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How to remove a colour cast


littletank

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I am interested in exploring ways of removing colour casts and would welcome your advice and expertise. I am aware that there are several ways that one might try but I have never had a real discussion around the problem.

As an example here is an image obtained by scanning an old 35 mm colour slide which has a marked red cast how would you go about removing it?

crown.jpg
 

Tom Mann

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The usual way to fix a print or transparency that has turned red is to bring in both left and right end points of each of the R, G, and B sliders in a Levels adjustment layer to the point where the histogram is just starting to show significant counts.

Unfortunately, I rarely see cases of color fading that are this bad, and when I try the above procedure, not only are the results unsatisfactory, the starting histogram does not look anything like what I would expect in this type of age fading.

This leads me to wonder if the image that you posted came from either a faded (color) negative (in which case, we should be working on that, not a positive from it), or a color correction has already been attempted on this image (in which case, we should be working on the pre-CC version).

If neither of the above possibilities are true, then my suggestion would be to convert it to a B&W image and colorize it (which is a separate topic, LOL).

Attached is a quick BW conversion (probably a bit over-sharpened in my attempt to pull out some detail in the featureless areas like the cloth).

Cheers,

Tom M
 

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Eggy

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I remember having to correct an orange cast slide scan some time ago.
I think (using my tablet now) I duplicated the layer, average blur and inversed color.
I can't remember what blend mode I used on that layer and then played with the opacity slider.
I'll have to check it tomorrow.
 

IamSam

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My method is similar to Eggy's.

I duplicated the layer.
Inverted the layer with Cmd/Cntrl + I
Screen Shot 2016-11-07 at 8.55.38 PM.png

Set the blending mode to color.
Screen Shot 2016-11-07 at 8.55.55 PM.png

Then lowered it's opacity to 50%.
This pretty much takes care of the color cast.
Screen Shot 2016-11-07 at 8.56.49 PM.png

I then added three different Hue & Saturation adjustment layers set to colorize.
I made color specific changes for different areas of the crown.
I inverted their masks and used the Brush Tool to add back or augment missing color.

Mind you this is fast and not color accurate.
Screen Shot 2016-11-07 at 9.00.10 PM.png

I then added a Curves adjustment layer to increase the overall contrast.
Screen Shot 2016-11-07 at 9.01.04 PM.png

There is a ton of more work to be done here, but this is only a sample so I will stop.
 

Tom Mann

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Finally, as I suggested in post #2 in this thread, I colorized my earlier B&W version. For ease of comparison, I used colors similar to those that Sam selected and in the same limited locations on the crown.

As might be expected, the result is not all that different ... maybe a bit sharper (at least at low magnifications, LOL), a bit cleaner and brighter background, and a bit better ability to see into the murky shadow areas, but all of these features are there only because I paid specific attention to them, whereas Sam's method is much faster to do & is much simpler.

T

PS - I'm still interested in knowing the history of this image, specifically, is what we see the raw scan of the slide, or did you manipulate it?
 

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Tom Mann

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littletank -

Besides the obvious technical and appearance / quality issues, another factor in deciding on an overall approach is how you like to work.

I didn't feel that there was a good reason to preserve whatever colors might have been present in the original, so I desaturated the image almost immediately and spent a good bit of time working on the B&W tones to come up with a decent looking BW image. In contrast, Sam's method has the possibility of making highly faded colors once again visible, and from there enhances / colorizes them as needed. A method such as his to help determine the original colors can be very helpful in subsequent colorization steps.

Tom M
 

littletank

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Some time ago I bought a job lot of old, commercial colour slides to experiment with. Amongst them were slides produced by Woodmansterne and I wrote to them, telling them what I was hoping to do and asking for their permission to publish anything of interest. They replied saying that they no longer were in the business of making and selling slides and gave their permission allowing me to go ahead, provided I mentioned their name.

The image I posted is from one of their slides, scanned using a Nikon Cool Scan IV scanner and ViewScan software. The only digital changes made was to re-size the scanned image to meet the forum requirements. I have no idea of the age of the slide but it was possibly part of a set issued around the time of the coronation of Queen Elizabeth II.

