What’s Tone Mapping?

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Tone mapping is a process by which an image, usually a photograph, is adjusted so that a high range of tones appear correctly in a medium not necessarily designed to handle them. This process involves understanding two basic concepts: what are high dynamic range (HDR) images and low dynamic range (LDR) images. HDR images have a large difference between low and high tones, shadows and highlights, while LDR images have a much smaller range between tones. A photographer or photo manipulator can use tone mapping to alter the data in an HDR image to be better viewed on a monitor or other medium that is LDR in nature.

The basic way tone mapping works is somewhat complicated, but understanding high and low dynamic range images can make things easier. An HDR or high dynamic range image has a wide range of tones, which are degrees of lightness and darkness. This means that someone viewing an HDR image can see a vast distinction between the lightest tones and darkest tones of a color, so the range in “blues” or “reds” can be quite dramatic in this type of image.

Conversely, an LDR or low dynamic range image has less distinction between different tones. Someone viewing an LDR image may notice that the lightest and darkest tones of a single color resemble each other more closely than in an HDR image. Many digital cameras can capture HDR images, but computer monitors typically can only display LDR images. This means that someone viewing a sharp, powerful photograph on a computer monitor might see it as dull and lacking in tonal range. The brighter and darker colors of the HDR original are effectively lost in the LDR medium, which retains only the midrange and “cuts” the highest and lowest tones.

Tone mapping, however, allows someone to manipulate a photographic image so that the range of tones available in HDR is displayed correctly through an LDR context. This is typically accomplished by creating 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 wider range of tone, light and shadow, captured in each image.

A photographer can then use tone mapping to essentially combine these different images into one final image. This creates a resulting image that can be viewed on LDR media, such as a computer monitor, but has the clarity and tonal range of an HDR image. Using tone mapping, an artist can create a final photograph that captures the range of tones that would otherwise be lost in direct transfer between the two formats.

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