Tone mapping

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HDR Tone Mapping Example

Tone mapping is a computer graphics technique used to approximate the appearance of high dynamic range images in media with a more limited dynamic range. Print-outs, CRT or LCD monitors, and projectors all have a limited dynamic range which is inadequate to reproduce the full range of light intensities present in natural scenes. Essentially, tone mapping addresses the problem of strong contrast reduction from the scene values (radiance) to the displayable range while preserving the image details and color appearance important to appreciate the original scene content.

1 Purpose and methods
2 Gallery
3 References
4 External links
- 4.1 Tone mapping algorithms

[edit] Purpose and methods

Tone Mapped High dynamic range image example showing stained glass windows in south alcove of Old Saint Paul's, Wellington, New Zealand.

The six individual exposures used to create the previous image.

The goals of tone mapping can be differently stated depending on the particular application. In some cases producing just nice-looking images is the main goal, while other applications might emphasize reproducing as many image details as possible, or maximizing the image contrast. The goal in realistic rendering applications might be to obtain a perceptual match between a real scene and a displayed image even though the display device is not able to reproduce the full range of luminance values.

Various tone mapping operators have been developed in the recent years ^[1]. A simple example of tone mapping filter is $L = Y / (Y + 1)$ . This function will map scene radiance values $Y$ in the domain $[0, \infty)$ to a displayable output range of $[0,1)$ .

A more sophisticated group of tone mapping algorithms is based on contrast or gradient domain methods. Such operators concentrate on preserving contrast rather than mapping luminance values. This approach is motivated by the fact that the human perception is the most sensitive to contrast or ratios between light patches. Those tone mapping methods usually produce very sharp images, which preserve very well small contrast details; however, this is often done at the cost of flattening an overall image contrast. Examples of such tone mapping methods include: gradient domain high dynamic range compression ^[2] and A Perceptual Framework for Contrast Processing of High Dynamic Range Images^[3] (a tone mapping is one of the applications of this framework).

An interesting approach to tone mapping of HDR images is inspired by the anchoring theory of lightness perception ^[4]. This theory comprehensively explains many characteristics of the human visual system such as lightness constancy and its spectacular failures, which are important in the perception of images. The key concept of this tone mapping method (Lightness Perception in Tone Reproduction^[5]) is a decomposition of an HDR image into areas (frameworks) of consistent illumination and the local calculation of the lightness values. The net lightness of an image is calculated by merging of the frameworks proportionally to their strength. Particularly important is the anchoring -- relating the luminance values to a known brightness value, namely estimating which luminance value is perceived as white in the scene. This approach to tone mapping does not affect the local contrast and preserves the natural colors of an HDR image due to the linear handling of luminance.

[edit] Gallery


	Nave in Old Saint Paul's,Wellington, New Zealand. Eight exposures ranging from 1/20th of a second to 30 seconds	Two different exposures (at 4 stops' difference) can be combined and the dynamic range subsequently compressed to make the image viewable on a standard display	An HDR sunset. Taken as an exposure bracket of three images 2 stops apart.
A nighttime picture of San Francisco taken from an 8th floor hotel room using 3 exposures bracketed two stops apart.	HDR composite image of the Cornell Law School tower in Ithaca, New York.	A HDR image of the Inner Harbour in Victoria, BC