Methods for Removing a Color Cast

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mojosam
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Methods for Removing a Color Cast

Post by mojosam » Thu 16 May 2019 08:13

I'm planning to write a few tutorials for how to use PhotoLine. It spurs me to learn the program better.

The first one (or two) articles will be on how to remove a color cast from an image. I have been experimenting and reading how people do this in other programs.

The following is a brief description of the methods I've discovered.
1. Which of these methods are preferred?
2. Are there any refinements or corrections you folks would suggest?
Thanks!

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A. AUTOMATIC FILTER METHODS
1. Duplicate layer.
2a. Filter > Automatic > White Point Correction.
AND/OR
2b. Filter > Automatic > Brightness Correction.
AND/OR
2c. Filter > Automatic > Color Correction.
3. Set blend mode to Normal (to block the underlying) or Hue (to blend with the underlying).

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B. CURVES, FREEHAND (my current favorite)
1. Layer > New Adjustment Layer > Curves.
2. Change color mode to Lab.
3. Select the Green-Magenta or Blue-Yellow color channel that corresponds to the color cast.
4. Grab a point on the line in the region over the peak with the largest area. Drag up or down, depending on which color you want to eliminate.
5. Optional: Adjust the Contrast, Luminance, and Gamma sliders to taste.

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C. CURVES, COLOR PICKER
1. Layer > New Adjustment Layer > Curves.
2. Use the black, white, or gray color picker to pick a spot that is supposed to be black, white, or 50% gray.

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D. INVERTED BLUR
1. Layer > Management > Duplicate Layer.
2. In Layers Attributes panel, change Type to Lab.
3. Filter > Blur > Calculate Average.
4. Tool > Color > Invert.
5. In Layers panel, change blend mode to Overlay.
6. If desired, adjust the Layer Intensity for a greater or lesser effect.
NOTE: Tim Grey on YouTube says to change the blend mode to Color instead. That requires changing the Opacity to somewhere around 50%. Then add a Hue/Saturation layer and boost the saturation.

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E. SELECTIVE COLOR CORRECTION
1. Tool > Color > Selective Color Correction.
2. Select Whites in the Colors popup menu.
3. Adjust the appropriate color.

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F. HISTOGRAM CORRECTION
1. Layer > New Adjustment Layer > Histogram Correction.
2. Select the color channel corresponding to the color of the cast.
3. Click on Auto Mode for Active Channel. If the correction isn't strong enough, adjust the appropriate upper or lower bound further.
4. Adjust the gamma, if necessary.
5. Tweak the other colors, if necessary.

Martin Huber
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Re: Methods for Removing a Color Cast

Post by Martin Huber » Thu 16 May 2019 10:32

The first ones that come to my mind are:
- Filter > Digital Camera > Whitepoint
- Filter > Digital Camera > Color Temperature
- Filter > Digital Camera > Color Correction

They are also available as adjustment layers.

In certain situations, "Filter > Quality > Dehaze" removes a color cast, too.

Martin

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russellcottrell
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Re: Methods for Removing a Color Cast

Post by russellcottrell » Thu 16 May 2019 17:00

For a photo with a really bad color cast, such as an old faded print, the automatic methods are shortcuts that can get you into the ballpark, but I would still finish it with curves. In addition to Lab, RGB curves can be used with the blend mode set to Color. Since they are curves, you can adjust as many points as you want, especially key elements like skin tones, or the sky.

http://www.russellcottrell.com/photo/Ph ... Curves.htm
http://www.russellcottrell.com/photo/Ph ... ration.htm (Lab curves about 2/3 of the way down)

pixel8tor
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Re: Methods for Removing a Color Cast

Post by pixel8tor » Thu 16 May 2019 18:37

Seems to me, first you need to determine if there is a color cast. Some images, like sunsets and others, have a natural cast and trying to correct that isn't helpful.

Anyway, here are a few things I look for:

1) do shadows and highlights have a similar color bias?

2) is there something in the image that should be gray/neutral, but isn't?

3) are skin tones believable?

