Tonemappingoperators

Tonemapping operators are algorithms used to convert high dynamic range (HDR) images into low dynamic range (LDR) images suitable for display on standard devices. They aim to preserve perceptual brightness relationships and important details while compressing dynamic range. Tonemapping can be global, applying a single function to all pixels, or local, adapting to the surrounding luminance to preserve local contrast.

Global tonemapping operators apply the same mapping to all pixels. A well-known example is the Reinhard global operator, which maps luminance L to Ld using a simple formula such as Ld = L / (1 + L) (with optional scaling) and then applies the same scale to the color channels. This approach is fast and tends to produce natural-looking results in many scenes. Other global approaches use logarithmic or perceptually motivated functions to control overall brightness and contrast.

Local tonemapping operators adjust the mapping based on the surrounding neighborhood of each pixel. They aim to preserve or enhance local contrast and detail by compressing the base luminance while preserving edges in the detail, often using edge-preserving filters or multi-scale processing. Local methods can reveal hidden details in shadows and highlights but may introduce artifacts such as halos or color shifts if not tuned carefully. A representative strategy uses a base-detail decomposition (often via a bilateral or similar filter) to achieve this effect.

Usage and considerations: tonemappers are applied after rendering HDR content in computer graphics pipelines or in image processing workflows. The choice between global and local operators involves trade-offs between speed, realism, and artifact risk. Evaluation typically considers perceptual quality, fidelity of detail, and consistency with the viewing environment.