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Thread: Color seperation filter used in Drum Scanners

  1. #41

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    Re: Color seperation filter used in Drum Scanners

    Quote Originally Posted by Sasquatchian View Post
    Well, I know that there are three color filters after the splitter and before the PMT's, so my guess is that the ultimate gamut of the scanner is defined by those filters.
    IMHO, not at all.

    The ultimate gamut is defined by the spectral responses (r, g and b) from the particular film we use (or from the dyes on DSLR pixels). The image capture is the critical step.


    You can match the "ultimate gamut" of two scanners by a well done calibration procedure, if you use a 3D LUT for that you can get a perfectly matching result, even in the case that the fiters in the scanner are substantially different, you only need a different calibration for each film to equal result from different scanners.

    ...but you won't be able to make a "general calibration" to match Velvia with Ektar or Portra, because it won't work when spectral nature of light or subject changes.


    After image capture you have spectral color information of each spot reduced to 3 values, it can be 3 voltage levels in a sensor or 3 silver density levels after first developer. In the taking we define the ultimate gamut, while the scanning/edition is irrelevant for the ultimate gamut if we have the good software tools.


    There is advanced math demonstrating that.


    _________________


    Scanners are IT8 calibrated, but different scanners may require a 3D LUT calibration for each film for a perfect match.

  2. #42
    Drew Wiley
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    Re: Color seperation filter used in Drum Scanners

    I think you're oversimplifying this, Pere. I don't want to get personally involved with discussing specific scanners; but I do know a thing or two about color mapping and filters, and why it's virtually impossible to correctly replicate every kind of hue regardless of the technology involved. "Gamut" in the jargon of computer skills is a bit different than true color gamut of things in nature etc, which cannot always be predictably quantified. It's hard enough just film to film. Film dyes are imperfect. Then there are also real limitations of output, at the printing or repro stage. A skilled craftsman learns how to work within these serious limitations to his advantage. Denying them is a mis-step.

  3. #43

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    Re: Color seperation filter used in Drum Scanners

    Quote Originally Posted by Drew Wiley View Post
    I think you're oversimplifying this, Pere.
    Drew, I'm not oversimplifying, it is film or on pixels dyes what makes the simplification. In the taking spectral information disapears, and the spectrum is what had the richeness.

    Let me make a reasoning. Say that that an scanner measures the rgb values in an spot in the color film. For that r-g-b combination a film always has the same transmission spectrum, there is a single possible one.

    Now imagine that you build a 3D table, it would be a cube with a cell for each r-g-b value, and in each cell you store the transmission spectrum of film having that particular r-g-b scanner reading.

    This would allow to map an actual spectrum in the film for each RGB reading. This is something that's not necessary in a workflow... but with a few logic steps it can be demonstrated that always there is a 3D LUT that converts the output from an scanner to the output from another one. This is something that it's well known by proficient colorists having an scientific education, and because of it they say that what is critical is the capture, the rest can be maped.

  4. #44
    Drew Wiley
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    Re: Color seperation filter used in Drum Scanners

    Here's just a small clue about something you're missing. The ideal narrowness or broadness of the bandwidth of RGB capture, individually, is not an nm fixed space, but potentially differs not only between films, but within a given film itself. For example, the red segment of Kodachrome ideally needs to be nm broader than that of green or blue. This is because of the specific dyes involved. Old Ektachrome 64 had a lot of red contamination to its greens, so if you want to reproduce that as is, you'd need different capture filtration than if you were hypothetically going to try to correct that kind of response or scan later Ektachromes without the same problem. Fujichromes differ among themselves too, with Velvia being a bit different than the others. In other words, one shoe size does NOT fit all. Scanner programs can be optimized for just so many given film categories, namely, certain ones extant and popular when the scanning software was developed. That's one of the reasons Kodak updated Portra films for sake of scanning parameter commonality; but then Ektar still has idiosyncrasies. Then you have sampling size issues which are related to dye curve shape. The better the scanner and bigger the sample, format-wise, the better the end result; but it's rarely ideal. People like to post colorful images of how good things are, but to a truly trained eye they often look contrived. I am a proficient colorist in my own right, Pere. I taught color matching. Any halfway decent watercolorist can mix in a matter of minutes complex hues which would utterly baffle the entire photographic and scanner industry, no matter how many tens of millions of dollars they've already spent on color R&D. Color mapping goes back to the 1920's. Of course, doing it on graph paper was extremely slow. But here's a question for you: why do inkjet printers use a 4-axis color mapping model, while film uses only a 3-axis model?

  5. #45

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    Re: Color seperation filter used in Drum Scanners

    Quote Originally Posted by Drew Wiley View Post
    why do inkjet printers use a 4-axis color mapping model, while film uses only a 3-axis model?
    First, because commercial CMY inks are not enough. Second because it's cheaper to obtain a grey level or an unsaturation level by using black ink.


