What's up with CITemperatureAndTint having vector inputs?

Question

OK, so the Core Image filter Temperature and Tint has two inputs, neutral and targetNeutral. However, my biggest issue is the fact that they're both two-component vectors, meaning each has two numeric inputs. I would expect the first to be from say 2500 to 10000. What would the vector be for?

Answer 1

The essential purpose of performing temperature and tint adjustment is to correct the white balance of a captured image: to account for the ambient illumination of the scene and adjust colors so that the image appears more like it was shot in "white" light (roughly 6500K).

Temperature relates to the warmth or coolness of an image, and is normally characterized qualitatively as orange-ish or bluish.

Tint refers to the deviation toward green or magenta of colors at the same temperature. Note that tint (defined as such) is mostly independent of color temperature. (Take a look at a CIE diagram with the Planck locus and isotherms drawn on it to develop your intuition about this. Here's one: http://en.wikipedia.org/wiki/File:Planckian-locus.png).

So, when you are interested in performing white balance adjustment (whether to make an image appear more realistic, or for artistic purposes), there are four different parameters you must supply: the temperature of the initial image, the tint of the white point as it appears in the initial image, the desired color temperature of the output image, and how tint-shifted the "neutral" tones should appear in the output image. The temperature/tint combination of an image is a function of the ambient light in the scene and the response of the material being imaged, and both temperature and tint are necessary to meaningfully characterize the white balance of a captured image.

This is why CITemperatureAndTint takes two vectors: it wants the two pairs of (temperature, tint) just described.

Now, if you want to create a UI for controlling white balance, you don't actually have to give the user control of all four of these values. Instead, hold the second vector (TargetNeutral) at a constant (6500, 0) and allow the user to adjust the other vector (Neutral). With this arrangement, the user will be selecting the perceived color temperature and tint shift of the original image. (You might choose instead to hold the Neutral vector constant and allow the user to tweak the Target Neutral vector; this may be more appropriate in contexts where the user wants to adjust the white balance artistically, but the correlation between the selected values and the resulting image is not as obvious).