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White point vs. Gray
Authored by: ngb on Feb 10, '05 12:20:47PM

Has any one tried this hint by clicking on something that they know should be gray, such as a gray card, or the grayish asphalt of a road? I don't have a copy of iPhoto 5, so I can't test this out myself.

However, I do have experience with Adobe's Camera Raw converter plug-in for Photoshop, as well as Nikon and Canon's RAW file conversion tools. A tool like this is generally called a white balance picker, but it's really geared towards mid-tones. When you use it to click on an pixel, it should adjust the color temperature and tint to make the red, green and blue values of that pixel equal.

There are a couple problems with clicking on something that is supposed to be white (instead of gray), such as:
• if all three color channels are already at 255 (i.e. blown out highlights), no change will happen
• if any one channel is is at 255, drastic color shifts in the other two channels may be required to bring them up to 255, which will make your image look really screwy

I'd like to hear the results if anyone can try this out.



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White point vs. Gray
Authored by: wisechoice on Feb 10, '05 01:12:37PM

iPhoto is not setting the white point, it is balancing the point you click on to be colour-neutral, and adjusting the other colours in the image in relation to it. This has nothing to do with brightness. So if you command-click on something that should be neutral black or grey or white, it will make adjustments.

Photoshop lets you set independent white point, mid-grey and black points and takes luminance into account. If you have all three of those in a photo, you can correct both colour and density perfectly.

iPhoto is adjusting the tint and temperature only. Quite useful, but it may take a few tries. The thing to keep in mind is that the point you command-click on does not need to be white, it can be grey or black as well.

If your photo has more than one type of light source (sunlight, flourescent, incandescent), this will not work perfectly because the different colour "temperature" of light sources is what causes photos to be off in the first place. If you don't have a uniform colour temperature from the light sources in your photo, some things that should be white will have a blue, yellow or green cast.



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White point vs. Gray
Authored by: stcanard on Feb 10, '05 05:09:51PM

Yup, this is definitely a gray balance.

I've used it to rebalance a number of outdoor photos I took in the summer, with the white balance accidentally set to fluorescent.

It works really well, but you have to be careful what you pick because it's really easy to introduce a nasty colour cast. It works best if you can find a mid-tone gray somewhere.



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