    Quote Originally Posted by Drew Wiley View Post
    The ideal narrowness or broadness of the bandwidth of RGB capture, individually, is not an nm fixed space, but potentially differs not only between films
    Drew, there is always a 3D LUT that matches results from different scanners for a particular color film, probably the color matching you were teaching did not include 3D LUTs.

    But remember, each film requires a different 3D LUT for a perfect match.

  6. #46
    Drew Wiley
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    Re: Color seperation filter used in Drum Scanners

    If there's such a thing as a "perfect" match anywhere in this kind of technology, Pere, then go patent it. You won't find it, and clearly don't understand anywhere near all the major variables of the wider topic yet. Certain things always get lost in translation. One attempts to match the film to the scene, then the scanner to the film, then the scan to the printer output. Each of these speaks a different color dialect, and you're trying to juggle it all using yet another language - that of your software and monitor. It's amazing how far all this has come along in recent decades, but it's still far from perfect.

  7. #47

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    Re: Color seperation filter used in Drum Scanners

    Quote Originally Posted by Drew Wiley View Post
    and clearly don't understand anywhere near all the major variables of the wider topic yet.
    Drew, do you know what a IT8.7/1 target is ?

    Let's guess you know what it is... well, all colors in that table are exactly matched by all scanners with the same RGB values, in fact you can even download a calibration file for each individual target to account for the target manufacturing variations.

    Colors that are between individual cells are interpolated, but from cell to cell there is little color distance, so the interpolation is a very, very good match.

    If the taget was made with Ektachrome then that target would allow a perfect match between scanners for ektachrome scanning.

    For Provia 100F that calibration won't be as perfect, but then you can use a Provia 100F targets, ISO 12641-1, and then you have again a perfect match between scanners for Provia.


    If you take the Ektachome calibration and the Provia 100F calibration, then there is a 3D LUT that converts one to the other. You can also calculate a conversion 3D LUT for any color film.



    Drew, look, you cannot compare ancient color matching with what a 3D LUT does. 3D LUTs map any source color to any destination color, when scene-light-film spectral natures are removed from the equation a 3D LUT accounts for all.
    Last edited by Pere Casals; 6-Aug-2019 at 02:20.

  8. #48

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    Re: Color seperation filter used in Drum Scanners

    Quote Originally Posted by Sasquatchian View Post
    . The bottom line is that if your scanner can record pretty much everything that is one your film, .
    A monochrome sensor will capture it too :-) In the end it is about reducing metameric errors. None of these sensors old or new match the eye. They use one or more colour matching functions in their imaging pipeline. That IS the whole purpose of scanner target you mention. When you save a JPG or just open a RAW file on your PC a matrix or "3D LUT if you like" is used to transform the data to match the spectral output of your display. (modern imaging programs will do this in several steps). In the case of plain JPG it is hard coded to sRGB by your camera when you hit the shutter.

    What seems to be the case is that drum scanners are set up to measure CMY dyes and transparency very well, to be printed with CMY dyes on a piece of paper... This should not be a huge surprise. (transparencies at least, not so much colour negative which is a bit different).

    As an example the cones in the Human eye that measure the long wave radiation have there peak at about 566nm this is a bit different, from the spectral peak of the red you will find on a drum scanner and also on most if not all DSLRs. Its also different too the spectral peak of a cyan dye, or the red phosphor of a CRT.

    Its doesn't seem to make sense trying to reduce metameric errors for colours that CANNOT be displayed using a CMY dye system, but instead make the most of inherit capabilities of CMY dyes. i.e. build a scanner for scanning CMY dyed film for printing CMY dyes on paper.
    Last edited by Ted Baker; 6-Aug-2019 at 09:03.

  9. #49
    Drew Wiley
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    Re: Color seperation filter used in Drum Scanners

    Interpolation. Sometimes it tries to pole-jump a ditch, sometimes something too wide to be unrealistic. I'm done with this thread. Too little knowledge of how dyes and inks and so forth actually work, which is never ideally.

  10. #50

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    Re: Color seperation filter used in Drum Scanners

    Quote Originally Posted by Drew Wiley View Post
    Interpolation. Sometimes it tries to pole-jump a ditch, sometimes something too wide to be unrealistic.
    Click image for larger version. 

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    This was an IT8 specific for Provia, Astia and Sensia. With it different scanners do deliver same result for those films. Colors in the array are exactly matched, other hues would not be far from calibrated points, so interpolation also gives an excellent match.

    Then add what it can be done with 3D LUTs for matching other films.

    Drew, IMHO it would be interesting you play with 3D LUTs, in the digital workflow it gives a clear knowledge about how dyes and inks actually work.

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