Posted tagged ‘New Hampshire’


January 19, 2011



Another HDR photo: Hanover UCC Church

February 12, 2010

The previous example of HDR photography resulted in a somewhat surreal looking sky. Photographers using the HDR technique often seek out that appearance. I agree that it looks cool. However, one can also obtain more realistic results, as I tried to do in the above example that I took when I was in graduate school. This church stood across the street from my office in the physics building:

Wilder Hallan HDR image of Wilder Hall, Dartmouth College

The actual church scene didn’t offer the dramatic lighting of the UCLA building at sunset. However, the lighting around the church spanned a wider total dynamic range between the sky and the shadows under the trees. So each exposure of the church is separated by 2 stops, rather than 1.5 as in the previous photo.

The thumbnail images next to the tonemapped church picture above can be clicked to view them more closely. I find the differences to be really striking here. In the darkest exposure (1/100 second), there’s basically no information in the shadows under the trees. Things just look black. Meanwhile, in the brightest exposure (1/6 second) the church almost disappears into the sky because it has been so overexposed. The tonemapped image shows detail in all these areas while remaining more or less realistic in appearance.

How do image histograms work?
The diagram above shows RGB histograms for the three original images, and the histogram for the tonemapped composite below them. The horizontal axes of the histograms indicate brightness or saturation. The black distribution represents brightness; the colored distributions show saturation of each color channel at a given brightness.

So, the left side of each histogram represents the darkest resolvable brightness in a given image. Anything darker would simply look black in that image. The right side represents the brightest resolvable luminosity; anything brighter would appear to be pure white.

The height of the distribution at a given location represents the number of pixels with that brightness (or color saturation for the colored distributions). The vertical lines represent “stops,” or factors-of-two differences in brightness.

You can see from the amount of overlap of the three histograms that they differ by two stops, or a factor of four in brightness. Thus the composite image contains information gleaned from a range of 9 stops, or almost twice the range that could be depicted in a single exposure. The bottom histogram shows how the data from those nine stops has been compressed back down to 5 stops through the tonemapping process.

Okay, enough photo geekery. I think I’ll get back to baking, knitting, and spinning related posts now.

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