Photographs taken outdoors tend to be a bit hit and miss. Sometimes you get a great one, but occasionally they are not so good. The auto-exposure of modern digital cameras and mobile phones can be fooled and you get a dark underexposed image. Here is how to fix exposure problems in Linux.
Faults often occur when there is a large bright area in a photo or a large dark one. The camera or phone sets the exposure to either the bright or dark area and the result is a poor photo. A bright patch of sky in a photo can make the rest of it appear too dark, or simply dull and lifeless. It can be fixed though.
For this example I will be using Linux Mint 17 and a photo editor called Fotoxx (kornelix.com/fotoxx.html). If you don’t have this, go to the Software Manager or Package Manager and search for it. Mint isn’t the only Linux with Fotoxx and others do too, Mint just happens to be a distro I like a lot.
Load a photo with File, Open Image File, then go to the Retouch menu. There are several ways to change the exposure of a photograph and the first I will look at is Gamma Curves.
Select it on the menu and an adjust image gamma control appears. It has a diagonal line from bottom left to top right and there is a dot at each end and one in the middle. It isn’t important what gamma is, just click and drag the three dots until the photo looks right. Here you can see I have dragged the middle one the most and the dark areas have become lighter, although the dark sky has become a bit washed out.
The image is better, but not perfect, so let’s take a look at another option on the Retouch menu. The obvious tool to use is Brightness/Color and this displays another control in a pop-up window. Selecy Brightness in the bottom half of the control. This time the line is horizontal, but there are three dots as before and you can click and drag them to adjust the image brightness.
There is also a row of buttons labelled Darker Areas on the left and Lighter Areas on the right. Click the buttons to adjust the line in small steps. You might find it easier than clicking and dragging the dots. For example, clicking the +++ under Darker Areas on the left raises the whole line and brightens the image. Experiment with the buttons to get the best exposure improvement.
The effect wasn’t too good and the sky changed to a light blue colour. Cancelling this and returning to the Retouch menu, the best result was obtained from a tool called Flatten Brightness Distribution. Turning this up to the maximum value of 100 produced the best exposure for this photo.
What do you think? Is this better than the original? The dark castle and foreground are certainly lighter, but without losing too much sky detail. After fixing the image exposure, go to the File menu and select Save to New Version. Never overwrite an original photo, you never know when you might to return to it.