Kens Photoshop Blog

(Yes I know the images are messed up, damn wordpress won’t leave my html layout alone grrr.)
For those times when you have shot photos without your Macbeth color chart or a gray card and have a color cast because your camera got confused and screwed up the white balance, this technique comes in very handy.

To correct a color cast, you need to find the white, black and gray areas of your image and make sure the RGB values line up, for example if you have a white area and the histogram tells you that the values are 245,240,253 at the whitest area of your image then you know you have a color cast, the problem lies in finding these areas, just hunting around your image and pecking at the colors by looking at them is a tedious and error prone process.

Fortunately photoshop can help us here. First go to your layers palette and select ‘threshold’

Once the threshold dialog comes up, pull the slider all the way to the left, this will turn your image white, now *slowly* move it towards the middle, the first colors you see come back is your darkest areas, so click OK to add the adjustment layer.

Now go to your toolbox or hit ‘I’ a couple times until you get the color sampler tool, place a marker at a splotch of black, you have now located the black point that we need for later.

Now double click on the threshold adjustment layer to bring up the dialog again, this time move the slider all the way to the right, then slowly move it back towards the middle, the first colors that comes back now will be the whites areas, click OK.

Now place a color marker again at a white splotch of color, you have now located the white point in your image.

Now comes the tricky part, locating the gray area of your image, the threshold layer won’t be able to help us here unless we give it a little help ourselves.

Make sure you delete the old threshold adjustment layer since we are done with that.

Add a new blank layer above the image, fill it with 50% gray and set this layers blending mode to Difference.

Now we can add a threshold adjustment layer above the gray layer, move the slider all the way to the left, then pull it towards the middle, the first colors that comes back now will be our grays. Note that some images will not have a gray area, so if you find you need to pull all the way to the middle there is probably not any good gray area in your image.

Add a color marker at the gray area, and we are ready to neutralize our white, black and gray (If any) points, make sure you delete the gray and threshold layer before you continue.

You probably noticed an annoying window popping up every time you added a color marker, this window is very important so move it to the side so it won’t be obscured by other windows and have a look at it.

You will notice your color markers are displayed here, numbered #1, #2, #3, which corresponds to the black, white and gray markers you placed. These colors needs to be neutral, the RBG values needs to be the same or else there is a cast in your image, this is where the Curves dialog comes in handy.

Add a Curves adjustment layer. Let’s start with the blacks, look at the RGB values, we have a cast here in the reds, you can see it says 7 and the green and blue values are at 1, so wee need to bump down the reds, select the red channel from the drop down at the top of the curves dialog.

Move your mouse cursor out into the image area, now press and hold the control key down while also pressing the left mouse button and move exactly to the color marker labeled #1, you will notice while you move the cursor around a little bouncing ball flails around in the curves dialog box, and soon as you let go of the control key, a point will be placed in the curves dialog. Now that you have placed a point in the reds curve we need to ‘lock’ the affected area down so we don’t touch other colors near the reds, you do that by placing another point in the grid above the point you placed.

Now you can press tab and shift-tab to select the first point, then move the point downwards by pressing the down arrow key, watch the red level in the info palette for the black marker, and stop when it’s a 1 like the 2 others. Note than sometimes you will need to place more than 1 lock down point a little higher or lower in the curves grid, if you find that the slope affects to much of the image, watch the other color markers while you change the values, if the others are changing to you need more lock down points.

From here on it’s simply a matter of repeating the above process for the white and gray values, I suggest you allways start with the grays, as fixing the grays usually brings the blacks and white closer together and takes a lot of the work out.

But how do you find out which value is the correct if all 3 (RBG) values are different? Well, that’s where you need to use your eyes, does the image look to blue (Cold) or to warm (Red) or is there a tint to the image that looks wrong? This is why I suggest that you always fix your grays first as that will usually fix a weird cast and bring the blacks and whites closer together.

Be aware that this is a very mathematical approach, it is a ‘by the numbers’ technique after all, 100% correctness (Is that a word?) is not always flattering, but it is a very good starting point for adding back warmness or other curves corrections like the standard S curve and L*A*B steepening of colors.

Color Correction must always be done before you try to boost colors and contrast. If you have a cast and you try to boost colors you will make the cast even worse. My workflow is fix image dimensions, then crop if needed. You always want to crop before you do any color correction.