Synthetic HDR is a technique that generates extra dynamic range in photographic images by trading off detail for bit depth. The name comes from Synthetic High Dynamic Range -- synthetic in the sense that the extra dynamic range (though quite real) is being synthesized, and high dynamic range, as in, well, high dynamic range.
The principle underlying Synthetic HDR is the fact that it is possible to trade off resolution for dynamic range. There's a little more background theory on the Synthetic HDR Proof of Concept page.
Photoshop CS2 Plugin
I have written a CS2 plugin that implements the Synthetic HDR algorithm -- this version works far better than the do-it-yourself Photoshop proof-of-concept. Currently, it's an early alpha release. Please read the Synthetic HDR Plugin FAQ before sending me email about it. It is entirely free (as in free beer). It's not entirely free as in free speech -- I'd like to release the source code, but the Photoshop SDK license terms make this impossible, unfortunately. I can, however, give away a binary version of the plugin, so go and get it and have fun. But please, please read the FAQ!
Support for the plugin is provided via the mailing list. Please send email to it rather than me personally. It's a low bandwidth moderated group, and spam free.
Sample output from the plugin
It's important to state that the plugin does not give equivalent results to the Photoshop method mentioned elsewhere in these pages -- rather, it goes to extremes to exactly preserve the tonality of the original image. For example, here's an original image, straight from the camera:
Running the plugin over it (with settings Amount = 50%, Noise Reduction = Strong), you get:
There are no prizes for pointing out that there is no appreciable difference between the two. This is the general idea -- the algorithm has near-negligible effect on the tonality or detail present in the original image. However, if you look into the shadow areas (with the brightness and contrast enhanced with Levels) you see what the plugin has actually done:
Unmodified image, 100% crop from the bottom right corner
Result from the Synthetic HDR plugin, same 100% crop and Levels adjustments
Whilst the original image shows unacceptable levels of noise and posterisation (at least to my eyes, your mileage is welcome to vary), the synthetic HDR image looks more like a deeper dynamic range version of the same image, with a little film-like grain. This effect covers the whole image through all tones, not just the shadow areas.
Relationship to 'traditional' HDR
The intent of the Synthetic HDR plugin is a little different to the HDR photography community, though it's certainly possible to borrow techniques in both directions. The main aim of synthetic HDR is to give images more usable dynamic range, allowing greater manipulation of contrast in post-processing than would normally be the case. It is certainly possible to borrow traditional HDR techniques like tone mapping, however:
Tone mapped image created by applying local adaptation in CS2
Though this image isn't particularly to my taste aesthetically, it does show off the smooth shadow detail pretty well.