Beep6581
commented
5 years ago How does anyone (photoshop, raw digger, our cameras..) know if an exposure value V at a specific pixel (X, Y) is considered saturated (a burned highlight) or not?
In RawTherapee, which does not use LibRaw, we measure raw files based on a set of clipped white-frames and register the white levels in a JSON file which RawTherapee checks every time it loads a raw file, see:
- https://github.com/Beep6581/RawTherapee/blob/dev/rtengine/camconst.json#L32
- http://rawpedia.rawtherapee.com/Adding_Support_for_New_Raw_Formats
HDRMerge uses LibRaw and here things are different. From my very limited understanding, LibRaw reports the maximum possible value as supported by the raw format, regardless whether the actual values reach that level or are even capable of reaching that level, I suppose leaving it up to the calliung program (HDRMerge) to figure out what the real white levels are.
Don't cameras have a brightness value cap that they can't record beyond?
Yes, a point at which a photosite saturates, but with other possible complications - see the long intro in the camconst.json file.
fanckush
heckflosse
This may not be the best place to post this question but it's at least relevant to how saturation levels are computed in HDRMerge.
I wanted to contribute to this project so I've been looking into the code and trying to understand it better. One part I found particularly strange was: https://github.com/jcelaya/hdrmerge/blob/3ad6d36c517ddb7f164799c98800bdccb2e5b9f2/src/ImageStack.cpp#L54 If I understand this correctly, then it's a problem. The brightest image in a set will always be considered over exposed even when it's not.
Of course I was clueless about EVERYTHING, for example I didn't know that Raw images only store brightness values. HDRMerge took the easy approach of assuming there is always over-saturation (over exposed pixels).
Now my questions are:
How does anyone (photoshop, raw digger, our cameras..) know if an exposure value
Vat a specific pixel(X, Y)is considered saturated (a burned highlight) or not?Is it always just an approximation? in that case does each software compute the threshold differently?
Don't cameras have a brightness value cap that they can't record beyond? If so, then we could set the saturation threshold properly knowing that for example 6845839 is the max brightness of camera C
I am probably mixing the words "exposure", "brightness", "white level" and "saturation" together. What I am ultimately referring to is the registered value from the sensor at any pixel in any space.