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Colour Correction using Curves by Big Al
Submitted By: karenc Date: March 7, 2010, 5:19 PM Views: 708

Colour Correction using Curves

by BigAl

In this 'How To' I'll show you how you can use Curves in Photoshop (6, 7 or CS) to correct a colour cast in your photos. No matter how good your camera is at White Balance you will often be amazed at how much this technique will improve the colours that you have thought to be correct.

Changing the Defaults
Before you actually make your first colour correction, you will need to do a one-time change to the Adobe default settings (in Curves) that will then remain set for all subsequent edits until you change it or reload/reset Photoshop.

Open any image and select Image > Adjustments > Curves… (or Ctrl+M).

In the lower right of the dialog are three 'eyedropper' icons which are used to set the monochrome points (from left to right) Black, Gray and White.

Unless you have altered it in the past, the "Gray" value will be ideal at a value of 128 for all three of the RGB colours. But Adobe have defaulted Black and White to the extreme values. If you were to use those values in this technique it would result in the image having too strong a contrast. You can try it out for yourself if you want. I'm going to suggest some values that will produce rich, punchy colours without giving too much contrast. You, on the other hand, can adjust these values up or down to suit your tastes. The more you take them away from their extreme values the lower the contrast will be.

Altering the Black and White Set Point Values
Double-click on the Set Black Point eyedropper. The Color Picker dialog is displayed and it shows that the RGB values are all set to zero (pure black).

Select the number in each of the boxes are overtype them all with a value of 16.

It is important that you give the same number in each box or it will introduce a colour cast into the picture of whatever is the higher number. "OK" the Color Picker to return to the Curves dialog.

Now double-click the 'eyedropper' on the right (Set White Point) and change its RGB values to 239.

This is the same amount (16) away from the extreme value as was set for Black, but you could choose values which are not so 'balanced'. The higher you set the values, the brighter the image becomes. The values I have given will serve to retain detail in the darkest and lightest parts of the image.

My Target Image
To demonstrate the Curves facility most dramatically I deliberately set the camera White Balance to "Tungsten" yet shot indoors using natural daylight. The camera, expecting a red glow from a normal light bulb, has increased the blue in the picture to compensate.
Furthermore, I took a shot of my hand so that we can recognise when the skin tones look right. Finally, I included a sheet with a black/grey/white pattern to help with selecting what should be neutral shades.

Locating the Lightest and Darkest Pixels
The Curves facility's eyedroppers stretch the brightness levels of all the RGB values proportionately towards the target values that you defined in the 'Set Black/Gray/White Point' operation previously. To use the facility correctly you need to know where, in the image, are the brightest and darkest pixels. Fortunately Photoshop has a facility to do this. If, however, you are confident at identifying the blackest and whitest pixels, you can skip this part.

In the Layers palette you need to create a Threshold adjustment layer. Click the button that looks like a circle divided diagonally into black and white. From the drop-down menu, select "Threshold…"

A Threshold layer is created and a Threshold dialog box pops up. The value defaults to midway (128) and the image now becomes pure black and white. Pixels in the image which have an average RGB value below 128 are shown as black and those of 128 and above are shown white - there are no grey areas.

Slide the slider towards the left and, as the threshold approaches zero, the black areas start to appear white. When the image is almost completely white, the small remaining black part is located at the darkest part of your image. In my example it happened at a threshold of 5.

You can now "OK" the Threshold dialog. Don't worry that your image is now 'white with a tiny bit of black' because the change is controlled by a layer so it will not be permanent.

You now need to 'mark' this darkest spot for later.

To do this you bring up the Color Sampler tool and click the cursor on the black area. A small circle appears with a "1" against it.

Now you need to locate and mark the brightest area. Double-click the Threshold adjustment layer to re-invoke the Threshold slider. Now drag the slider to the right until the image is almost total black with just a small white area. Click the Color Sampler cursor on the white area to mark it with a "2" circle.

You can now close the Info palette.

You now have the two extremes of brightness marked and can drag the Threshold adjustment layer to the "Delete layer" trash can to remove it and restore the look of your image.

The adjustment layer only had a visual effect on your image and did not edit any pixels. Its purpose has been served.

Applying the Curves
You can apply the Curves colour correction either as a single edit or a (temporary) visual modification that can be applied as an edit later. To perform it as an edit you will need to select Image > Adjustments > Curves… (as before) and, once you've made the adjustments (explained shortly) you click OK to apply them. However, if you plan to do other edits to the image you might do better to apply Curves via an adjustment layer. To do this you select Curves from the 'Create new adjustment layer' drop-down in the Layers palette.

Changes made through this method can be tweaked over and over until you are satisfied with the results and flatten the image.

When you have the Curves dialog box up, start by clicking the Set Black Point eyedropper (the one on the left of the three) and then clicking the eyedropper cursor on sample point 1 (the darkest pixel). Next click the Set White Point eyedropper cursor on sample point 2 (the lightest pixel). This will normally stretch the range of colours out and make the image look punchier.

Now you need to select the Set Gray Point eyedropper and click it on a point in the image that is supposed to be a mid-grey colour. Usually this will be a shadow falling on something white. Any colour cast in the grey area is corrected as its RGB values are forced to 128 on each. But, in the process, it takes out the imbalance (colour cast) from every other pixel in the image. If you cannot find a mid-grey area on your image then you will need to use a lighter or (preferably) darker grey. If you have no (supposed to be) grey areas then you will have to skip this step.

If you choose too light a grey to click on you may well find that you pick up on reflected colours from things outside of the image that changed the grey. In the example I show what happened when I picked a part of the grey card that wasn't in the shadow of my hand. It made the image go a horrible green-yellow.

Fortunately I did not have to Undo (Ctrl+Z) the mistake, just click other places until it looked right.

Now that the colours look right, the image may appear slightly darker or lighter than desired (depending on the relative brightness of the pixels you chose for the 'set points') and will need to be corrected. The Curves dialog does this as well. Click and hold the mouse pointer in the middle of the diagonal line running through the 4 by 4 grid of the Curves dialog. Drag the line upwards (to brighten) or downwards (to darken) and release the mouse button to see the effect. If you have overdone the change, click and drag the new dot (on the line) up or down till the brightness looks right.

Click the OK button to close the Curves dialog with the changes in place. At this point you can click the Clear button of the Color Sampler settings and the circles numbered 1 and 2 will go.

Reusing the Adjustment Layer
If you have taken two or more photos at the same location and with the same lighting conditions, you can use a useful feature of Photoshop to duplicate the Curves (or any other) adjustment layer.
Once you have made a successful adjustment layer in one image and before you flatten the image to save it as a JPEG, you can open all the other images that need the same amount of adjustment. Simply drag the adjustment layer from your current image and drop it on to any image that is opened in the Photoshop window. Instantly, a copy of that adjustment layer is added to the target image so it becomes 'corrected' by an identical amount. If the adjustment doesn't totally suit the target image you can double-click the copy of the adjustment layer and change some of the parameters to suit. For 'colour correction layers' it will normally mean just dragging the curves line up or down to affect the brightness.
You will need to Flatten the layers to apply the adjustment layer changes and enable saving the image in JPEG format.

The colour correction is complete.

Preparing for a Shoot
If you like the effect that 'colour correction using Curves' does, then you might think about preparing yourself for a shoot to make the job easier.

Simply create and print a sheet like the one shown underneath my hand. Then, before you start shooting the objects or model properly, take one photo of the black-grey-white sheet as part of the scene (e.g. get your model to hold the sheet). Then, once you have an image on which to create your adjustment layer, you can use the layer copying technique to correct all the photos in the session.

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