Saturday, December 6, 2014

"Color spot" RAW white balance mode in PhotoFlow

Sometimes, getting a correct white balance for a shot taken in non-standard conditions (like for example below the trees in a deep forest) is not straightforward, unless you had the precaution of shooting a neutral grey card in the same lighting conditions... I don't know about you, but I never have this precaution! So I tend to end up with several shots that are all different, without an obvious neutral object to use as a reference, and with a camera white balance that changes from one shot to another.

It is with this kind of situations in mind that I have decided to code a new white balance mode that is not found in other open source RAW editors: the "color spot" mode. This mode is used to adjust the white balance based on known colors that are not neutral. But before discussing that in detail, let's see briefly what are the white balance options currently available in PhotoFlow. First things first.

PhotoFlow offers for the moment only three RAW white balance modes:

  1. "CAMERA WB" applies the white balance coefficients stored by the camera in the raw file at the time of shooting
  2. "SPOT WB": this tool requires you to click on the image with the mouse in order to select a certain region that is supposed to be gray. The image data is then averaged over a small (15x15 pixels) area around the clicked point and the WB coefficients are adjusted to neutralize the corresponding color.
  3. "COLOR SPOT WB": this tool works in a similar way as the normal "SPOT WB", except that it lets you specify a non-neutral target color for the selected area. The target color is given in terms of Lab "a" and "b" values, so that the result is independent of the camera and working profiles being used.
The available options are clearly still quite limited, and all standard white balance presets (daylight, shadow, incandescent or fluorescent light, etc...) are badly lacking (but planned in some near future).

The first two methods are found in any raw converter, therefore I skip their detailed description and jump directly to the third one, which is something quite new. To describe how it works, I'll use a portrait shot that Patrick David made freely available on is blog and I will try to set the white balance based on the tonality of the model's skin.

To activate the "COLOR SPOT" tool, you have to open the raw developer dialog, select the "White balance" tab and chose the "Color spot" mode in the "WB mode" drop-down list. For more details on how to develop RAW images in PhotoFlow, you can have a look at this previous blog post.

It is time now to decide what color you want to match in your picture, and put the proper "a" and "b" values in the corresponding boxes; for this picture, I was able to come very close to the camera white balance using a=17 and b=16. This values make quite sense, as they correspond to a color tonality that has as much magenta as yellow... you can slightly increase the "b" value to get a more "tanned" aspect, depending on what you want to achieve. Decrease both values if you want to obtain a "paler" skin tone.

In order to sample a uniform skin region, I have chosen a point in the middle of the front head just above the eyebrows. Then I adjusted this point with three different settings for "a" and "b": the one that matches the camera WB, a "sun-burned" setting with "a" quite larger than "b", and an "extra-tanned" setting with "b" quite larger than "a". The last two settings are obviously far too extreme, and are there only to illustrate the range of results that can be obtained.



Source image: Mairi by Patrick David (cc by-sa)
Click type to see: Camera WB - Color spot (a=17,b=16) - Color spot (a=22,b=16) - Color spot (a=16,b=22)

That's it! Once you know the approximate "a" and "b" values that are needed to get the desired color tint, the procedure becomes quite fast and intuitive. And might save you a lot of time trying to get the right white balance "by eye".

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