iOS White point/white balance adjustment examples/suggestions

Question

I am trying to change the white point/white balance programmatically. This is what I want to accomplish:
- Choose a (random) pixel from the image
- Get color of that pixel
- Transform the image so that all pixels of that color will be transformed to white and all other colors shifted to match

I have accomplished the first two steps but the third step is not really working out.

At first I thought that, as per Apples documentation CIWhitePointAdjust should be the thing to accomplish exactly that but, although it does change the image it is not doing what I would like/expect it to do.

Then it seemed that CIColorMatrix should be something that would help me to shift the colors but I was (and still am) at a loss of what to input to it with those pesky vectors. I have tried almost everything (same RGB values on all vectors, corresponding values (R for R, etc.) on each vector, 1 - corresponding value, 1 + corresponding value, 1/corresponding value. RGB values and different (1 - x, 1 + x, 1 / x).

I have also come across CITemperatureAndTint that, as per Apples documentation should also help, but I have not yet figured out how to convert from RGB to temperature and tint. I have seen algorithms and formulas about converting from RGB to Temperatur, but nothing regarding tint. I will continue experimenting with this a little though.

Any help much appreciated!

Howardhowarth

White point adjustments aren't something you can explain in some lines. I wrote a whole chapter in my Bachelor's thesis about this. Of course you might not need all the background info. I just tried CIWhitePointAdjust and I can't imagine what this would be used for. Can you tell what the exact use case would be in your application? – Porkpie

I am trying to build a simple photo editor. I would like to pick a photo taken with the device in low-lit situation (for instance of a white plate of food in a restaurant) that has wrong colors (the white plate will usually be yellow) and transform the photo so that the plate will be white and other colors will be changed accordingly. – Howardhowarth

I think you would need to look into chromatic adaptation. There's a bit of math needed but all formulas are available. www.brucelindbloom.com is your best friend here. As a fair warning: this is not trivial! – Porkpie

Thanks, my reasearch has already gotten me this far. Already visited Bruce Lindblooms site and looked into the math. – Howardhowarth

Answer 1

After a lot of experimenting and mathematics I finally got my app to work almost the way I want.
If anyone else will find themselves facing a similar problem then here is what I did.
I ended up using CITemperatureAndTint filter supplying a color in Kelvins calculated from the selected pixels RGB value and user suppliable tint value.
To get to Kelvins I:
- firstly converted RGB to XYZ using the D65 illuminant (ie Daylight).
- then converted from XYZ to Yxy. Both of these conversions were made using the algorithms found from EasyRGB.
- I then calculated Kelvins from Yxy using the McCamry's formula I found in a paper here.

These steps got the image in the ballpark but not quite there, so I added a UISlider for the user to supply the tint value ranging from -100 to 100.

With selecting a point that should be white and choosing values from the positive side of the tint scale (all the images I on my phone tend to be more yellow) an image can now be converted to (more) neutral colors. Yey!

I supplyed the calculated temperature and user chosen tint as inputNeutral vector values.
6500 (D65 daylight) and 0 as inputTargetNeutral vector values to CITTemperatureAndTint filter.

Howardhowarth

Hey, I'm trying to see if this will work for me, but your link to the Kelvin conversion is broken. It's a Google Doc that looks like it's no longer available. I also couldn't figure out from the Apple docs how to control the tint, as you describe. Would you be able to post some (or all) of your code? – Neille

This DSP answer gives an abbreviated calculation that's pretty useful. – Neille