What's Wrong with 'Flickr HDR'?
Creating an HDR Image
The image you see above was created from nine exposures, each spaced one f-stop apart. They were then combined into a high dynamic range image, and saved in the popular .hdr format. So far so good. There are many programmes now that can do this, including Photoshop, HDRShop, Photosphere, Photomatix, and Artizen HDR. I'm sure there are more, but these are the ones that I am aware of. It should be noted that at this stage, we have successfully combined several exposures into a single image. The rationale is that each of the exposures contains information over a limited range of intensities, but that together we have good information for all pixels. The algorithm to combine exposures essentially determines for each pixel which exposures contain useable information.
Displaying an HDR Image
Of course, just as we could not capture the scene with a single exposure, we have a similar problem when we want to display the image. We now have a range of values that is much larger than our poor monitors can handle. We therefore have to reduce this range of values again, a procedure known as tone reproduction, or dynamic range reduction. Of course, this means that we have to create a normal 8-bit image. I hope you can imagine that this means that somehow we have to get rid of information. But what information should that be? In short, the answer is that we don't know exactly. It depends on many factors, including what was in the image to begin with, the dynamic range of the display, the lighting in the room that contains the display, and on the specific state of the human visual system of the observer who will be looking at the image.
In essence, we can simply scale the high dynamic range image from whatever it was to a displayable range of 0 - 255. The result is the image seen here on the right. As you can see, this is largely an unsuccessful rendition of the scene we are interested in. However, if the purpose of dynamic range reduction is to make an HDR image displayable, then we have succeeded: all pixels are within the monitor's range of displayable values. Unfortunately, most pixels are too dark to be meaninful for human inspection.
This example illustrates that in addition to bringing all the values within the range of our monitor, we should try to preserve some other attribute(s) of the high dynamic range image, which is our representation of the scene that we have photographed. This is where it gets tricky. We are now interested in the nature of the attributes that we should preserve. It is generally accepted that we are creating images for the purpose of being viewed by humans (as opposed to, say, image analysis or computer vision). Hence, we want to create an image that is matched to our own perception of the scene. In practice, we might want to try to preserve the general appearance of the scene, or alternatively brightness, or contrast, or ... There are many possibilities.
We cannot in general preserve all attributes of a scene at the same time. We therefore have to make choices. This is all perfectly fine. However, it should be realised that dependent on which choice we make, we get an image that has a certain 'look' or 'feel'. Now, the problem here is that these choices are in principle not made by you, the photographer, but by the designers of the programmes that you use to manipulate high dynamic range images. Sometimes you do get some parameters to tweak, but these only give a limited freedom to steer the final result.
The Problem
Now look again at the image at the top of this page. It is displayable on your monitor, so it means that all the values are within range. The designers of the programme used to achieve tone reproduction have instilled a particular look and feel. Some people like it and some, like me, don't. For me, the problem with this image is that it is no longer a natural representation of the scene that I so caringly photographed. I lugged a heavy tripod around, and took the trouble to take a collection of nine photographs. I then ran them through software to create a single high dynamic range image. After this much trouble, I expect my tone reproduction software to produce an image that looks as close as possible to the scene I witnessed with my own eyes.
In other words: tone reproduction should produce a natural image. If I wanted the image to have a specific 'look' or 'feel', I am perfectly capable of running the image through a set of Photoshop filters. Tone reproduction has a single purpose only: reduce the dynamic range so that a good natural image can be displayed on a monitor without making it look funny in any way.
The image at the top is not what I expected to happen. It does not look natural. The tone reproduction operator has done something that it shouldn't have. Unfortunately, the image above is not an exception. The Flickr site has oodles of HDR images that were butchered by incapable use of a tone reproduction operator. I would argue that the vast majority of images tagged HDR on this site are seriously messed up. I am not surprised that there now exists a Flickr group called 'I hate HDR'. Photographers are split as to whether HDR is a good thing or a bad thing. This is a real pity.
HDR: Good or Bad?
