18.12 HDR TLDR🔗⧉

• Scenes routinely span 100,000:1, about 17 stops, without the sun in the frame; a large sensor at base ISO records 12 to 14 stops to a noise floor of SNR 1, fewer at the 10 or 20 dB a photograph needs, and even values below the clipping point can be clipped by electronic noise (Chapter 17.1, Dynamic range and multiple-exposure capture).
• Test a single raw first. Add a bracket when it supplies important unclipped highlights or cleaner shadows. Keep aperture and focus fixed; varying shutter also varies motion blur (Bracketing, and which parameter to vary).
• With raw files, calibration is exposure ratios; with rendered files, recover the response curve first (Chapter 17.2, Camera response and radiance calibration).
• Register and calibrate before merging. Weight in radiance units, mask invalid samples and flag regions unsupported by every exposure. All-clipped pixels supply bounds, not recovered highlight detail (Chapter 17.3, Merging exposures).
• A burst of short, identical exposures merged on a phone reduces noise and motion; in bright light a single image may be enough. It does not raise the per-frame clipping ceiling (Chapter 17.6, HDR+ and burst imaging on phones).
• Store radiance maps in half-float OpenEXR or floating-point DNG; deliver photographs as gain maps and video in PQ or HLG (Chapter 17.7, Encoding HDR images).
• Tone mapping has three goals, range compression, appearance and look; say which one you serve before judging a result (Part 18, Tone mapping and display).
• A reproduction needs a system gamma of about 1.2 to 1.5, for surround and absolute level, a toe for flare and a chroma boost for the Hunt effect; the effects do not multiply (Chapter 18.1, Tone mapping and perception).
• Start with a deliberate exposure and a global curve, in log luminance; a single curve cannot keep local contrast while compressing range (Chapter 18.2, Global tone mapping).
• For local control, compress an edge-aware base and keep the detail, or use a local Laplacian filter; check halos, noise in lifted shadows and the overprocessed look (Chapter 18.3, Two-scale and bilateral tone mapping, Chapter 18.5, Local Laplacian filtering and other multiscale methods).
• Exposure fusion skips the radiance map and is often the right tool for a finished photograph Mertens et al. (exposure fusion) (Chapter 18.4, Exposure fusion).
• A rendering depends on the display and the room; display-adaptive mapping and HDR display mapping make that explicit (Chapter 18.7, Tone mapping as perceptual optimization, Chapter 18.10, HDR displays and display mapping).
• Hue and chroma through the compression are part of the problem; per-channel curves skew hues toward the primaries (Chapter 18.8, Display rendering transforms and color).
• Evaluate against a stated goal, on a stated display, with people when claiming superiority (Chapter 18.11, Evaluating tone mapping).