The Grumpy Editor's guide to HDR with Linux

A while back, your editor stumbled across the Flickr HDR pool; some of the photos in that pool were sufficiently amazing to inspire an immediate "I wanna do that" reaction. The better part of a year later, it finally became possible to learn a bit more about the process behind those pictures. HDR (or high dynamic range) photography is a set of techniques for overcoming the limitations of contemporary hardware and, in the process, generating images which better represent a scene as viewed by the human eye - or which appear to come from a work of fantasy art.

The sensors in today's digital cameras have gotten good, but they still fall short of the human eye in a few ways. In particular, the range of light levels which can be captured by the sensor is not yet up to what film can handle, and is far from what the eye can do. Anybody who has spent any time taking pictures is familiar with this problem: one can take a beautiful landscape picture, but, in the end result, the wild cloud formations are washed out completely and the shadows just go black. Being unable to capture a scene that one can see quite well can be most frustrating.

The idea behind HDR, as it is used with photography, is to extend the available dynamic range by taking multiple shots at different exposure levels. For a given exposure, there will be a range of light levels which will be captured with good resolution by the sensor; everything else gets compressed at one end or the other. If one has a series of images at different exposures and a reasonable model of the camera's response curve, one can generate a composite image by using the parts of each source image which are in the good part of the curve. So, in that landscape picture, a very dark exposure can be used for the bright parts of the scene - clouds, for example - while a bright exposure yields low-light details. By mixing them together, the HDR algorithm can produce an image with full sensor resolution across a much wider dynamic range.

As an example, consider the photograph below, taken from your editor's dining room:

Original	HDR

(Larger versions of the images are available.) In the original, parts of the plant in the foreground are entirely lost in the shadows. Meanwhile, the breathtaking view of Colorado suburbia (with mountains in the distance) is washed out entirely. The HDR version brings all of that detail back.

HDR is not applicable to all situations. It has a tendency to turn people into cartoon characters. Beyond that, the need for multiple exposures generally implies setting up a tripod and taking some time for the entire process. It is thus not well suited to changing scenes, sports photography (though baseball, perhaps, can be expected to stand still for the requisite time), etc. It can work well for relatively static scenes: landscapes, buildings, the SCO case, and so on.

Most of the people playing with HDR seem to be using proprietary plugins for a proprietary image manipulation program running under a proprietary operating system. That is, needless to say, not your editor's preferred mode of operation. Thus your editor began a search for tools which would perform HDR processing under Linux. It turns out that there are a few such tools around; there is no need to use proprietary software for this task.

HDR generation

Next, one should come up with a set of source images. Ideally, these images are taken with a tripod-mounted camera and cover a range of at least two f-stops above and below the nominally "correct" exposure. Varying the exposure time is preferred over changing the aperture; if nothing else, this ensures that all of the images will have the same depth of field. One can start with images taken without a tripod, but it will be necessary to register them before continuing. Your editor did not get into that aspect of the task; tools like hugin and hdrprep can be used for this job. These tools may be a good topic for your editor's attention in a future article. One can also apply HDR techniques to a single image, especially if it is in the camera's raw format, but multiple exposures give much better results.

With the images in place, one can look at combining them into an HDR image. This is a two-stage process (two user-visible stages, at least): creating a set of response curves and using them to map the images together into a single dynamic range space. The response curves are a mapping between some sort of real-world light levels and the resulting sensor values on all three color channels. When combined with information on the relative exposure times of two (or more images), the response curves allow the HDR program to map pixels from all of the images into the same space. The response curves can be generated directly from the source images; they don't normally change, so they can be saved and reused later.

The first HDR-generation tool to look at is cinepaint, once known as "Film Gimp." This tool is a fork of the GIMP which is aimed at use by movie studios; its floating-point image data support makes it useful for HDR processing as well. The generation of HDR is done with the "bracketing to HDR" plugin which is, happily, packaged with the cinepaint source distribution. There is a detailed explanation of what this plugin does and how to use it. Be warned that it makes for somewhat difficult reading - and it would even if it weren't originally written in German.

