Posted by: David Vernon | May 27, 2009

Photoshop Friend #2 – Removing Color Cast

The Tree 39For new shooters, there is often some hesitation to shoot in RAW. A lot of folks just don’t need to do any real level of post-processing and for them JPEG is a pretty acceptable and realistic format to work in.

One of the major benefits of shooting in RAW however is the ability to render color correctly – even after the fact. It’s far easier to correct your white balance in RAW then in JPEG – but that doesn’t mean you can’t take a JPEG image and get any color cast out of the way. First let’s examine what a color cast is…

COLOR CAST

A Color Cast is pretty simple really. It’s a tint – applied overall to a photo – in an undesired way. A white balance misadjustment is usually the primary cause for this. This is one of the big reasons for shooting a “grey” card as a reference image for your lighting conditions. We know that a grey card is completely neutral (the RGB values should all be the same if the color cast is removed) so we can accomodate for that – adjusting the color in the image until the grey card reads neutral again – and then we know we’ve got the color right. If we shoot in RAW, we can simply use the white balance adjustment tool in Adobe’s Camera Raw engine to remove the cast. We just click on the grey card in our reference image with the tool and WHAM! the color cast is removed. BUT if we shoot in JPEG we need to come at this from another direction. Let’s start with an image again – and let’s go through a number of ways to correct the Color Cast – because in Photoshop there’s always a few ways to do this…

REMOVING COLOR CAST WITH ADOBE CAMERA RAW

Cast01 Here’s another image from Ireland – a sign, if you will, of things to come if you drive the wrong way. There’s definitely a blueish-green cast to this image. But how can I tell? I opened this file in Adobe Camera Raw (yes – it was a JPEG but you can still open JPEGs in ACR) and I went searching for the important telltale sign – I went in search of something NEUTRAL. What’s neutral? Something white is neutral (the RGB values should all be pretty close). Same with something black and something grey. But we tend to stay away from black when we make adjustments – they may just be too subtle. But look – I have a white sign on the left-hand side of the image. Mousing over the white in the sign I see my color values.

Cast02 Look at the RGB values in the image (R: 140, G: 166, B: 162). Yeah – for something I expect to be neutral, the red is really low and the blue and green is pretty high in comparison. Hence a blueish-green cast. Now even though this is a JPEG, I can use the ACR White Balance adjustment to correct temperature and tint. I won’t be setting it to a specific temperature but I will be neutering my neutrals (so to speak). How do I do this?

The quickest way – and frankly it’s a pretty good solution for beginners – is just click the drop down and change “As Shot” for white balance to “Auto.” This is going to get you in the ballpark immediately. If you want to tweak it from there, that’s completely your call but there is no shame in using the Auto key! These tools are pretty darn good nowadays. So let’s do just that. Here’s the shift:

Cast03

And a closer look at our white balance panel shows:

Cast04Sure enough, our cast has been removed – and our white is almost completely neutral. I tweaked my temperature and tint sliders just a tad to my liking but I stayed within a point or so of neutral.

Now this is all fine and dandy if you have Adobe Camera Raw. You can open your file from here (or just save it) and move on. But what if you don’t have ACR – is there a way to do this inside Photoshop (or something like Photoshop)? And of course the answer is yes.

REMOVING COLOR CAST WITH CURVES

After I open the original non-color corrected image, I’m going to take the Eyedropper Tool and SHIFT-click it on something neutral (my sign) and set a reference point (I’m going to set the eye-dropper to a 3×3 Average – not as a Point Sample). When you do this, Photoshop leaves behind a small cross-hair symbol with a small number attached to it. In our case we see this:

Cast05Sure enough, our neutral white is far from neutral. The blueish-green cast is still there.

Now I’m going to do something interesting to prepare to remove the color cast. I’m going to create an artificial grey card so that I can “stick it in my image” to remove the color cast. And I’m going to do that by finding the white and black points in the image (the same basic white and black points from yesterday’s Levels lesson). And I do this with a temporary “Threshold Level”.

