Alan Corey <alan01...@gmail.com> writes: > I've been doing digital photography (amateur) for about 15 years but > finally bought a Nikon D5200 so I can have RAW files. I've been > programming on and off since 1968, got introduced to graphics through > doing scientific plotting. I'm a retired network administrator, still > program for fun, mostly in C. I have a Color Munki and I've read > quite a bit about having everything calibrated.
welcome to ufraw > The current Ufraw homepage at http://ufraw.sourceforge.net/ says it's > going to get into more detail later about color matrices but as far as > I can tell never does. > > The only photography text I ever read cover to cover is Mastering > Photographic Composition, Creativity and Personal Style by Alain > Briot. He's a big fan of using gray cards, Macbeth color charts, I > have great hope for white plastic lens caps but I don't have one yet. > The idea being that white balance needs to be more dynamic than > manufacturer presets like cloudy and incandescent. When I use the > cloudy setting the pictures are too yellow, then again there are dark > clouds and light clouds. Indeed. You might want to read Color Science by Wysecki and Stiles. > I wrote a program that takes a JPEG shot of a gray card and outputs an > averaged hue and RGB values. Because it might not be full frame or > there might be a thumb holding the card I do: Take an overall mean > hue, calculate a standard deviation, then a second mean hue of pixels > within 1 standard deviation of the mean, then mean RGB values for > those pixels. It's just a command line thing, the only dependency is > the common JPEG library. To deal with this, what I do is to use only raw, and to take an image of a grey card (or the white card on the back). As Udi said, JPEG has the camera's WB worked into it, and what you really want is the transform from raw to correct JPEG. > What I'd like to do is output numbers that can plug into Ufraw's > channel multipliers and green and temperature settings. Those 5 > numbers seem linked in ways I don't understand yet. I don't like > using the eyedropper to sample a neutral area of the real image > because it assumes there is one. There also doesn't seem to be a > provision for using a full-frame shot of a gray card and applying the > numbers to a different image. Then I use the selector on the grey/white section of the grey card shot, and use the 'neutral' button to adjust color balance to make the *converted* pixel values equal. I then save this. Then, when I open a real picture, it inherits the WB setting from the gray card adjustment, and things work out. In theory, the relationship between channel multipliers and color temperature/green is straightforward math, but really it's based on the multipliers the camera reports when setting particular temperatures and then interpolating. Partly this is because the gain calibration has to be worked in. But regardless, if you shoot a white card in the same light, and balance it, you should get quite close.
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