Sometimes you have an under-exposed photograph that you would like the salvage. However, you really want to keep the image low-key, portraying sedated feeling. In that situation, give Gimp's Auto-Normalize feature a try. Auto-Normalize brightens the highlights, while keeping the shadows dark. It's possible to salvage under-exposed photos by making manual adjustments of brightness, contrast, and etc. But nothing is quick than one click on the auto-normalize function. Take the following step to auto-equalize your photograph.
Auto-Normalize creates low-key photographs. To keep your photograph high-key, see the "Gimp by Example: Auto-Equalize" article.
The following is an under-exposed photograph. It's under-exposed by at least two stops.
Using the Auto-Normalize functionality of Gimp renders the following photograph and keeps the photograph low key.