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Thread: tone mapping

  1. #1

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    tone mapping

    Playing with HDR imaging in the digital world made me curious about how the process works beyond dragging sliders for "tone mapping" properties and seeing what happens, a quick trip to wikipedia yielded:
    http://en.wikipedia.org/wiki/High_dynamic_range_imaging
    anyway the gist of it is there are mathmatical equations that I don't understand going on behind the scenes, but how is tone mapping done with film?
    Wikipedia says it has been done on some relatively well known photos
    High dynamic range imaging was originally developed in the 1930s and 1940s by Charles Wyckoff. Wyckoff's detailed pictures of nuclear explosions appeared on the cover of Life magazine in the mid 1940s. Wyckoff implemented local neighborhood tone remapping to combine differently exposed film layers into one single image of greater dynamic range.
    but I'd like to know how not just that it has been done. Anyone?

  2. #2
    Eric Rose's Avatar
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    I think Sandy King was playing around with HDR using film. You might ask him.
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  3. #3

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    Quote Originally Posted by bob100684 View Post
    but I'd like to know how not just that it has been done. Anyone?
    I haven't read up on Wyckoff, but a guess is that complementary film masks were put to use.

    As a real quick primer, imagine that you photographed a night scene with a brightly lit white building surrounded by dim areas. And you made sure to get a bit of shadow detail in the exposure. Although the film is holding all the detail, straight printing is not too successful. When you print light enough to see some of the dim-light detail, the white building is blown out (all white).

    Now, the idea of the mask: imagine if you made a contact print, on film, that was the perfect opposite of your negative. That is, if the two were sandwiched, in perfect alignment, they would cancel out. Where your negative was almost clear, the contact print was almost perfectly dense, and vice versa. No image could be printed. BUT... if the contact print were exposed and developed very weakly, it would only partially cancel out the original negative. And finally, if instead of being perfectly sharp, so that it cancelled out all levels of detail, it was made blurry. Sandwiching a fuzzy contact print with the original neg could ROUGHLY cancel (partially) larger areas, but NOT interfere with fine details in those areas. Voila! An unsharp mask.

    A person who has good examples of some rather extreme effects is astronomer David Malin. He has sample images and diagrams of a masking setup here: http://www.aao.gov.au/images/general/technical.html

    Malin's image here: http://www.aao.gov.au/images/image/cenA_usm.jpg shows original negative (upper left), the unsharp mask (upper right) and the both sandwiched, at bottom.

    I'm not comfortable with the term "tone mapping" for this, but I guess it is. The "localized" part just means that certain parts of the negative have their own special masking effect.

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    "Contrast masking" is perhaps a better description to use as search terms if you want to look for what you can do with film.
    Mind you, i think you may have to go through masses of digital/Photoshop techniques on the net to find descriptions of the old, analog way of doing things.

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    I went on a bit of a search for information about Charles Wyckoff's Life magazine photos. I didn't find any cover photos in the 1940s per the Wikipedia article. Of course, maybe I just overlooked it. I did find some information inside (not on the cover) of a 1966 issue. There is a sequence of 20 high speed photos of "...an explosion conducted by the AEC before the 1963 test ban treaty."

    Following the photo sequence, it says, "Charles Wyckoff, of a research company called EG&G, Inc., devised a special film called "XR", for extended range. XR film is comprised of three panchromatic layers: an extremely slow layer (color-coded cyan blue) to record the brightest phenomena; a medium-speed layer (magenta) for intermediate brightness; and a high speed layer(yellow) for faint phenomena."

    Perhaps this is the source of the "...differently exposed film layers..." aforementioned in the Wikipedia article.

    That same Wikipedia article has other problems, in my book, so I'm now skeptical about it. But those things were not part of the question, so no sense wasting anyone else's time. At least I learned a few things along the way.



 

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