At the moment I use the method outlined by IamSam to remove colour casts but my colour recognition is not sufficiently acute to let me be involved with colour restoration therefore, I remove the cast prior to a monochrome conversion. Perhaps the crown was not the best example to use and I have many other examples of colour casts and colour degeneration including landscapes. My intention of experimenting with these old slides is to see if, from what was clearly a record shot, an image of interest can be produced in monochrome.

Thank you all for taking the time and trouble to respond and, if anyone is interested in continuing the discussion I would be delighted to co-operate.
 

Tom Mann

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Please don't go to any great lengths to find it, but if it's easy to do, I certainly would be interested. The reason is that I've done a lot of restoration from scans of faded color slides and prints over the years (ie, well into the hundreds), and I just don't ever remember any looking this bad, either to my eye, or, more importantly, in the histogram. OTOH, the largest group I have ever done was from the late 1950's, with only a handful earlier than that, so it certainly is possible this one is much older than what I'm used to.

With respect to keeping the discussion going, I would indeed be interested, but I have a couple of family obligations later in the week, and 2 big events to shoot this weekend, so I will likely not be as available as I would like, but I'll keep checking in.

To start the discussion, are you familiar with the technique of using a curves adjustment layer, finding a few places in the photo that you know are neutral (or some known color), and which cover the full range of brightness, then adjusting the curves so that each of those points come out r=g=b?

That's another of my favorite techniques.

T
 
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littletank

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The unknown quantity about these commercial slides is the film onto which the copies were made and the age and storage conditions of them, some are faded and others have varying amount of colour cast and degradation. The only reason I use these slides is that I have a shortage of my own slides and negatives. Most of my slides show only very little colour cast which tends to be blue on Kodachrome 25 rather than red and the negatives seem to be trouble free.

I have tried methods similar to that which you mention but my main problem is that I am not very good with colour recognition. This is why I am not interested in the restoration of coloured images but prefer the monochrome route. However, I do believe it is necessary to get the image as near cast free as possible and colour and luminosity balanced before converting to mono.
 

Eggy

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This is why I am not interested in the restoration of coloured images but prefer the monochrome route. However, I do believe it is necessary to get the image as near cast free as possible and colour and luminosity balanced before converting to mono.
In that case why not use the free NIK plugin bundle, containing Silver Efex Pro 2 which could make your job easier and faster
There's room for manual adaptation while using that plugin.

crown A.jpg
 

littletank

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I have both NIK and Macphun but time is no problem to me I have very little else I can do. That's a very nice conversion you have produced, Eggy, and gives me something to use as an example. I intend to have another look at some of my own slides and negatives to see if there are any worth playing around with making selective enlargements as I used to do in the old days in the dark room.
 

Tom Mann

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The unknown quantity about these commercial slides is the film onto which the copies were made and the age and storage conditions of them, some are faded and others have varying amount of colour cast and degradation. The only reason I use these slides is that I have a shortage of my own slides and negatives. Most of my slides show only very little colour cast which tends to be blue on Kodachrome 25 rather than red and the negatives seem to be trouble free. ...snip...
Ahh. Good point. I thought you had the originals. I didn't realize that you had copies of the true originals. I'll bet that they might have tried to do some color correction before the images ever got to you. This would explain what I consider to be the bizarre histogram.


...I do believe it is necessary to get the image as near cast free as possible and colour and luminosity balanced before converting to mono.
I agree completely with you that to get the very best results, especially in difficult cases, one should try to minimize problems with hue, saturation and luminosity before doing the B&W conversion step. I use that method quite often, and, in fact, on your crown image, I tried to repair the odd luminosities and contrast in the different areas of the crown image (eg, the cloth area had almost no contrast in the version you posted) before I did my conversion to B&W before re-coloring. I would caution that in some cases, the required adjustments are so extreme, you wind up introducing noise and other artifacts into the image, so one is often better off trying to get better, but not perfect colors before conversion.

Also, don't forget that for less severe situations, to save you some time, PS's "Black & White" adjustment layer does at least provide a quick and dirty way to make luminosity adjustments when converting. And, as I recall, some of the non-native PS tools like NIK's and Power Retouche Pro's B&W conversion plugins provide features like simulation of various B&W filters and other fast and easy-to-use in-line adjustments before the B&W conversion takes place.

Cheers,

Tom M
 
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