How I learned to love the GIMP Home » Blogs » tomfriedel's blog

This article is a work in progress.

I was down to my last Photoshop activation. I had two laptops stolen, and Adobe kindly granted me a third activation. So when I went to Ecuador in June, I decided not to transfer my last activation to my laptop, and installed a copy of GIMP for windows.

The main feature of Photoshop I had found missing in GIMP was the convenient “Shadows and Highlights”. I knew with Levels and Curves I could do much of the same type of image manipulation, but I never new that with Curves I could in some cases do more than with Shadows and Highlights.

I grouped images into two types. The first group had highlights and shadows that were out of range. That is, pixels that would have exceeded 255 or gone below 0 if the scale was not 0-255. In these cases, some of the data in the image is lost; a block of pure white or black has no information about the scene. But you can still do the best you can, and try and get the colors in the image back into a normal range. Shadows and Highlights does this well, but Curves in Photoshop or the GIMP can also be used.

For the case of an image with ‘blown’ or overexposed highlights, open Curves and move the top right point on the diagonal line down. This moves the highest values in the image from 255 to a lower value, say 245. Then save. You might be happy with how the image looks now, but you can also open Levels and notice that there is a empty area 10 pixels wide on the right side of the graph. Your image has no pixel values between 245 and 255. You can move the right slider left 10 pixels, and redistribute all of the pixels up to the value 255. The middle slider moves the ‘midtones’ higher or lower, and should be tested to see if the improves the image. The same logic applies to images which are underexposed, or which have many pixels with a value of 0.

Although every image is different, in my opinion a general guideline is that an image with very few pixels from 0 to around 15 and around 240 to 255, and then a bell curve in the center is an image with good color balance. Of course some images, say a dark bird against a light sky, will not conform to this model. Once all of your image pixels are within range, you can continue to use Levels and Curves to do the work of “Shadows and Highlights”. Simply adding a point and raising the curve over the low value pixels (left side) or lowering the curve over the right side might be all you need. Raising the entire curve is the same as shifting the center tab in the Levels graph.

“Shadows and Highlights” has three other useful options: “Color Correction” , “Midtone Contrast”, and “Black Clip”, Midtone Contrast can presumably be simulated with the Hue/Saturation (by desaturating an image that became oversaturated when color values were increased). “Midtone Contrast” can presumably be closely matched with “Contrast”, and for “Black Clip” the best I can suggest is carefully adjusting the curve in Curves of the black pixels. I believe you can do as much or more with Curves than “Shadows and Highlights” despite the convenience of the latter.