The good news is that actually using this plugin is easy. One selects "bracketing for HDR" from the File->New from menu, then selects the set of source images from a simple dialog. The plugin will then import them. There is no provision for obtaining the relative exposure information from the image files themselves; instead, the plugin sorts the images by brightness and applies an assumed (adjustable) exposure difference between them. It attempts to feed each image to dcraw for decoding, but your editor was not able to get raw images to work despite the fact that dcraw supports his camera just fine; it looks like the raw import plugin was written for an older version of dcraw. That problem is likely to be easily overcome; your editor just didn't want to spend much time on it. So TIFF files were used instead.

Once the images are in, the user can check the exposure values, then hit the "compute response" button. That yields the two plots shown in the screenshot. By messing around with the buttons, one can look for the reference image which yields the smoothest set of response curves - or one can just accept what the plugin does by default. Then a click on the "generate HDR" button creates the final product, which can then be saved out in the OpenEXR format.

Your editor set out to take some amazing pictures for this article. The area in which your editor resides is widely held to be beautiful, but, frankly, Colorado is not at its best in early March; perhaps this article should have been written in June. Nonetheless, the effort was made. Below is a rather mediocre shot of the Boulder foothills in original and HDR (with cinepaint) forms (larger versions available).

Original	HDR

The HDR image above shows a halo effect (the bright sky above the mountain) which is characteristic of some tone mapping algorithms; we'll get into tone mapping shortly.

An alternative approach is PFScalibration, a command-line HDR generation utility based on pfstools. These tools work as a netpbm-like pipeline; their use requires a fair amount of typing, though much of the work can be scripted. The steps are the following:

• Run jpeg2hdrgen to generate a description file for the source images. It reads the EXIF information from the source files to get the relative exposures and outputs it in a simple file. There is a dcraw2hdrgen tool as well, but the subsequent stages in the pipeline are not able to work with raw files. Your editor suspects that TIFF files could be used by creating the hdrgen file by hand, but the whole process seems to be intended for use with JPEG files. A lossy file format is not the most auspicious starting point for somebody interested in high dynamic range imagery, but that's how it is.

• The pfshdrcalibrate utility can then be used to create a set of response curves; gnuplot can be used to visualize them. This process can take some time (it's significantly slower than cinepaint), but the resulting file can be saved and reused with different images in the future.

• Another pfshdrcalibrate run then uses the response curves to create the HDR image. Piping the output into pfsoutexr generates an OpenEXR file.

Here's an example generated from a series of pictures of your editor's dungeon office (larger versions):

Original	HDR

As a general rule, HDR images generated with cinepaint and PFScalibration tend to look identical. The generation of HDR is not where the real magic lies, so the results should be close.

For those who don't like command-line HDR processing, the qtpfsgui utility may be worth a look. It is a graphical wrapper around PFScalibration based on QT4; it handles both HDR generation and tone mapping. On the HDR side, it puts up a file selection dialog for the source images followed by the "HDR creation wizard." The user is asked to select a "creation configuration," from a list of configurations helpfully named "Configuration 1" through "Configuration 6". The advice to stick with Configuration 1 was hard for your editor to ignore; simply hitting "next" generated the image.

Said image appeared in a display window; like exrdisplay, this window can only show the image in full resolution. Your editor, lacking a 7 megapixel monitor, was thus unable to view the entire image at once. Even worse, qtpfsgui is one of the family of (generally KDE-based) graphical tools which feels the need to implement its own window manager. The display window lives within the larger qtpfsgui window; it cannot be resized with the usual shortcut your editor is used to. In summary, qtpfsgui gets the job done, but writing a simple script around PFScalibration seems like an easier way to go.

