Tone Mapping - Visual Effect

Visual Effect

Local tone mapping produces a number of characteristic effects in images. These include halos around dark objects, a "painting-like" or "cartoon-like" appearance due to a lack of large global contrasts, and highly saturated colours. Many people find the resulting images attractive and these effects to add an interesting new set of choices for post-processing in digital photography. Some people believe that the results stray too far from realism, or find them unattractive, but these are aesthetic judgements, and often concern the choices made by the photographer during the tone mapping process, rather than being a necessary consequence of using tone mapping.

Not all tone mapped images are visually distinctive. Reducing dynamic range with tone mapping is often useful in bright sunlit scenes, where the difference in intensity between direct illumination and shadow is great. In these cases the global contrast of the scene is reduced, but the local contrast maintained, while the image as a whole continues to look natural. Use of tone mapping in this context may not be apparent from the final image:

• Regions of direct illumination and shadow on the Grand Canyon

Tone mapping can also produce distinctive visual effects in the final image, such as the visible halo around the tower in the Cornell Law School image below. It can be used to produce these effects even when the dynamic range of the original image is not particularly high. Halos in images come about because the local tone mapping operator will brighten areas around dark objects, in order to maintain the local contrast in the original image, which fools the human visual system into perceiving the dark objects as being dark, even if their actual luminance is the same as that of areas of the image perceived as being bright. Usually this effect is subtle, but if the contrasts in the original image are extreme, or the photographer deliberately sets the luminance gradient to be very steep, the halos become visible.

• Tone mapped composite image of the Cornell Law School tower in Ithaca, New York. Note the 'ringing' or 'halos' around the tower.

• Nave in Old Saint Paul's, Wellington, New Zealand, using eight exposures bracketed one stop apart.