High Dynamic Range Video

高动态范围视频 (High Dynamic Range Video, HDR Video) is a class of video systems that expand the range of brightness that can be encoded and displayed, with the goal of presenting brighter highlights and reflections while preserving shadow detail. ITU-R BT.2100 specifies two main pathways for television production and program exchange—Perceptual Quantization (PQ) and Hybrid Log-Gamma (HLG)—which are used in conjunction with wide color gamut and higher bit depths.

HDR is not simply a matter of brightening the entire image. During production, decisions must be made regarding how scene brightness is mapped to the signal, what level the diffuse white should be set to, how high the highlights can go, and how to compress the signal when the target display lacks sufficient capability. A wider brightness range also expands the presentable color volume, as higher brightness allows for the display of more saturated colors. PQ is based on absolute display brightness, with a curve covering a wide brightness range; HLG adapts to different environments by relative scene signal and display system gamma. Consumer formats such as HDR10, HDR10+, and Dolby Vision are primarily based on PQ, though they differ in metadata and compatibility rules; HLG is commonly used in broadcasting and live production. HDR is an umbrella term and cannot be used as a substitute for the names of these specific formats.

Consumer HDR typically uses at least 10-bit quantization to mitigate banding artifacts in wide gradients, but neither the 10-bit, BT.2020, nor 4K labels alone can prove that a video is HDR. Conversely, BT.2100 allows for 1920×1080 HDR; HDR does not necessarily require 4K. Resolution, dynamic range, color gamut, frame rate, and encoding format should be documented separately. Tone mapping occurs when a display cannot reach the master’s peak brightness; different televisions may produce different results based on static metadata, dynamic metadata, or their own analysis. The peak brightness claimed on packaging, the capabilities of the master monitor, and the actual pixel brightness in the program are not the same thing. HDR labels indicate the signal system but do not constitute a guarantee of image quality or the source of the master.