Tone mapping is a process by which an image, usually a photograph, is adjusted so that a high range of tones appear properly in a medium not necessarily designed to handle them. This process involves the understanding of two basic concepts: which are high dynamic range (HDR) images and low dynamic range (LDR) images. HDR images have a large difference between the low and high tones, darks and lights, in them, while LDR images have a much smaller range between the tones. A photographer or photo manipulator can use tone mapping to alter the data in an HDR image to be better displayed on a monitor or other medium that is LDR in nature.
The basic way in which tone mapping works is somewhat complicated, but an understanding of high and low dynamic range images can simplify things. An HDR or high dynamic range image has a wide range of tones, which are degrees of lightness and darkness. This means that someone seeing an HDR image can see a vast distinction between the lightest tones and the darkest tones of a color, so the range in “blues” or “reds” can be quite dramatic in this type of image.
In contrast, an LDR or low dynamic range image has less distinction between different tones. Someone seeing an LDR image might notice that the lightest and darkest tones of a single color more closely resemble each other than in an HDR image. Many digital cameras can capture HDR images, but computer monitors can typically only display LDR images. This means that someone viewing a crisp, powerful photograph on a computer monitor might see it as dull and lacking in tonal range. The brightest and darkest colors of the HDR original are effectively lost in the LDR medium, which retains only the middle range and “cuts off” the higher and lower tones.
Tone mapping, however, allows someone to manipulate a photographic image so that the range of tones available in HDR are properly displayed through an LDR context. This is typically accomplished through the creation of three or more photographs, rather than just one image. Each of these images is captured at different exposures, which alters the time that light is received by the camera. This creates a series of images that are essentially the same, but with a wide range of tones, lights and darks, captured in each picture.
A photographer can then use tone mapping to essentially combine these different images together into one final picture. This creates a resulting image that can be viewed on an LDR medium, such as a computer monitor, but has the clarity and range of tones of an HDR image. By using tone mapping, an artist can create a final photograph that captures the range of tones that would otherwise be lost when transferring directly between the two formats.