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Is "exposure fusion" distinct from HDR compositing techniques, or is it essentially a marketing term for a specific HDR process. Or is it just a synonym for HDR?

Or, to put this in "meta" terms, should exposure fusion be treated as a distinct tag from HDR, or should it get lumped in with ?

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"Exposure Fusion" just refers to a certain type of the tone mapping of an HDR image. It doesn't include all HDR tone mapping algorithms, and is rightly tagged as "HDR".

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    This sounds right to me. I have this impression that there's a formal logic that would show that all HDR processes are transformations of a set of aligned images into a single image of the same color depth, and one can show that "exposure fusion" is just a special case of such suitably characterized transformations. – feetwet Sep 7 '14 at 2:39
  • Bingo. You said better than I did. – JenSCDC Sep 7 '14 at 8:00
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Whether Exposure Fusion is a form of High Dynamic Range Imaging (HDR) or not depends on how you define HDR.

If you have a broad definition of HDR Imaging that includes techniques that have been around since the 1850s when Gustave Le Gray first used parts of two differently exposed images to create photos of seascapes, then Exposure Fusion is a form of HDR. If you include the tone mapping done in the darkroom using dodging and burning when printing from negatives that Ansel Adams raised to an high art form in the mid-20th century, then Exposure Fusion is a form of HDR.

If you choose to restrict the term High Dynamic Range Imaging (HDR) to the technique developed in the late 20th and early 21st centuries to deal with the limited dynamic range of digital cameras compared to existing films at the time by producing a 32-bit floating point image file that is then tone mapped into an 8-bit image then Exposure Fusion is not a from of HDR.

See http://en.wikipedia.org/wiki/High-dynamic-range_imaging for more on the history of High Dynamic Range Imaging.

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    that's a great answer - often, I tend to hyper focus on the little post processing world on my computer screen. Your thoughtful answer helps to remind me of the long heritage of photography – B Shaw Sep 7 '14 at 8:34
  • Yes, thanks for the excellent answer! Would you care to address the "meta" question of whether, in current practical use, "HDR" should be expected to include "exposure fusion"? Or do photographers now generally expect that "HDR", including in-camera HDR, specifically refers to the process of compositing a higher-depth image and then tone-mapping it back to an 8-bit image? – feetwet Sep 7 '14 at 15:14
  • I disagree about Ansel Adams. Tone mapping is not the same thing as HDR. – JenSCDC Sep 7 '14 at 16:00
  • @AndyBlankertz It is if you define HDR as High Dynamic Range Imaging. Dodging and burning was all about getting the additional dynamic range the negative could contain into prints on paper that couldn't contain the same dynamic range. Read Adams' trilogy The Camera, The Negative, and The Print. The zone system he developed was entirely about squeezing the wider dynamic range in the scenes he photographed into the narrower dynamic range of the photographic paper on which he printed. – Michael C Sep 8 '14 at 2:43
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HDR and exposure fusion (as done by a tool like enfuse) are two different ways of attacking the same problem, but exposure fusion is NOT an HDR technique and does not require HDR file formats or tone mapping.

Enfuse's main algorithm does not involve remapping tones along a larger dynamic range, but selects and weights values from the original pixel data of member images, based upon three criteria. From the enfuse website:

The basic idea is that pixels in the input images are weighted according to qualities such as proper exposure, good contrast, and high saturation. These weights determine how much a given pixel will contribute to the final image. A Burt & Adelson multiresolution spline blender is used to combine the images according to the weights. The multiresolution blending ensures that transitions between regions where different images contribute more heavily are difficult to see.

Instead of mapping out all the tones in the member images along a high dynamic range, and then tonemapping back down to an LDR format, weighted averaging is done on the original member pixel values to create the final image pixel values. Essentially every value in the final image is going to be within the range of the original member image values, not necessarily the result you get with HDR and tonemapping, which can manipulate the luminance and saturation of pixels outside the original range.

In more manual methods of exposure fusion, simple masking allows for pixel selection from member images into the final image--again, no mapping to an HDR file values or tonemapping is required. You can also think of enfuse as masking and selecting at the individual pixel level.

See also:

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    Your definition of High Dynamic Range Imaging is awful narrow. HDR imaging goes all the way back to the 1850s using a variety of techniques to expand dynamic range beyond the conventional capability of the time. In my view both exposure fusion and 32-bit floating point files are slightly different forms of HDR. – Michael C Sep 7 '14 at 6:54
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    Yes, it's clear we've got some contention of terminology going on here. But I think it's also clear we have two different algorithms and that while broadening the definition of HDR could include both, that does not make them the same algorithm, or the same thing by two different names. By the broadness of definition others are insisting on, one could also call reverse-S Curves adjustments or fill flash HDR. – inkista Sep 7 '14 at 8:14
  • @MichaelClark if you refer to this en.m.wikipedia.org/wiki/… , this is exactly what we are talking about. it is exposure fusion, not HDR. you confusion may come from the fact that it is mentioned in the history of HDR section, as precursor of HDR. – ths Sep 7 '14 at 13:29
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    Why does HDR depend on the existence of a file with a larger than normal dynamic range? Isn't HDR a process where one takes multiple images with different exposures and turns them into a LDR? – JenSCDC Sep 7 '14 at 15:51
  • Because to digitally represent an HDR, you can't do it with an LDR file format--the possible numerical value range is too small. That's why you need a new file format with a larger bit-depth. JPEGs, for example, typically use 8-bit or 16-bit depth per channel. Some HDR file formats use 32-bit floating point values. – inkista Sep 7 '14 at 16:05
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Exposure fusion means blending of differently exposed images. This resulting image will not have an expanded dynamic range, but is basically a composite of the dark parts of the overexposed image, and the light parts of the underexposed one.
HDR on the other hand is an image which was created by combining all the data of the source images, resulting in an image with expanded ("high") dynamic range, potentially far more than can be displayed by any devices or papers.
Those HDR images can then be tone mapped to a narrower range so it can actually be viewed; this tonemapping result is not an HDR image anymore, although it is often erroneously called so.

So, no, they are not the same, but different techniques with similar applications.

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    I'm sorry, but you're wrong. You can start out with the same sequence of bracketed exposures, combine them into one HDR image, and then tone map it as you like- and "Exposure Fusion" is one of the available algorithms. Do you understand the difference between a technique and an algorithm? You don't seem to. – JenSCDC Sep 6 '14 at 23:59
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    i most certainly do. maybe you should take a look at the relevant wikipedia entry: en.m.wikipedia.org/wiki/Exposure_Fusion just because some tone mapping program chooses to call one of its options so the much older technique doesn't disappear. – ths Sep 7 '14 at 0:05
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    Nonsense, as has already been pointed out. – user28116 Sep 7 '14 at 0:17
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    If an algorithm takes a high dynamic range image and maps its value to a LDR image, it is by definition a tone mapping algorithm. – JenSCDC Sep 7 '14 at 0:53
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    @speising: Are you just making a semantic argument? Of course, 16- or 32-bit images literally have a higher dynamic range than traditional 8-bit images. And the product of HDR processing is an 8-bit image with no more theoretical color depth than an unenhanced image. But when photographers talk about "HDR" are they not talking about images that have been processed to incorporate more dynamic range than a sensor can capture, regardless of whether they explicitly pass through a stage in which higher color depth is stored? – feetwet Sep 7 '14 at 1:16

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