I think that HDR is unequivocally a good thing --- if done right. The general technology allows photographers to take photographs that they would otherwise not dream of. The image shown here is a point in case. Without multiple-exposure techniques we would never ever shoot into the sun (and yes, I know that this particular photograph has a flare problem --- in fact, it was taken for the purpose of demonstrating flare in my new book on colour imaging. It just happens to also be a good example to demonstrate what typically goes wrong with tone reproduction). The image to the right here shows the best single exposure: it is clear that a single exposure does not adequately capture this scene, as there are both under- and over-exposed areas in this one image. As a further example, we also have problems taking photographs of indoor scenes, if there are windows visible. I hear that real-estate agents encounter this problem regularly!
So HDR is a good thing --- if done right. Unfortunately, the Flickr HDR community does it wrong on a massive scale. My aim here is to get you to understand that your choice of software probably has the single largest impact on whether you will be able to produce natural looking images. The second issue is that you should realise that as soon as the image looks funny in some way, then this is wrong. It is an artifact that should be avoided! If you want to deliberately produce an artificial looking image, then by all means do so. However, please do not abuse tone reproduction to get there. And please do not then go and call it 'HDR'.
If anything, the image at the top looks sufficiently unnatural that I would call it 'NPR', which stands for 'non-photorealistic rendering'. I consider it an example of HDR gone wrong.
Of course, after such a rant, I should now provide an alternative. Luckily I do have one on offer. The image shown below was created with a tone reproduction operator I developed several years ago with the help of Mike Stark, Peter Shirley, and Jim Ferwerda. It is used widely in various industries. It is aimed to provide a natural look. This means that there is less emphasis on preserving every tiny detail, but instead on giving a good overall impression. It is not meant to produce a spectacular result, just a natural one. I think that the image below achieves that. You can almost sense that it is 6:30 in the morning, and not too warm. It matches my memory of the scene I photographed pretty well. This image is not perfect, but it does not stand out as obviously wrong.
Oh, before you ask. This tone reproduction operator is available for free, and everybody is invited to experiment, use it, and build it into their own software. I do not require royalties for this, given that my boss has already paid me to do the work (the University of Utah in this case). There are also several others that are available for free, which I would encourage the technically minded reader to experiment with. See for example PFSTools.
Below is another image. This was produced with a slightly more obscure operator, which I worked on in 2005 with Kate Devlin. It is more directly inspired by what we know about the human visual system. It includes a rudimentary form of white balancing, which is seen here as an image which has a little less yellow in it. However, where I really think that this image improves over the one above, is that the sun was allowed to burn out. This has the effect of producing an image with a truly bright sun. Do you see that the sun appears even brighter than the white background of this page? That is very difficult to get with coloured light sources such as the sun in the image above.
Now, before you start thinking that all of this is a shameless plug, I fully acknowledge that there are many other tone reproduction operators in existence. Several of these produce excellent results. They are described in the scientific literature, as well as in technical books on high dynamic range imaging, including the one I was involved with, together with Greg Ward, Sumanta Pattanaik, and Paul Debevec. Several papers have compared them by means of psychophysical experiments, not leading to a fully clear answer as to which operator is best. It depends a little on the application. You can imagine that for instance in medical visualisation, a tone reproduction operator does not have to look natural.  Instead it should produce a visualisation that avoids mis-diagnosis.
However, these pages discuss high dynamic range imaging and tone reproduction for the purpose of photography. In my opinion, this means that visual artifacts should not be introduced anywhere in the imaging pipeline, unless the user specifically requests it. This is not what I see happening at the Flickr site. Photographers have to tonemap their HDR images to prepare them for display, and inadvertently introduce many ugly compression artifacts. The 'look' and 'feel' associated with these compression artifacts are now being taken as representative of HDR: tone reproduction and aesthetics are hopelessly confounded. This is wrong. The purpose of this micro-site is to show that it is also avoidable. All it takes is a little common sense and a little awareness. I would encourage you to have a look at the comparison page to see what I mean.