Dumb question about color casts...

[e.rose]

So I've googled and I've come up with at least 10 different methods for getting rid of color casts... and that's all fine and dandy, but what I'm *not* finding which is what I *actually* want to know... is how do you know when you've effectively gotten *rid* of it without having created another cast of a different color?

I know there are those color checker charts, but for the sake of my question... let's assume I forgot to use it because it's fairly new to me and I keep forgetting I have it... (Actually, no assumption needed. That's *exactly* what happened. )

When I look at the image I can tell there's too much of a red cast. So I use curves to try and get rid of it... but then when I flip back and forth between my "corrected" version and the original, I can't tell if there's ACTUALLY a blue-green (dare I say cyan) cast, or if my brain is over-compensating for the fact that I was just staring at an overly red image and now I'm not...

WAAAAAAAAAY way back once upon a time... when I was first learning how to use GIMP, I sort of remember some method of matching numbers or something in curves in order to get rid of a cast... but at this point I'm not sure if I'm making that up in my head or if that's an actual thing.

Does anyone know a sure fire way of getting rid of a cast without creating a new one? Or rather, a sure fire way of TELLING that you effectively got rid of a cast?

 

[skieur]

I tend toward speed rather than complexity, so I use Nik Software: Remove Colour Cast. Viveza by Nik as well can cure some colour casts too.

skieur

 

[e.rose]

I tend toward speed rather than complexity, so I use Nik Software: Remove Colour Cast. Viveza by Nik as well can cure some colour casts too.

skieur

Thanks for the suggestion. I appreciate your taking the time to answer... however lets also go with the assumption that I don't own any Nik Software.

 

[Derrel]

Go to selective color in PS, and use that to remove the color cast or casts.

 

[e.rose]

Go to selective color in PS, and use that to remove the color cast or casts.

Thanks, I'll check it out.

 

[clanthar]

When I look at the image I can tell there's too much of a red cast. So I use curves to try and get rid of it... but then when I flip back and forth between my "corrected" version and the original, I can't tell if there's ACTUALLY a blue-green (dare I say cyan) cast, or if my brain is over-compensating for the fact that I was just staring at an overly red image and now I'm not...

Your brain can be counted on to do that. The more stressed and/or tired you are the more your brain will convince you that you're seeing the cast removed when it's not or that you made things worse when you haven't. You have to train your brain to stop it. It takes years.

WAAAAAAAAAY way back once upon a time... when I was first learning how to use GIMP, I sort of remember some method of matching numbers or something in curves in order to get rid of a cast... but at this point I'm not sure if I'm making that up in my head or if that's an actual thing.

You can use numbers to help. For example we know that certain colors are fairly consistent. In the sRGB color space a clear afternoon blue sky has a Hue value of 208/209. Go to Photoshop's Color picker and click on the blue sky in a photo. You'll see numerical values for H, S, B -- H is Hue. Hue values range from 0 to 360. 0 is red. Likewise we know that average adult skin should have a Hue value ranging from the high teens to mid twenties. Click around a person's face and if you get Hue values like 18, 22, 20, 24 etc. odds are you've got good color balance. Infants are redder and have average Hue values in the low teens to occasional single digits. If you get single digits on an adult face you've got a color cast or someone with a really bad sunburn or cyanosis -- call a doctor. If you get Hue values above 30 on an adult face your subject probably has hepatitis.

Does anyone know a sure fire way of getting rid of a cast without creating a new one? Or rather, a sure fire way of TELLING that you effectively got rid of a cast?

Yes, you have to do it. You have to develop the skill and the confidence to know you're right. If you're looking for a software solution you're wasting your time. Software can't do it. You can set a grey point in a photo if you have one. But not every photo you take will have one. Lately, I've been walking around my neighborhood photographing some of the 100 year old houses. Most have direct TV dishes on them -- perfect grey point. But what do you with a photo like this:

(Taken by one of my students -- nice portrait). There's no grey point. You might think the white stripes of her jersey are a white point you could use, but that's often a bad idea. You really want to color balance with a grey point if you have one -- if you don't have one do it manually.

The camera was set to auto white balance which should tell you a huge amount about this problem. Auto white balance on digital cameras has a 100% failure rate. If they can't make it work in the camera how could they make it work later in post processing? There's no software solution where an algorithm will do the job. Here's Photoshop's software solution to this photo:

It was bad and now it's worse. You can expect the same from various 3rd party products that claim to be able to remove color casts or otherwise white balance with a mouse click. So I took the original image and checked for skin tone Hue values. I got values in the single digits and even the high 350s! That's a serious blue/magenta color cast.

