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Thursday, July 16, 2009

••◊ Highlight recovery

I've been trying my hand at poster printing lately. I really want to print and frame a picture I took at the World Cycling Championships in Italy last year, but I'm having "issues" with how the print is coming out. Since this is going in my house of course I'm going to be overly critical and picky.

One of the issues is that the print comes out too dark, which would normally indicate under exposure, but in this case the cyclists were riding in a bit of mid-day shade and they are about 0.5 stops under what I would deem a pleasing brightness to the picture. Since I always shoot in raw format it seem simple enough to just up the exposure half a stop...but, now the highlights are blown out.

First let's take a look at the original under exposed looking picture. I obviously want to make this brighter and more vibrant for a large print.

The first step is to bring up the exposure to a level that represents what I want the print to look like. I think +0.5 stop of exposure compensation does the trick. One caveat of displaying pictures on the web is that they don't quite appear the same as a thumbnail, so you'll have to trust me that this picture is actually quite brighter in full resolution. One side effect of using this exposure compensation is that the backs of the cyclists are getting blown out to white. I also included the histogram showing how the pixel values are cramming their way to the right side, which indicates overexposure. We can fix this.

The next step is to create an under exposed version of the same picture. Again, since this is a raw format picture I can just slide the exposure control down -0.33 stop in my raw converter to eliminate most of the pixel cramming to the right of the histogram. This gives me a version of the picture with "recovered" highlights. I personally found that the difference between the plus exposure and minus exposure needs to be within 1 stop for this process to work. Any greater than that and the picture gets weird banding artifacts in the last step of the process. Below is the underexposed picture and the associated histogram. Notice that we now have minimal blown out pixels. Whatever is still blown out is gone forever. How did I determine this optimal -0.33 stop value? ...by watching the histogram and seeing when the pixel values at the right stopped moving as I moved the exposure control down.

Now we have to combine the two images to recover the highlights into the over exposed picture from the underexposed picture. We'll do this using a mask and 2 layers.
  1. First put the underexposed picture into your editor (Gimp, PSP, Photoshop...whatever) as the background layer.

  2. Next insert the overexposed image as the top layer and add a layer mask using the overexposed picture itself. This should automatically convert the mask to monochrome.

  3. Then invert the mask layer on the overexposed image layer. This should make the overall image look "hazy." This is normal.

  4. Select the mask on the over exposed image layer and open the "curves" control in your editor. You want to slide the right side of the curve over until only about 10-15% of the curve is exposed. This should leave you with a mask that is black where the image is overexposed. The under exposed image now shows through any part of the over exposed mask layer that is black. The picture below shows this control in action.

We have now made the blown out areas transparent, via a layer mask, in the over exposed image. The under exposed image show through the mask and the highlights are recovered. Yeah! Results below. Note that the change is very subtle, especially in a small picture. However you can mostly see the results on the cyclists backs. On the larger image the change is more obviously. This technique works well in a number of exposure situations, so I thought it was something good to share.

I finished my family picture archiving project, so that's probably next to share. More soon.

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