carol.gimp.org

gimp and photography

Clarity

Almost every photograph has a problem with "all over grayness"; just a few easy steps with one of the gimp color tools and your photographs can be sharper and look much better Scans of photograph prints are much worse about this grayness; I was surprised to see the same problem with digital photographs as well, just not as much.

This tutorial is about a method that worked with gimp-1.0, gimp-1.2, gimp-1.3 and now with all versions of gimp2. It is a simple fact I learned from my friend who was learning how to use Photoshop, so I know that it worked with Photoshop LE as well. It is a fact about digital images and the solution deals with color mathematics and other such facts that the computer needs to work. The nice thing about this is that it is easy to see it work and the knowledge, so far, is independent of the graphical user interface (GUI), the graphics application used (it simply needs to have this tool), the operating system or even the manufacturer of the digitising device -- scanner or camera.

This tutorial is about the Levels Tool. In gimp-1.0 and gimp-1.2 the tool was accessed via menus only: <Image> -->Colors -->Levels. Gimp2 has several different ways to access the tool: menu, the toolbox and the toolbox dialog.

The photograph here shows what the tool can do -- it is a digital photograph, taken in lowlight conditions (outdoors at sunset), with a hand held camera that was warning about the long exposure time that would be needed. Feel free to use this image to follow the tutorial and familiarise yourself with the levels tool.

	Levels Dialog The Levels Dialog looks quite complicated, but it actually represents complicated color information in a relatively simple way. In a mess of photographs (more than 2000) I used the "Input Levels" for all except one photograph. I also found that the eye droppers that can be found on the Gimp2 Levels dialog (and did not appear in the gimp-1.2 Levels dialog were more confusing rather than helpful. What they do accomplish very well is to make the face of the dialog appear more like the face that people who are using other apps expect to see it. More interesting is how the gimp-1.2 Levels displays a different histogram (the black and white graph associated with the Input Levels part of the dialog) than Gimp2 does. When I ask about this I generally get some developer mumble jumble about how it doesn't matter. The dialog does do its job nicely regardless of the developer dribble and the differences in the histograms. Removing the all over "gray" involves just using the triangles directly below the histogram.Keeping the "Preview" toggle toggled for this dialog will allow you to see the changes you are making.
	Levels Dialog:Input Levels Use the information on the histogram The method is shown here in three steps (I selected the "Ok" button three times to get the results of each step), feel free to work on your image in one step and select the "Ok" button at the end of the color tweaking. With this portion of the dialog, the colors are manipulated by moving the little triangles below the graphical display of the images colors.
	Move the outside triangles Using the mouse, slide the outer-most triangles to where the color starts to display itself on the graph. Don't worry about the ups and downs of the histogram display for most photos.
	Move the center triangle The center triangle is not so easy to make into a procedure like the outer two; so much of it depends on what you would like to see in your image. If you move it back and forth a little, you can see how it affects the contrast. Moving it towards the right makes it darker and towards the left makes it lighter. The "rule" I heard was to move it 10% in the direction that the greater change had occured on the edges. I could never make this make sense or work predictably.
I tried to pick a spot where this child and the photographer (me, better than a signature I think!) both look fairly good. This might be the problem with formulating where the center pointer is to land; people have different preferences about the contrast in their images. Look at the preview and make the photograph look the best it can to your expectations.

the original

outer pointers adjusted

center pointer adjusted

	Levels Dialog:Output Levels The subject of this photograph (a statue, Invocation by Stephen Fitz-Gerald) is the one photograph I have used the Output Levels portion of the Levels Dialog successfully on. Using the Input Levels as described earlier in this tutorial brings the colors of the statue out, however it also will make the already too light background lighter. Nudging the Output Levels down on the white side just a touch helped the photograph. Once again, feel free to use this original image to work through the instructions here.
	Use the information on the histogram The image was taken inside, behind it is an extremely bright day. The camera adjusted for the whole image; it would have been much better if I could make the flash go off on the camera. Once again, I am showing what I did in steps -- when I determined the right combination, I used worked with only one instance of the Levels Dialog.
	adjust the Input Levels This image needs more lightness on the subject and less in the background.
	adjust the Output Levels Moving the light pointer to towards the center makes the white less bright. Just a little bit makes the subject of the photograph easier to see.

the original	Input Levels adjusted	Output Levels adjusted
Another option for fixing this photograph would be to replace the sky with the method described in this method I learned from Jenny Drake in an exchange on the gimp-user mail list. Many of the details in the background of this one didn"t seem to need the special treatment, however.