Tone mapping

While the tools above will generate a fine HDR image, one problem remains: the dynamic range in that HDR image far exceeds the range of your editor's monitor (or printer). Turning that image into something which can be displayed requires a step called tone mapping. This is where the serious magic comes in: somehow the vast amount of information in the HDR image must be scaled back in a way which does not compromise the image quality that was the whole point of this exercise in the first place. Several tone mapping algorithms exist, and most of them have a number of mysterious knobs to tweak. While the generation of HDR can be mostly automated, tone mapping inherently requires experimentation and human judgment.

The bulk of the action appears to be in the pfstmo package, which implements several tone mapping algorithms as separate, standalone filters. One can use pfstmo with the rest of the pfstools package to construct pipelines which generate tone-mapped images. Given the iterative nature of the task, however, it would be nice if there were a better way.

That better way is qpfstmo, a Qt-based graphical interface to pfstmo. The interface feels a little clunky at times, and it would sure be nice to have some online documentation on what the various parameters do, but qpfstmo does what is really needed: it lets the user play with tone mapping algorithms and compare the results. A small image size can be used for trying out algorithms and parameters - a real time saver, since some of the algorithms can take a long time on a full-size image - and multiple versions of the image can be on the screen at once. When a final configuration is found for a given image, it can be generated in a larger size and saved in any of the usual image formats. When applied to a large image file, this step can be rather hard on the hardware; your editor discovered that 1GB of memory was not really enough.

The qtpfsgui tool mentioned above has the ability to drive pfstmo as well. It is, in fact, clear that this tool shares a lot of code with pfstmo. The interface is far less friendly, however: everything happens within the One Big Window and it does not appear to be possible to see the results from more than one algorithm at the same time. It resets the display image size every time the user changes algorithm. One assumes that this (fairly new) tool will improve over time. For now, though, qpfstmo seems like a much better way to go for tone mapping control.

A different set of tone mapping operators is supplied with the exrtools distribution. Your editor tried them all; each one is a cumbersome, multi-step process. It can take a long time to process an image, only to find that the parameters need quite a bit of tweaking. The tools seem like they will do quality transformations, but they just cry out for a qpfstmo-like interface which allows experimentation with smaller-size images and comparison of results. For what it's worth, here's a shot taken from the hill above your editor's house mapped with the exrtools non-linear masking method:

Original	HDR

See the larger versions for more detail. Doubtless one could get good results from these tools with enough effort, but your editor found it easier to get quality images with psftmo.

Conclusion

For the generation of HDR images, your editor found cinepaint to be faster and simpler to work with. This does not count, however, the long and frustrating experience of building the HDR plugin on a Fedora Rawhide system; one gets the sense that the plugin's author uses a rather older, less picky version of g++. Longer-term, however, the PFScalibration suite may prove to be the way to go. It is far more compact and easy to install on a new system; why lug the weight of cinepaint if one is not going to use its other features? A bit of scripting will easily turn PFScalibration into a single-command HDR generation tool.

It's worth noting that there are a couple of other HDR generators for Linux out there. MakeHDR is where a lot of it started; one of its authors is Paul Debevec, who did much of the early research in this area. The code was last touched in 1999, however, and it comes with a "educational purposes only" license. One can also look at HDRgen, but it is a binary-only, free-beer tool. Your editor did not actually try either one of them; given that the free tools do the job so well, there didn't seem to be any point.

For tone mapping, pfstmo (and qpfstmo) are the best tools at this point. It is hard to be entirely satisfied with the state of the art in this area, though. Tone mapping will always be an exercise in compromises, so it's not surprising that the results are rarely perfect. There is likely to be room for improvement - in both the algorithms and the interface to them - for some time to come.

As is the case in many areas, Linux has the tools one needs to play with high dynamic range imagery. One just has to work a little harder to get started than on some other systems. HDR has found its way into your editor's photographic toolkit; look for the results in the reporting from some conference in some exotic part of the world. When playing with this stuff, your editor is far from grumpy.

http://www.mpi-inf.mpg.de/resources/tmo/NewExperiment/TmoOverview.html

http://www.cgg.cvut.cz/~cadikm/tmo/