But that is different - moving the slider is a re-assignment of pixel values to bins, not the direct application of a linear function to pixel values. If there is no clipping, the net effect may be close to the application of some linear function - because the re-assignment is based on a linear function - but there are discretization effects in the re-binning that make it not quite the same even in that case. You may want to take a look at the Maxim article that I referenced earlier.

    The main point of the discussion is not what the definition of a linear function is but concern about the effects of discretization during the various subsequent stages in the workflow. Linear stretching by the STF (re-binning for visualization) is a way of visualizing operations on data with low signal content without actually changing the pixel values. This is perceived as being beneficial because it allows users to keep the discretization effects related to visualization outside the first stages of data processing.
    It all depends what software we are talking about; in PixInsight when you move a Histogram Slider it executes a 64-bit floating point PixelMath function on the data. There is no "rebinning" - it's not doing low-bit depth integer maths!

    This is one reason why 8/16-bit software is such a crude tool for use on AP data. You need the mathematical precision of 64-bit FP to accurately represent the results of maths functions on the data without worrying about "discretization".
    Telescope: SWEquinox ED80 Pro (Schott/Ohara Fluorite)+Baader Steeltrack, C8 XLT Mount: Skywatcher HEQ5 Pro, Rowan belt drive on Pillar mount+EQDIR+ADM saddle/dovetails Camera: JTW1100D mono , CentralDS 600, ASI120MM. ASI120MC, IDAS & Baader NB 2-inch filters, TS Filter Drawer System Accessories: TV Nagler T4 12 mm, Baader 8-24 MkIII, Ortho 5mm, Aspheric 31mm, Meade 12mm reticle, TV Powermate 2x & 2.5x, JMI+BAST Motorfocus+FCUSB, TV 0.8x FR/FF, Celestron 0.63FR, Orion RACI & Mag Mini, Canon IS 15x50 Bins
    Software: EQMOD/Stellarium/Autostakkert!/Registax/BackyardEOS/PixInsight,PHD2,AlignMaster, AstroTortilla, Straton

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