Chapter 4. Tone Mapping

Photo: Dieter Bethke

Tone mapping is a technical necessity as well as a creative challenge.

It's pretty much the opposite of what we did in the Chapter 3: In HDR generation, we expand the dynamic range by assembling it slice by slice from the source images. That's very straightforward, and the outcome is either right or wrong. In tone mapping, we reduce the dynamic range again so it fits back into a single LDR image. Here the outcome is not so clear, and there are a million ways to do it. Technically, it is impossible anyway. We're trying to squeeze something very big into a tiny container.

There is no ideal result. It just won't fit completely. So we have to make compromises. The whole trick is to make good use of the tonal values available and redistribute them in a way that makes sense. This redistribution is what tone mapping is about. We map the tonal values from an HDR image to an LDR image. How exactly that is done depends on the image content itself and how high the range really is. Sometimes it just takes a little squeeze; sometimes it is like pushing an elephant into a phone booth.

Back to normal: Read this out loud: high-dynamic-range-to-low-dynamic range conversion! After we're done with tone mapping, we don't have an HDR image anymore. It's completely bogus to point at a JPEG that just happened to be made via tone mapping and call it an HDR image. You don't call a JPEG a RAW image just because you shot it that way. Some may argue that the dynamic range of the scene is still contained in the JPEG, but that is not the case. Applesauce might contain everything that was an apple, but it is not an apple anymore. Maybe there are whole pieces mixed in, maybe it contains all the best of 20 apples, but there is no doubt that the resulting sauce is not an apple. A real apple wouldn't fit in the jar and you couldn't scoop it out and enjoy it as you could a well-done applesauce. Same goes for JPEGs made from an HDR image via tone mapping: They are processed for easy consumption via ordinary devices, and they are low dynamic range by definition. If you read and understood the first chapter of this book, you should know better than to label a tone mapping as an HDR image.

Figure 4-1. Tone Mapping looks for better ways to map the HDR pixel values into an LDR image.