Love it or hate it, HDR is here… and it’s on Linux, too

These days, it is hard to avoid HDR when you are interested in photography. Just for the record, High Dynamic Range is a way to combine different exposures of the same image: you end up with an image that has a wider range of shadow and highlight that what you normally see on printed paper or on your monitor. You then rearrange this extra dynamic to fit into a file that you can see on your monitor or print.

The result is an image that has a lot of details in both shadows and highlights – in my opinion, most of these images look very unnatural (not to say ugly) and very rarely do I come across “good taste HDR”. Like every post-processing, it is only when you don’t notice it that it is done right. Other people, of course, will disagree and process every single one of their pictures in HDR.

To know more about HDR, have a looks at the wikipedia article as well as the HDR Flickr gallery.

Although I won’t cover HDR in all details, I decided to give it a go out of curiosity, especially since Linux does it – via Qtpfsgui, that’s right GUI means: “no command line” 🙂

Getting the images

You need to start with 3 (or more) bracketed version of the same view. You need to use a tripod and vary the shutter speed since bracketing a hand-hold picture will probably produce misaligned images. Although you can realign them in software, it doesn’t work too well in practice often resulting in a blurry final result.

An (easier) way is to process a RAW file with -1, 0 and +1 exposure compensation. I decided to go that way and had a RAW file processed into JPEG 3 times from UFraw in the command line – that avoids alignment issues as well. Although there is a way to manually enter this information, Qtpfsgui will process your files more easily if they include exifs.

Creating an HDR file

Once you have these files, you can fire Qtpfsgui – there is a detailed documentation in their sourceforge wiki.

Creating an HDR file is dead easy, just click on (surprise, surprise) create HDR and go through a few steps; if your files have proper exif datas and are aligned, this is straightforward – it basically boils down to choosing the files you want to use to produce your HDR image. You can also manually align your files if needed. I left all the settings to default and clicked next a few times. This is the result:

Save your image as test.exr (an open source HDR format).

ToneMapping

Now this is where the good fun starts. Tone Mapping is about squeezing back the HDR image to a “standard” dynamic range, ie one you can show on your monitor or print on paper. When clicking the ToneMap button, you are presented with a new window including the Tone mapping Panel where you to chose the image size you want (start with small sizes, since tonemapping is CPU intensive), the pregamma setting as well as your tonemapping algorithm and its settings. Once you have made your choice, click apply and you get a window with your image:

The next step is to adjust the levels (like in any image manipulation program) – this step makes quite a difference in the final output:

I created an image with each of the different tonemapping algorithms (settings as defaults) and then saved another version with the levels adjusted – if I could get a usable output from the previous step. This may look a bit tedious, but it shows how much difference there can be in the final result of an HDR image. In adjusting the levels, I just tried to get a pleasing output: saturated and without burning the whites (too much). By the way, Open Source Photography has a page describing the different algorithm in more detail.

Ashikhmin algorithm (no level adjusted version):

 Drago algorithm:

Durand algorithm (no level adjusted version)

Fattal (sic!) algorithm

Mantiuk algorithm

 

Pattanaik algorithm

Reinhard 2002 algorithm

Reinhard 2004 algorithm

Conclusion HDR or Not?

My favorite output is Reinhard04 – I think it keeps the white unburnt while allowing a high saturation of the image and maintaining a relatively natural overall look. So I had a more careful processing of my image using this algorithm, and here is the HDR result which I like most for this image:

For comparison’s sake, here is the same image processed classically (with RawStudio) from a RAW file:

I thought this image would be a good candidate for HDR since it has snow that needs to be very white without burning (I didn’t burn it when shooting, yeah!) with a lot of color nuances up to pitch dark (for the trees).

And indeed, when comparing the classical version to the HDR one side by side my great certainty from the beginning of this entry starts to fade: I have this feeling that the “truth” is somewhere between the two versions…

Let’s go back to trying more things, different algorithms 🙂