Can an HDR or tone-mapped image be reversed to recover the original exposure(s)?
Asked 11/12/2013
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I created an HDR image in Photoshop CS6 and want to know whether it can be converted back to the exact original image or images it came from. I understand HDR combines multiple exposures, and sometimes the result is then tone-mapped for normal display. Is there any inverse operation that can recover the original exposure(s), and why or why not mathematically?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
13y ago
2 Answers
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It is very important to understand the difference between a high dynamic range image, and a high dynamic range image that has been tonemapped back down to low dynamic range for display on a standard monitor.
This is a high dynamic range image that has been produced from multiple exposures:
Looks dull and lifeless because you're viewing it on a low dynamic range monitor. What you can do is locally adjust the contrast, so as to maximise the dynamic range of your monitor in every part of the image. This process is called tonemapping:
If you've done this tonemapping step (which is easy to detect as it makes your images look like the above), then you're out of luck, you'll never get back to the original(s), as it's a highly nonlinear filter that takes whole neighbouring image areas into account.
However if you've merely done a merge-to-HDR-image, then that process is entirely reversible, you can get back any of the original images by just truncating the dynamic range at the appropriate points:
Originally by user1375. Source · Licensed CC BY-SA 4.0
user1375
13y ago
0
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Usually, no. Once multiple exposures are merged into an HDR image, and especially once that HDR image is tone-mapped for normal display, the original exposure(s) cannot be recovered exactly.
Why: HDR merging combines information from several images into one representation, and the original separate exposures are discarded. Tone mapping then applies nonlinear, often local contrast and brightness adjustments across neighboring areas to compress the scene’s dynamic range so it can be shown on a standard monitor. That process is not generally reversible.
Mathematically, the mapping is many-to-one: different input images can produce the same HDR/tone-mapped result, so there is no unique inverse. Local operators also depend on surrounding pixels, which further destroys the possibility of reconstructing the exact originals.
If you still have the original bracketed photos or the untouched HDR file before tone mapping, you may be able to reprocess those. But from only the final HDR/tone-mapped image, you cannot recover the exact original image(s).
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