All tone mapping techniques are applied to a raw image. Unfortunately, only a few rendering packages today offer the possibility for storing a raw image. Most rendering packages apply a tone mapping, and only the final image can be stored. In this way, a lot of information is lost. We have seen how different display media have different characteristics. An image mapped once for a particular monitor, does not have to be the optimum solution even for another monitor, and especially not for a printer or some other device. If a raw image is stored, mapping can be done for each particular device.
Furthermore, when better display devices become common in the future, raw images can be mapped optimally to the new media.
The reason why raw images are rarely stored is the large storage space requirement. It is true that storing a raw image as the array of float triplets will demand a huge amount of storage place (96 bits per pixel - four times the usually necessary 24 bits). Compression techniques applied on such files do not help a lot due to the very poor entropy characteristics of such files. Fortunately there are currently three freely available raw image formats that overcome the size problem, at the cost of reduced accuracy. They are RADIANCE [Ward94a], the log format proposed by Pixar, and finally a newly proposed extension to the TIFF file format by SGI, logLuv. We will describe each of these three formats next.