2015-07-30, 20:54
Why Use HDR Tone Mapping for Your Display?
Most current true HDR display panels with at least 500 nits or more of peak luminance do a reliable job of following the PQ EOTF 1:1 up to PQ reference white (100 nits) and can provide accurate source luminance tracking for the majority of the source values — It is an unknown fact that 90% or more of PQ HDR video is encoded below 100 nits. Where most HDR10 displays struggle is with the trade-offs involved in handling the smaller number of brighter source values that are mastered with brightness levels that are well above the capabilities of the display. This is mostly a consequence of the static nature of the open-source HDR10 standard that provides the display with a single HDR10 metadata value to summarize the mastered peak nits of the entire video runtime — The Maximum Content Light Level (MaxCLL), or if the MaxCLL is missing, the mastering display maximum luminance.
Many HDR displays will assume the brightest source pixels captured by the source metadata only represent a limited portion of the overall picture or a limited timeframe of the full movie and will choose to use a tone curve that clips only the brightest specular highlights in many scenes. For most displays, these are highlights mastered above 1,000 peak nits, but some HDR displays may clip highlight detail as early as 600-700 nits. This keeps the overall Average Picture Level (APL) consistently brighter, but loses all specular highlight detail mastered above the display's clipping point.
Other HDR displays will attempt to retain all specular highlight detail in the source file and will use a harsh tone curve that captures the full MaxCLL by setting a lower roll-off point on the display curve that darkens all scenes with frame peaks that are well below this MaxCLL. This presents much of the movie as darker than intended with distorted contrast and less optimized tone mapping for dim and mid-bright scenes.
By selecting tone map HDR with pixel shaders and checking output video in HDR format, madVR will compress any source levels that are mastered above the real display peak nits entered in madVR and send all compressed HDR PQ source values to the display. As the source levels rise above the real display peak nits, madVR smoothly curves them back into the display range. For any HDR display 480 nits or brighter, this tone mapping is only applied to the specular highlights mastered above 100 nits. The HDR metadata sent to the display to trigger its HDR mode is also altered to reflect the source peak after tone mapping, which is matched to the provided real display peak nits.
Compressing all source levels to match the real display peak nits benefits the display in two ways. It keeps all specular highlight detail within the display range at all times without any highlight clipping while not impacting most of the remaining image. It is also prevents the display from choosing a harsh tone curve for titles with high MaxCLLs, such as 4,000 nits or 10,000 nits, by reporting a lower source maximum frame peak luminance to the display. Tone mapping is applied dynamically for each movie scene, so only specular highlights with levels above the entered real display peak nits are compressed back into range and the rest of the image remains identical to HDR passthrough. The ability to compress HDR highlights too bright for the display is very similar to the HDR Optimiser found on the Panasonic UB820/UB9000 Blu-ray players.
Spears & Munsil UHD HDR Benchmark 10,000 nits MaxCLL Demo Footage (HDR10 Passthrough)
LG C8's Static Tone Mapping:
LG C8's Static Tone Mapping + madVR's Dynamic Tone Mapping:
LG C8's Static Tone Mapping:
LG C8's Static Tone Mapping + madVR's Dynamic Tone Mapping:
LG C8's Dynamic Tone Mapping:
LG C8's Dynamic Tone Mapping + madVR's Dynamic Tone Mapping:
Pixel shaders with HDR output checked can benefit most HDR displays by assisting the display's own internal tone mapping. The benefits of this tone mapping somewhat depend on how the display handles the altered static metadata provided by madVR (lowered MaxCLL and mastering display maximum luminance) that is often used to select which static tone curve is used by the display.
Some HDR displays will not recognize that the compressed source fits within the display's peak brightness and will apply an additional tone map where a second tone curve is used to further compress the source values to some extent. This double tone map effect could make madVR's dynamic tone mapping less beneficial if the primary display curve used is overly harsh. Pixel shaders also cannot correct displays that do not follow the PQ curve, like those that artificially boost the brightness of HDR content or those with poor EOTF tracking that crush shadow detail. Displays with a dynamic tone mapping setting don’t typically use the static metadata and should ignore the metadata in favor of simply reading the RGB source values sent by madVR.
Determining the usefulness of madVR's dynamic PQ tone mapping can require some experimentation with the real display peak nits and movie scenes with many bright specular highlights. The brightest HDR movies mastered above 1,000 peak nits tend to be the titles that most often lose some detail in the specular highlights with the default display tone curve. A good way to test the impact of the source rescaling is to create two hdr profiles mapped to keyboard shortcuts in madVR that can be toggled during playback: one set to passthrough HDR to display and the other pixel shaders with HDR output. To trigger the altered HDR metadata sent by madVR after switching to pixel shaders, uncheck and recheck the output video in HDR format checkbox in the control panel during playback.
