Tone mapping

[2] The introduction of film-based photography created issues since capturing the wide dynamic range of lighting from the real world on a chemically limited negative was very difficult.

Early film developers attempted to remedy this issue by designing film stocks and print development systems that gave a desired S-shaped tone curve with slightly enhanced contrast (about 15%) in the middle range and gradually compressed highlights and shadows [1].

The advent of the Zone System, which bases exposure on the desired shadow tones along with varying the length of time spent in the chemical developer (thus controlling highlight tones) extended the tonal range of black and white (and later, color) negative film from its native range of about seven stops to about ten.

Despite this, if algorithms could not sufficiently map tones and colors, a skilled artist was still needed, as is the case with cinematographic movie post-processing.

Computer graphic techniques capable of rendering high-contrast scenes shifted the focus from color to luminance as the main limiting factor of display devices.

[7] This theory explains many characteristics of the human visual system such as lightness constancy and its failures (as in the checker shadow illusion), which are important in the perception of images.

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.

One simple form of tone mapping takes a standard image (not HDR – the dynamic range already compressed) and applies unsharp masking with a large radius, which increases local contrast rather than sharpening.

This method also allows for more output adjustment and saturation enhancement, making it be less computationally intensive and better at reducing the overall halo effect.

The manipulation of film and development process to render high contrast scenes, especially those shot in bright sunlight, on printing paper with a relatively low dynamic range, is effectively a form of tone mapping, although it is not usually called that.

Local adjustment of tonality in film processing is primarily done via dodging and burning, and is particularly advocated by and associated with Ansel Adams, as described in his book The Print; see also his Zone System.

The normal process of exposure compensation, brightening shadows and altering contrast applied globally to digital images as part of a professional or serious amateur workflow is also a form of tone mapping.

There are now many examples of locally tone mapped digital images, inaccurately known as "HDR photographs", on the internet, and these are of varying quality.

This popularity is partly driven by the distinctive appearance of locally tone mapped images, which many people find attractive, and partly by a desire to capture high-contrast scenes that are hard or impossible to photograph in a single exposure, and may not render attractively even when they can be captured.

Although digital sensors actually capture a higher dynamic range than film, they completely lose detail in extreme highlights, clipping them to pure white, producing an unattractive result when compared with negative film, which tends to retain color and some detail in highlights.

This avoids the artifacts that can appear when different exposures are combined, due to moving objects in the scene or camera shake.

For this method, models for the Human Visual System (HVS) and the display are first generated, along with a simple tone mapping operator.

With these models, an objective function that defines the tone curve can be created and solved using a fast quadratic solver.

The images on the right show the interior of a church, a scene which has a variation in radiance much larger than that which can be displayed on a monitor or recorded by a conventional camera.

Relative to the church interior, the stained-glass window is displayed at a much lower brightness than a linear mapping between scene radiance and pixel intensity would produce.

Local tone mapping technique of HDR image processing often produces a number of characteristic effects in images such as bright halos around dark objects, dark halos around bright objects, and sometimes a "cartoon-like" appearance due to extremely vivid colors and lack of large-scale color variations.

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.

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.