Manipulating Contrast to Reveal Details
Sometimes, our eyes can see details that our photographs don’t show because of insufficient contrast in the scene. Other times, the contrast is so low that even our eyes can’t see the details. Several years ago, while in a class on Computer Vision and Image Processing, I learned first-hand how remapping the values in an image can reveal features in a low-contrast image that our eyes can’t see. The image’s histogram (a visual map showing the count of the number of pixels at each tonal value, dark values to the left and light values to the right) was bunched up into a sharp peak, showing that most of the values were in a very narrow range. The technique we used in that class was Histogram Equalization, which attempts to redistribute the values in an image until the histogram is level – that is, every value is represented by an approximately equal number of pixels in the image. Histogram Equalization is a useful function offered by many image processing applications such as Photoshop, but you don’t always need (or want) an even distribution of the tones to reveal the features of interest.
These petroglyphs are on a canyon wall along the Colorado River, a few miles south west of Moab, Utah. We were given instructions by a forest ranger on where to find these, and we drove to the prescribed location, but did not see the petroglyphs from the road. Exercising a little faith that the forest ranger would not have led us astray, we got out of the car, walked right up to the canyon wall, and then, magically, the petroglyphs appeared. The scene was rather low contrast, as indicated by the top image; the camera did nothing to make the petroglyphs more visible. Using the tone controls in Adobe Lightroom, I accentuated the “Exposure” and “Blacks” by moving their respective sliders toward the right, essentially spreading apart the values in the image. The resulting histogram is not level — there still is a peak — but the values have been spread apart (widening the peak) to the point that we can see the petroglyphs much more easily.
Looking at the histogram from the manipulated image, you’ll notice that there are many values at the left (dark) end of the spectrum. This is a clear indication that a lot of shadow details have been lost. Normally, you would not do this to your photographs (preferring, instead, to keep the toes of the peak just barely inside of the left and right boundaries of the histogram). For this image, I made a conscious choice to widen the peak of the histogram farther than normal; making the petroglyphs more visible was more important to me (especially for this example) than preserving shadow detail. This is one of those times when I prefer using more manual tools for manipulating the histogram than the automatic tool of Histogram Equalization — I get more control over the results.
Although I used the tools available in Adobe Lightroom, the underlying technique of manipulating the histogram to spread out the values is supported by many image processing applications, including Photoshop.
Tip of the Week
2007.04.02
