Removing Colour Casts from Jpeg images

REMOVING A COLOUR CAST FROM A JPEG IMAGE – Tutorial

Although I normally use either my Pentax K100D or Cannon 300D in RAW mode, I always carry my Fuji 2600Z 2Mb pixel point and shoot at all times.

This sample image was taken of the Belen (Nativity scene) which was in a marque in the Ayuntamiento Plaza in Mula during Xmas holidays, it was taken in the early evening and the lighting in the marque was a mixture of sodium and fluorescent lamps, as a result the image had a very magenta /red colour cast. This tutorial is a combination of methods as described in Scott Kelby book “The Photoshop CS2 book for Digital Photographers¨.

The first step is to open the image in Photoshop, then CTRL+J to create a duplicate layer.

With the duplicate layer active, go to the bottom of the layers palette and click on the 4^th item from the left, (Create new fill or adjustment layers). Scroll down the list to THRESHOLD and select it. The Threshold box will appear and your image will turn to a black and white negative.

Move the white arrow all the way to the left and the image will turn white, now move the arrow towards the right and stop when the first solid black area appears. Photoshop is now showing you where the darkest area of the image is. Click OK to close the Threshold dialog box.

You can now mark this spot….. Press SHIFT + I, this will select the colour sampler tool. On the top tool bar, in the box - sample size – select 3 by 3 average. Now with the colour sample tool, position over the darkest black and click, this will put a marker on the image. On the layers palette, double click on the active threshold layer thumbnail, this will again bring up the threshold dialog box.

This time move the arrow all the way to the right, the image will turn black, now move the arrow back towards the left until the first solid white appears. Click OK, this closes the Threhold dialog box. Making sure you still have the colour sampler tool active, click to mark this point.

You can now discard the Threshold layer by dragging it to the recycle bin at the bottom of the layers palette.

CTRL+M will njow open the Curves window, select the “Set black point” eyedropper, the left one below the options box and position it exactly over the #1 marker and click, now select the “Set white point” eyedropper, position this exactly over #2 and click, you will immediately see a marked improvement in the image. Click OK to close the curves window. You can now clear the markers by going to the top tool bar and clicking “Clear”

The next step is to set the Grey point.

Click on Create a New Layer Icon at the bottom of the layers palette, it’s the one next to the trash can, this will create a blank new layer.

Go to EDIT menu and choose FILL. Under the Pop up menu, in the contents box where it say´s Use, select 50% Grey then click OK, this fills the new layer with 50% Grey

On the layers palette, with the new layer active, change the Blend mode to Difference, don’t worry that your image looks strange, its only temporary.

From the Create New Layer icon choose Threshold again, again and move the pointer all the way to the left, again your image will turn white. Slowly move the pointer back to the right and stop when the first area of black appears, this is your neutral Grey point. Click OK. Then Shift + I to select the colour sampler tool and position the eyedropper over the black area and click. You have now marked the Grey point and you can discard the Threhold layer and the 50% adjustment layer by dragging them into the trash can. You now have the image as it was previously and you should see the maker.

Press CTRL + M to select the curves window and this time use the centre eyedropper “Set Grey point”, position this over the marker point and click, you have now set the Grey point, click OK and the image colour cast is removed. On the top toolbar click “Clear” to remove the marker.

This technique should work 99.99% of the time

I would appreciate any comments or suggestions on this tutorial

John