[Warner306]

How the Dynamic Display Target Nits Works

INTRO

Scene Peak <= real display peak nits:
No tone mapping is required, so the PQ luminance values are output 1:1 with the real display peak nits.

Scene Peak > real display peak nits
Low Frame FALL:
When a scene has a high scene peak brightness but a low frame FALL, a small increase is made to the display target nits to restore some specular highlight detail without compressing the scene to the point of losing visible shadow detail near black.

Scene Peak > real display peak nits
High Frame FALL:
When a scene has a high scene peak brightness and a high frame FALL, a larger increase is made to the display target nits to capture even greater specular highlight detail and aggressively compress any pixels near the target display peak white that might otherwise clip at the top of the display range and wash out the scene contrast.

FINISHING STEP HISTOGRAM SHAPED TONE MAPPING

The display target nits is selected at a ratio of 2x the scene FALL:

Active Display Target Nits = Max(100, min target / real display peak nits) + [2 x scene FALL × (dynamic tuning% (0-100) / 50)]

Sidenote: Definition of a Movie Scene (% change in the brightness histogram)
madVR detects movie scenes using a brightness histogram that is capable of measuring the frame peak brightness, average peak brightness and pixel distribution in the lower, middle and upper ranges of each frame. This information is used to determine when a peak in the histogram combined with a change in frame peak brightness can be considered a change in scene. Without the benefit of dynamic HDR10+ or Dolby Vision metadata, these decisions must be made in real-time by predicting significant changes in the distribution of the average brightness in the image.

A movie scene as described by dynamic tone mapping is denoted on the basis of having a visible change in APL rather than having a distinct beginning and end. By this definition, scene changes can be frequent. For instance, a muzzle flash from a gun would not be considered a scene change, but a conversation between two people that starts in front of a bright window and moves continuously into another room would be considered a visible change in APL that could benefit from a change in display target nits. Changes to the display target nits occur often enough to keep scene and corresponding APL changes smooth and largely invisible to the viewer.