Tonemappers

Tonemappers, or tone mapping operators, are algorithms that convert high dynamic range (HDR) images into low dynamic range (LDR) images suitable for standard displays, while aiming to preserve the appearance of the original scene. HDR images contain a wider range of luminance than displays can reproduce; tone mapping compresses this range and adapts brightness, contrast, and color to produce a perceptually convincing result.

There are global operators, which apply a single function to all pixels, and local or adaptive operators, which modify the mapping based on a pixel’s neighborhood or multiple scales. Global operators often prioritize overall brightness and material contrast, while local methods emphasize local detail and edge preservation. Common goals include maintaining perceptual brightness relationships, preserving details in shadows and highlights, and avoiding artifacts such as halos or color shifts. Local methods may use multiscale decompositions, edge detection, or local contrast adjustments.

In practice, tonemapping is applied after retrieving or rendering an HDR radiance map. The resulting LDR image is then gamma corrected and color-mapped to fit the display’s gamut, often with perceptual or custom color management so that hue and saturation remain plausible.

Applications span photography, computer graphics rendering, and film, where real-time or offline tone mapping balances visual fidelity against computational cost. Limitations include potential artifacts and color or contrast distortions if the operator is ill-suited for the scene. Evaluation uses subjective viewing tests and objective metrics such as TMQI or other perceptual quality measures.