How to Tone Map an HDR Image
Everything else in high dynamic range (HDR) photography either leads up to or follows tone mapping. Tone mapping occurs when you convert a higher dynamic range image to one with a lower dynamic range, most often a JPEG or TIFF. The HDR image is, of course, created from bracketed photos with a wide range of exposures.
One aspect of the tone mapping process is illustrated here: namely, squeezing dynamic range. This is important because monitors, printers, and standard graphics files are incompatible with HDR images. The large dynamic range of the HDR image (taken just outside a large building with a very large portico) is mapped into the lower dynamic range during tone mapping (which you perform after creating the HDR image in your HDR application).
Notice that the EV range of the HDR image ranges from below -6 to above +9 EV, which is well beyond the capabilities of an 8- or 16-bit image. Tone mapping enables you to see something like the original dynamic range of the image, using a less-capable format. Aside from all the technical mumbo jumbo, without tone mapping, there is no point to HDR photography.
This is where you decide, based on the information available in the high bit-depth HDR image, what the final low dynamic range image looks like. And that is the point — creating a final image you can continue to edit, post on the Web, and print to hang on your refrigerator.
When monitors, printers, and computers work with 32-bit HDR images — like JPEGs — tone mapping might shrink in importance because there will be no need to tone map an HDR image to a low dynamic range space — it’ll be compatible with everything as originally shot.
And that’s not all that happens when you tone map.
Given a bracketed set of images, if you examine two points and compare their brightness (luminance), they have a distinct relationship with each other, as shown in this figure. In this case, the spot on the building is darker than the clouds. No matter what image of the bracketed set you look at, the two points have the same relationship to each other in terms of luminance.
Although the contrast between them diminishes as either point gets blown out or lost in shadow, the basic relationship stays the same. To reiterate, in this bracketed set, a point in the shadowed area of the building is always darker than the clouds, regardless of the bracket you look at.
Tone mapping breaks this relationship. As you tone map the HDR image, you don’t have to keep the same brightness relationship between the two points. You can alter and even reverse it so that the darker point becomes the lighter point. In fact, this is exactly what happened. In the tone mapped file, the same spot in shadow on the building is slightly lighter than the clouds!
This is why tone mapping is different from blending exposures and other techniques that simply compress the larger dynamic range into a smaller space. Tone mapping allows you to take the high dynamic range data and change the brightness relationship between different points of the image. This is where your photographer’s judgment and artistry come into play.
This figure shows the final tone mapped image. Irrespective of all the technical processes going on in the HDR application, the goal was to create an aesthetically pleasing finished image, which meant fiddling with the tone mapping settings in Photomatix Pro until the result was satisfactory. The wealth of exposure data provided by the bracketed photos gives you the flexibility to accomplish the same thing with your HDR images.