Specifies the measured peak brightness of the output display, in cd/m^2 (AKA nits). The interpretation of this brightness depends on the configured --target-trc. In all cases, it imposes a limit on the signal values that will be sent to the display. If the source exceeds this brightness level, a tone mapping filter will be inserted. For HLG, it has the additional effect of parametrizing the inverse OOTF, in order to get colorimetrically consistent results with the mastering display. For SDR, or when using an ICC (profile (--icc-profile), setting this to a value above 203 essentially causes the display to be treated as if it were an HDR display in disguise. (See the note below)
When using an SDR transfer function, this is normally not needed, and setting it may lead to very unexpected results. The one time it is useful is if you want to calibrate a HDR display using traditional transfer functions and calibration equipment. In such cases, you can set your HDR display to a high brightness such as 800 cd/m^2, and then calibrate it to a standard curve like gamma2.8. Setting this value to 800 would then instruct mpv to essentially treat it as an HDR display with the given peak. This may be a good alternative in environments where PQ or HLG input to the display is not possible, and makes it possible to use HDR displays with mpv regardless of operating system support for HDMI HDR metadata.
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