Instructables

Step 2: Theory- layered image editing

How do we do it?

HDR is where layered image editing programs such as GIMP or Photoshop are very useful. Using a technique called layer masks, we stack the photos with different exposures, rather like a stack of acetate sheets, and make the parts we don't want transparent so the best parts show through the stack. Photoshop/GIMP are doubly useful for this because we can make them do the hard work of actually making the layer masks.

Layer masks?

When stacking layers in an image editing program, it is necessary to specify how transparent you want each part of the image to be. Rather than have to do this by hand, image editing software lets you do this by providing a second greyscale image that specifies how transparent each part should be. White parts of the layer mask make the corresponding part of that layer opaque, so they show up on the finished image, and black parts make that part transparent so the lower layers show up. A couple of clever tricks in the software let you generate the layer mask from the original in a few clicks, rather than laboriously paint the entire layer mask by hand.
 
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EOBeav5 years ago
Comment about layer masks: When I was doing this at home, I experimented with one more step after desaturating the mask. I used a threshold filter on it so that I could get entire sections that were either opaque or transparent. This seemed to work when I had, for example, a cliff, part of a river, and a sky. Each one of those three items were exposed at different levels, and I wanted to get the best ones compiled into one picture. This gave me a more pronounced HDR effect, but not so much that it didn't look realistic. What are your thoughts on using the threshold filter after decomposing? BTW, this is a great tutorial, and one that has really got me going in HDR images. It's a great way to improve your own photographs.