Friday, January 7, 2011

Extreme contrast and skin detail using unsharp mask

The exams are approaching and it's time to study. Sometimes that makes me feel a bit desperate. Hence this self portrait:
Note: this one only really works in full resolution. Click for the full res image (hosted on deviantArt).

This was shot with self timer on a tripod with my old modified strobe in my right hand. I found the harsh lighting, desaturation and steep, dark contrast very fitting. A more in-depth view of how I got such contrast and skin-details (with the regular kit lens) after the jump!

I'll show you how I used the unsharp mask to get that high level of contrast and skin detail. A crop from the photo straight from the camera looks like this:

Editing the curves in ufraw to an aggressive S shape gave me some more harsh contrast to suit the mood:

The next thing I did was apply some unsharp mask. What an unsharp mask does is basically amplify the high frequency components, increasing the acutance or apparent sharpness. It does this by first blurring a copy of the picture (hence unsharp mask) and then subtracting some of this blurred image from the original. You usually need to provide three parameters: the radius of blurring, the strength of the filter (how much of the blurred image will get subtracted) and the threshold (if the difference between blurred and original is lower than this threshold, the filter will not be applied at that pixel).

You can use an unsharp mask in two ways. The first is increasing edge contrast, using a low blurring radius (1 to 10 pixels), a moderate strength (20 to 60 percent) and maybe a slight threshold (often somewhere between 0 to 10 percent). The small radius will only sharpen "steep" transitions (edges) and the threshold will make sure that even (non-edge) areas don't get sharpened (which could lead to exaggerated ISO noise).

The second way you can use an unsharp mask is to increase local contrast. One then chooses a large blurring radius (in the order of 100 pixels, larger than any small detailed features), a low strength (5 to 20 percent) and usually no threshold, or a very small one (say, 0 to 2 percent). This will bump up the "medium frequency" components of the image, and thereby increase the contrast on a larger scale. Note, though, that the bigger the blurring radius you choose, the longer the processing will take, as a greater area needs to be examined to blur each individual pixel.

For my photo, I chose to increase the local contrast first. I used the gimp to apply an unsharp mask with a 120 pixel radius and a strength of 15%. The threshold was set to 0. The result looks like:

As can be seen, there is more (local) contrast when compared to the photo above, giving it a sharper appearance. The fine details, however, are still a bit soft. Since I'm going for a very extreme result, I applied another unsharp mask. This time I used a radius of 5 pixels, a 50% strength and no threshold. That gives us:

Note that the hairs of my eyebrow are much more pronounced, and there is a lot more skin detail. As a last step, I applied another unsharp mask with a radius of 0.5 pixel, 50% in strength, no threshold. This brings out the very fine skin detail even more, as can be seen when inspecting the previous image with this last one:

Comparing this last image to the image we started with shows the amount of contrast and detail you can "recover" with a strong S curve and with using an unsharp mask. My final image was made with even less saturation to fit the bleak mood.

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