Understanding HDR part V – Understanding and handling tone mapping

Rothenburg ob der Tauber is one the most idyllic German medieval towns I have visited. Fot that reason I have been there three times. This split road is particularly lovely I think. Photo by: Jacob Surland, www.caughtinpixels.com

In the previous articles we have covered the theory behind HDR, and why it is necessary to take more than one shot. How to improve the image quality, by pushing the histogram to the right and various approaches to post-process HDR photos. This is fundamental knowledge to HDR photography.

If you did not read them, you might want to. You find them here:

The purpose and result of HDR software

Because you shoot several shots of the same scene, you have to use a tool to put them together. You can either do it manually, and blend the images, by stacking the photos in Photoshop or GIMP and then blend them into a final HDR photo using layer masks. Or you can use a tool that will do this automatically.

The automatic HDR software is probably the more common approach.

The problem of doing something, automatically is, that you are less in control, and the engine might not do want you want it do, or it will do more than you want it to do. Like in all other aspects of of life ‘there is no silver bullet’. There is no secret recipe, that will give you fantastic HDR photos, by pressing a button.

But making a fantastic HDR photo, does not have to be difficult, if you just know what to do, and have done a little practicing. I will help you understanding, what it is that you (need to) do.

As covered in part IV, the purpose of the HDR software is a two step process, first to merge the photos together to a 32-bit image file (the real HDR photo), and second to transform it into a 16-bit image, because you can’t see the 32-bit image properly. The 16-bit, is not really an HDR photo, but is usually referred to as the HDR, and this I also do, just bear in mind that the 32-bit image is technically the HDR photo.

A tone mapping algorithm will map the tones from 32-bit to 16-bit and a fusion algorithm will blend the photos.

The fusion algorithm does not have the same flexibility and artistic options as does the tone mapper, and for that reason I personally prefer to use a tone mapper, rather than use a less flexible fusion algorithm. But it is a matter of personal taste.

Tone mappers also come in many flavors, and to me, artistic flexibility is the most important thing.

Single exposure tone mapping

The tone mapper maps tones from one image to another, by passing the image through an algorithm. The merging of three images into the 32-bit HDR, does not have anything to do with the tone mapper itself.

You can take a single (well exposed) image, and put that through the tone mapper, and you will get a similar result, as had you used a 32-bit HDR photo. The viewer will see the photo, and recognize it, as what is commonly referred to as ‘an HDR photo’ (keeping in mind, that the only HDR really is the 32-bit image, that we can’t see properly).

Not all single exposure images gives great results in the tone mapping algorithm. It depends very much on how well exposed the image is.

A couple of examples of single exposures I have tone mapped:

University of Copenhagen

Lighthouse on the edge

To commoners these photos will look like HDR photos, because what is commonly referred to as HDR photos, really are tone mapped images. And the commoners, will not know the difference. I hope I have made it clear enough to you.

Side effects when tone mapping

Tone mapping is not without flaws, in particular if you push the gas pedal towards the floor. To me the tone mapper is a flexible tool, that you can bend, not necessarily to your will, but can bend into many interesting and artistic results, but if you are not careful, you can also get the worst image ever. This opens up for creativity and I do use this to great extend, and with great care.

Let’s walk through some of the common side effects from tone mapping.

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