Further Reading:
HDR10 Tone Mapping Explained
HDR to HDR: Advantages and Disadvantages
Static vs. Dynamic Tone Mapping
Most current true HDR display panels with at least 500 nits or more of peak luminance do a reliable job of following the PQ EOTF 1:1 up to PQ reference white (100 nits) and can provide accurate source luminance tracking for the majority of the source values — It is an unknown fact that 90% or more of PQ HDR video is encoded below 100 nits. Where most HDR10 displays struggle is with the trade-offs involved in handling the smaller number of brighter source values that are mastered with brightness levels that are well above the capabilities of the display. This is mostly a consequence of the static nature of the open-source HDR10 standard that provides the display with a single HDR10 metadata value to summarize the mastered peak nits of the entire video runtime — The Maximum Content Light Level (MaxCLL), or if the MaxCLL is missing, the mastering display maximum luminance.
Many HDR displays will assume the brightest source pixels captured by the source metadata only represent a limited portion of the overall picture or a limited timeframe of the full movie and will choose to use a tone curve that clips only the brightest specular highlights in many scenes. For most displays, these are highlights mastered above 1,000 peak nits, but some HDR displays may clip highlight detail as early as 600-700 nits. This keeps the overall Average Picture Level (APL) consistently brighter, but loses all specular highlight detail mastered above the display's clipping point.
Other HDR displays will attempt to retain all specular highlight detail in the source file and will use a harsh tone curve that captures the full MaxCLL by setting a lower roll-off point on the display curve that darkens all scenes with frame peaks that are well below this MaxCLL. This presents much of the movie as darker than intended with distorted contrast and less optimized tone mapping for dim and mid-bright scenes.
By selecting tone map HDR with pixel shaders and checking output video in HDR format, madVR will compress any source levels that are mastered above the real display peak nits entered in madVR and send all compressed HDR PQ source values to the display. As the source levels rise above the real display peak nits, madVR smoothly curves them back into the display range. For any HDR display 480 nits or brighter, this tone mapping is only applied to the specular highlights mastered above 100 nits. The HDR metadata sent to the display to trigger its HDR mode is also altered to reflect the source peak after tone mapping, which is matched to the provided real display peak nits.
Compressing all source levels to match the real display peak nits benefits the display in two ways. It keeps all specular highlight detail within the display range at all times without any highlight clipping while not impacting most of the remaining image. It is also prevents the display from choosing a harsh tone curve for titles with high MaxCLLs, such as 4,000 nits or 10,000 nits, by reporting a lower source maximum frame peak luminance to the display. Tone mapping is applied dynamically for each movie scene, so only specular highlights with levels above the entered real display peak nits are compressed back into range and the rest of the image remains identical to HDR passthrough. The ability to compress HDR highlights too bright for the display is very similar to the HDR Optimiser found on the Panasonic UB820/UB9000 Blu-ray players.
Spears & Munsil UHD HDR Benchmark 10,000 nits MaxCLL Demo Footage (HDR10 Passthrough)
LG C8's Static Tone Mapping:
LG C8's Static Tone Mapping + madVR's Dynamic Tone Mapping:
LG C8's Static Tone Mapping:
LG C8's Static Tone Mapping + madVR's Dynamic Tone Mapping:
LG C8's Dynamic Tone Mapping:
LG C8's Dynamic Tone Mapping + madVR's Dynamic Tone Mapping:
Pixel shaders with HDR output checked can benefit most HDR displays by assisting the display's own internal tone mapping. The benefits of this tone mapping somewhat depend on how the display handles the altered static metadata provided by madVR (lowered MaxCLL and mastering display maximum luminance) that is often used to select which static tone curve is used by the display.
Some HDR displays will not recognize that the compressed source fits within the display's peak brightness and will apply an additional tone map where a second tone curve is used to further compress the source values to some extent. This double tone map effect could make madVR's dynamic tone mapping less beneficial if the primary display curve used is overly harsh. Pixel shaders also cannot correct displays that do not follow the PQ curve, like those that artificially boost the brightness of HDR content or those with poor EOTF tracking that crush shadow detail. Displays with a dynamic tone mapping setting don’t typically use the static metadata and should ignore the metadata in favor of simply reading the RGB source values sent by madVR.
Determining the usefulness of madVR's dynamic PQ tone mapping can require some experimentation with the real display peak nits and movie scenes with many bright specular highlights. The brightest HDR movies mastered above 1,000 peak nits tend to be the titles that most often lose some detail in the specular highlights with the default display tone curve. A good way to test the impact of the source rescaling is to create two hdr profiles mapped to keyboard shortcuts in madVR that can be toggled during playback: one set to passthrough HDR to display and the other pixel shaders with HDR output. To trigger the altered HDR metadata sent by madVR after switching to pixel shaders, uncheck and recheck the output video in HDR format checkbox in the control panel during playback.
Further Reading:
HDR10 Tone Mapping Explained
HDR to HDR: Advantages and Disadvantages
Static vs. Dynamic Tone Mapping