A threshold level is also pretty simple. I’m going to pick an arbitrary value (let’s start with 128). Every pixel in the image with luminosity less then 128 will show up as black in my threshold level and every pixel greater then 128 will show up as white. It will look like this:

Cast06To find the black point, I’m going to slide that threshold slider all the way to the left until all the pixels in the image are white. Then I’m going to slide it back to the right just a tad – until some pixel (or pixels) falls beneath the threshold and turns black. And those pixels are the ones I’m after. I’ll take and do another SHIFT-click on that point and with that I’ve set my black point. Just so you can be sure – it will look like this:

Cast07Not much to look at – since almost all the pixels are now above my threshold. But eventually I did find the darkest pixels (the lowest luminosity) and I added my eye-dropper icon (#2) right over the top of those pixels.

Next – I do the same thing in the highlights. I run my threshold up to the other end until all the pixels turn black. I then back it off just a hair until I find my first white pixels. Then I do another SHIFT-click to set my white point and I’m ready. I can actually CANCEL this layer – I don’t need it. The white and black points will remain for my next adjustment. Let’s go do it.

First you need to create a Curves Adjustment Layer or open your Curves dialog. It’s going to look something like this:

Cast08First of all, if your Curves dialog only has a 4×4 grid, don’t sweat it – it won’t impact what we’re doing here (although you can always ALT-click on the grid – on a PC – to toggle between the 4×4 and 10×10 grids). If this is the first time you are correcting color cast with the Curves dialog, we have to do one extra set-up step. We have to set the eyedroppers to their appropriate levels. Start by double-clicking on the black eyedropper. It will present you one more dialog box – that looks like this:

Cast09Just set your H (Hue), S (Saturation) and B (Brightness) settings like you see here (0,0, and 5). Then Click OK. Repeat the process for the white eyedropper by double-clicking on it. Set your values for that one to 0,0, and 95. When you’re done with this correction – Photoshop will ask you if you want to save these settings for future corrections. You do.

Okay – with that one time set-up out of the way it’s pretty easy from here on out. Go back and single-click the black eyedropper. Then move out to your image (you’ll want to zoom in to make sure you hit your mark and not some neighboring pixels) and find the black point you established in your threshold level. Just click the black eyedropper on that black point. Then take the white eyedropper and click on the white point you established. You’re now most of the way there. Let’s take a look:

Cast10Alright – we’ve removed most of the cast. Remember, point #1 was our neutral reference point. It still reads with a slight blue cast. Point #2 was our black point. It’s pretty good – everything is within a point. But the white point – it’s reading slightly red. Let’s reopen our Curves adjustment layer and finetune it. We’ll go into the dropdown at the top (the one that says Channel) and we’ll pick the Red Channel.

Cast11It’s very easy to work with this – no matter how complicated it looks. I simply CTRL-Click on my white point (marker #3 from earlier) and this creates the little red square on my curve. I can then use the arrow keys to adjust my curve. Since I have a red cast in my highlights, I’m going to use the down and right arrows to move the curve away from red. Each down and right click adjusts the curve slightly and you’ll see the red indicator drop in your white point reference.

You may have to tweak all three color channels – but just keep repeating the process until your cast is gone. It usually just takes very small adjustments at the ends of the curves (upper right for highlights – lower left for shadows). To add a little color (red, green or blue), simply arrow up and left. To subtract those colors it’s again down and right. Just CTRL-click on your white and black points within the approrpiate channel you want to change. It doesn’t have to be perfect – remember where you started after all and it may drive you crazy trying to get everything neutalized.

If you get your white and black points perfect (or close) and there’s still a shift – then that shift that remains exists in the midtones. In this case, just find a point somewhere in the image where the values read somewhere in the middle. In this case I just finetuned my original reference point (it was just a little above middle grey) by opening my curves up and CTRL-clicking right on that point. When we’re all said and done we’ve removed the cast:

Cast12Now my black points and white points are pretty much right on – and my neutral white is also spot on. By using reference point #1 (on the sign), I was able to tweak the midtones to remove the rest of the red cast from earlier.

Obviously, using Adobe Camera RAW to tweak your JPEG is a one-click solution. But if you don’t have ACR, and are willing to spend a few extra clicks – you can pretty much get any color cast out of the image. If you’re shooting JPEG, take that extra moment to set your white balance before you shoot – it will pay off in the end. There’s nothing like a bad color cast to ruin an image.

Tomorrow: Let’s take one extra step – we’ve improved tonal range and corrected color cast – now let’s make the image a wee bit sharper.

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Responses

  1. Cool tutorial! I’ve never tried that before!

  2. […] I was all set to write two more Photoshop articles – to complement our lessons on levels, color correction and sharpening from last year. And then I come to find out that our own Bill Shaner had already […]


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