Here's a manual correction:

And when I check it I get skin tone Hue values right where they belong around 20.

You noted that you try and remove color casts using Curves. I would suggest you use Levels and convert your photo into Lab color mode. In Lab mode Levels will give you access to the Lightness channel and the two color channels "a" and "b." The "a" channel is magenta/green and the "b" channel is yellow/blue. To color balance a photo select either color channel and pull the midpoint slider left or right. Remember to convert the photo's mode back to RGB when you're finished.

This method removes an over-all color cast. It is possible, especially with camera JPEGs, to have a local color cast in a photo. In this case the work is more difficult as you need to remove a color error in a local color. Derrel noted the Selective Color dialog in Photoshop which can be used for that purpose. You can also use Photoshop's Hue/Saturation dialog.

Joe

 

[e.rose]

If you get single digits on an adult face you've got a color cast or someone with a really bad sunburn or cyanosis -- call a doctor. If you get Hue values above 30 on an adult face your subject probably has hepatitis.

Thanks for all the information. Very informative! :sillysmi:

 

[Overread]

clanthar - out of interest the values you state as averages are quite exact, is there a book/resource which gives a more comprehensive list of these (and other examples) or are the numbers based from your own experiences in photography? I wonder only as a resource which lists example values would be a great resource for people to be aware of.

 

[clanthar]

clanthar - out of interest the values you state as averages are quite exact, is there a book/resource which gives a more comprehensive list of these (and other examples) or are the numbers based from your own experiences in photography? I wonder only as a resource which lists example values would be a great resource for people to be aware of.

Wish I had a book resource. Way back in the day I used to work in a press shop (ran a stat camera and made offset plates). We had all kinds of color references and especially skin tone charts which always came with numbers. That early "paint by the numbers" experience stuck with me. I'm certain similar skin/flesh tone charts are still available. I do still have one that we photographers should all recognize. My Macbeth colorchecker is still in my desk drawer. The first two colorchecker squares are human skin tone (dark/light) and they both have the same sRGB Hue value of 19.

Here's something I made that I often find helpful:


sRGB

Adobe RGB

Those are not photographs. They're graphics that I created in Photoshop using the color values on the back of the colorchecker chart for each color.

edit: oops, forgot: here's a link to Macbeth's webpage: http://xritephoto.com/ph_product_overview.aspx?ID=820

All the colors in the chart have real-world corollaries.

Joe

P.S. By the way, if you're using a web browser that is photo capable those two charts will look exactly alike. If they look different, then you're using a web browser that can't properly display a photograph.

 

[Overread]

Aye sRGB is the format of the net - Opera is certainly showing your Adobe chart with more muted colours than the other chart.

Although one thing I am curious on is how far one goes between a colourcast and ambient light colour - for example if I shot a photo in early morning light I'd have a very strong yellowy colourcast (or reddy) over the shot, but it wouldn't be incorrect as such; but simply a function of the light at the time and also (most likely) part of the intent of the photo. So I'm curious how one classifies a colourcast that is undesirable against one that is desirable for general shooting (ie anything non-scientific).
I'm suspecting the answer is that its down to the photographer how far they take things

 

[clanthar]

Aye sRGB is the format of the net - Opera is certainly showing your Adobe chart with more muted colours than the other chart.

Although one thing I am curious on is how far one goes between a colourcast and ambient light colour - for example if I shot a photo in early morning light I'd have a very strong yellowy colourcast (or reddy) over the shot, but it wouldn't be incorrect as such; but simply a function of the light at the time and also (most likely) part of the intent of the photo. So I'm curious how one classifies a colourcast that is undesirable against one that is desirable for general shooting (ie anything non-scientific).
I'm suspecting the answer is that its down to the photographer how far they take things

Absolutely, that's the photographer's call. Assuming a grey card in a photo, it would be possible to use that as a reference and remove the wonderful warm glow of late afternoon; let's not.

So it's only a color cast if we don't like it.

How about this question: If I'm right and auto white balance doesn't work and yet 90+% of consumer cameras are all set to auto white balance, is it a color cast if they can't see it?

Joe