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Old Dec 1, 2004, 3:29 PM   #11
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Stevekin: You're welcome.

TimS: You're welcome, too. :-)

You're suspicion is correct. Just because an image has a true white point and a true black point does not mean it doesn't have a color cast. To be confident that you've removed all color cast, you'd need to establish and force a gray point, too. The more medium-gray that point is, the more confident you can feel that you've eliminated the cast. (FYI -- and you might already know this -- when working in RGB, any value wherein the Red, Green and Blue are all equal, is a gray tone.)

However, that's easier said than done. For me, the most challenging part of color-correction is finding that gray point. After all, I'm red-green color blind, so I'm not so confident in my ability to always judge what should be just gray and what should be gray with a brownish tint, for example.

Sometimes it's easy. For instance, if someone in the photo was wearing a gray T-shirt, then my question is answered. Sometimes you can find a gray rock, but that's tricky because many rocks are brown. To make this even more of a pain ... many photos don't even have a truly gray object in them. If you can find a white cloud that's not PURE white, that can help a little. So can a black sweater that, while black, isn't PURE black (not as black as a deep shadow). These extreme shades of gray can help you get at least a little closer to removing a color cast.

The other issue is that you may not want tocompletely remove a color-cast anyway. It's not uncommon to WANT a color cast. For example, I have a photo of Maine's Prospect Point Harbor Lighthousethat wouldn't have the same mood if I removed the blue cast; there's a blue sky reflected in blue water, making for lots of blue light bouncing around. It just wouldn't look right if it was fully corrected. The same can be said for sunset photos, many of which have red casts to them. If you removed that red cast, it wouldn't look like a sunset anymore. My point is (I swear I have one) not to get too caught up trying to find that gray point, because you might be better off leaving it alone anyway.

As for using software other than PhotoShop ... It's true that some photo programs don't have a Curves feature, which makes it hard to do accurate color corrections. Curves is a very powerful tool. You can also try to do acolor-correction using Levels, if your program's Levels dialog lets you break the correction down to Red, Green and Blue. On each channel, just move the white and black sliders in to just barely clip the ends of the histogram, and that will give you a very basic correction. It's worth experimenting with.

And yeah, I get the traffic light question now and then, too. How's this answer? "Lights are no problem at all, because they're three different colors. Orange, blue and brown."

--Chris Nicholson
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Old Dec 2, 2004, 11:24 AM   #12
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Re: Traffic light situation.

I always respond with.....The light at the top means "stop", the bottom one means "go" and the middle one means "go onI dare you".

Stevekin.
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Old Dec 2, 2004, 11:32 AM   #13
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Hi guys,
This is an interesting thread.
Now for the archives.
True color blindness is a lack of any functioning cones(L, M, S). Just as regular blindness you do get degrees of it. Such as Red-Green or Tritons(blue insensitive). The S cones are concerned with Blues and Violets, the M with Greens, and the L with reds and oranges. Arrangened along the spectrum you see as a gradient.

There are others though such as me who have three functioning cones, but do not see the same colors as other people do, we are (get out the dictionaries) Anomylous Trichromats meaning that we are not color blind but we do see the color spectrum differently. Personally I am hypersensitive to blue and into the UV So i see a view of color much closer to the camera's view. It is a weird one, and i feel very, very fortunate that i can somewhat turn things on and off, and distinguish shades and tints that others can't; but i cannot always be certain that the color i am seeing is the same as really anybody else.

Personally i just know which colors are likely to be wrong, and look at the RGB values for the portions of the image that they lie on and check that, or allow my color profiled camera to do the work for me.

Hopefully the rest of you will find this as fascinating as i do.
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Old Dec 2, 2004, 11:34 AM   #14
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Stevekin wrote:
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Re: Traffic light situation.

I always respond with.....The light at the top means "stop", the bottom one means "go" and the middle one means "go on I dare you".

Stevekin.
That usually works, but here in Seattle and portions of Wisconsin They are sometimes horizontal. Worse though are the red or amber lights which mean either caution or four way stop.
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Old Dec 2, 2004, 12:33 PM   #15
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Stevekin wrote:
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Re: Traffic light situation.

I always respond with.....The light at the top means "stop", the bottom one means "go" and the middle one means *"go on*I dare you".

Stevekin.
My driver ed. teacher in high school used to say, "Red means stop, green means go, and yellow means go like hell!"!!!!!!!!!!!!!:evil:
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Old Dec 2, 2004, 3:12 PM   #16
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Sounds like drivers ed teachers all say that kind of thing. I remember the most often uttered phrase for me was "The light is green, why are you still brakeing?" I guess i didn't trust the car gremlins to stop the car if i wasn't looking at the speedometer.:lol:
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