4) are other "memory"(sky, foalige, water, . . .) colors believable? Are they all "wrong" with the same color bias?

Those are a few things I look for - I'm sure there are others I do subconsciously.

What about you? How do you determine color cast?

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russellcottrell
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Re: Methods for Removing a Color Cast

Post by russellcottrell » Thu 16 May 2019 19:06

Also, remember that a literal rendition of the colors is not necessarily what you want in a photograph. A sunset might look better (to you) with the color cast increased. Under sun and open sky illumination, if the highlights are rendered neutrally, the shadows will be blue, which may be disturbing if they include skin tones. Warmer colors tend to advance and cooler colors recede; this can be used creatively. Consider an interior scene with a beam of sunlight coming through the window; a neutral background with a very warm beam of light is probably not literal but is likely to be more interesting. In general, I consider a “correctly” rendered photograph to be a point of departure. (Unless you are making scientific or forensic photos, or advertising.)

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mojosam
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Re: Methods for Removing a Color Cast

Post by mojosam » Thu 16 May 2019 19:11

What about you? How do you determine color cast?
Yes, color casts are often intentional. Photographers love to shoot during golden hour, for example.

What got me started down this path is I have a roll of color film that was processed badly. There's a nasty greenish tint to everything. I've had the hardest time removing it. It was a great learning experience, though!

For my forthcoming article, I have made a test shot with a digital camera in broad daylight. I set the white balance to various settings. I have been able to correct most of those to something approaching normal. The one I can't seem to fix is the tungsten-balanced shot. It has a nasty blue that I can't get rid of without ruining the other colors and the exposure level.

I want to write one or two articles covering the common, most effective approaches to fixing a color cast. If you look at YouTube, you will see that most people demo more than one approach. Different approaches for different problems, apparently.

The most common use case in videos and articles is fixing old photographs. I suppose the more common use case for digital shooters is somebody who only shoots JPG and has set the white balance on his camera incorrectly. The worst (yet common) case is if the scene was lit by two different light sources (e.g., indoor lighting, with sunlight coming in through a window).

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mojosam
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Re: Methods for Removing a Color Cast

Post by mojosam » Thu 16 May 2019 19:19

russellcottrell wrote:
Thu 16 May 2019 19:06
Also, remember that a literal rendition of the colors is not necessarily what you want in a photograph.
Yes. Many good photographs have been doctored to convey what the photographer felt, not what he saw. Also, cameras aren't good at reproducing the scene as perceived in our brain. We often want to "fix" them, even if the camera is more "technically" accurate.

The whole thing is subjective.

bruce1951
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Re: Methods for Removing a Color Cast

Post by bruce1951 » Fri 17 May 2019 00:38

It's a tricky subject. My wife and I often disagree on what we saw. Each of us have a different perception of what we are looking at. Complicate that with what we remember and it only gets more complicated.

bruce

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mojosam
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Re: Methods for Removing a Color Cast

Post by mojosam » Fri 17 May 2019 04:00

russellcottrell wrote:
Thu 16 May 2019 17:00
In addition to Lab, RGB curves can be used with the blend mode set to Color.
Aha! The RGB curve set to Color blend is probably the most effective method for fixing the tungsten-balanced photo that I've tried. None of the other techniques worked that well.

So apparently you have to go Layer > New Adjustment Layer > Curves, then click OK in the Curves dialog box without doing anything, in order to close it and create the layer.

Then you have to change the blend mode to Color.

Then you double-click the Curves diagram in the Layers panel to reopen the Curves dialog box. Now you can make your changes and see them taking place correctly in your image (if Preview button is on). Otherwise, any changes you make would be in Normal blend mode, which would be misleading.

That's not an ideal workflow. Any way to not have to close then reopen the Curves dialog?

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russellcottrell
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Re: Methods for Removing a Color Cast

Post by russellcottrell » Fri 17 May 2019 05:36

I had a feeling that would work the best.
I have always wished that the curves and hue/saturation dialogs had the blend mode on them for this very reason. If you want, you can record the layer creation with specified blend mode as an action.
You can also use the adjustment layer panel, but I find it cumbersome.