A First Look at Levels in GIMP (what are they and how do they work) Presented by Dave Mawdsley, DACS Member, Linux SIG Member March 20, 2013

Introduction 2 Using GIMP, the Levels adjustment feature is an easy, great way to tweak a picture. Levels can improve this very poor image to recover some details. The original bad image shown was taken in full moonlight with no time exposure, through a window and without a tripod. A great GIMP challenge! (DSCN1429.jpg: tree shadows on the snow)

The Original Levels 3 In GIMP from the Colors menu, select Levels. Shown are the values that apply to the original image. First we'll adjust the Input Levels. The histogram shows almost all black for the image. First we'll move the white slider to the left, click OK and then save as DSCN1429a.jpg.

After Moving the White Slider 2 Our slightly improved image was saved as DSCN1429a.jpg after moving the white slider to the beginning of the black portion of the histogram.

Next We'll Move the Black Slider 3 Next, reopening DCSN1429a.jpg in GIMP, we'll call up Levels again. You can now see an expanded histogram. We'll move the black slider to the right just a little bit, click OK and then save as DSCN1429b.jpg.

After Moving the Black Slider 2 Working with DSCN1429b.jpg, we'll adjust the black slider to the beginning of the black portion of the histogram and move the white one a bit more left where data begins and click OK. Then save this image as NSCN1429c.jpg.

Next We'll Move the Middle Slider 2 Working with DSCN1429c.jpg, we'll adjust the middle slider until It looks balanced with more white and less black and click OK. Then save this image as NSCN1429d.jpg.

Finally We'll Adjust the Output Levels 2 Working with DSCN1429d.jpg, at the Output Levels sliders, we'll adjust the black and white sliders until It looks balanced with over all lightness and click OK. Then save this image as NSCN1429e.jpg.

Last We'll Adjust Brightness and Contrast 2 Working with DSCN1429e.jpg, with Colors > Brightness and Contrast, we'll brighten a small bit and click OK. Then save this image as NSCN1429f.jpg.

4 End Remarks The graininess of the end picture is caused by extreme under exposure, not using a tripod, using .jpg as the image format and the tweaking of the picture with the various changes that were made. Too much tweaking usually makes things worse. It's best to pick an early stopping point. Ideally the original bad picture should have been taken with: * A tripod, * Outdoors with a Large-Lens Camera and * Using a Time Explosure of about 2 minutes. (longer and the Moon would have moved and blurred the image) Even a “hopeless” picture can be improved with GIMP!

A First Look at Levels in GIMP (what are they and how do they work) This OpenOffice.org Presentation 'gimplevl.odp' can be downloaded from http://madmod.com/freebies.html