2015-04-17, 02:04
HDR to SDR Gamma Compression (Continued...)
What Does Changing the Target Nits Look Like in Action?
I can show you some examples of what changing the display target nits looks like visually in terms of changing the shape of the highlight roll-off and how it alters the contrast of the image.
The sample display is an LED monitor set to SDR output that reaches a peak of approximately 175 real display peak nits.
Note: highlight recovery strength set to high
Avengers - Infinity War: 824 nits frame peak
500 target nits
275 target nits
Here, you can see both the difference in display luminance mapping and the space allocated to the highlight roll-off. The frame is paused so you can clearly see the detail of the bright starburst. A target of 500 display nits visibly lowers the APL of the image, particularly the background elements, but the more gradual roll-off provided by the larger target nits creates additional space to better render the fine detail of the starburst.
At 275 nits, the visible luminance steps are a little too close together at the peak of the roll-off, and as result, some clipping occurs at the display peak where portions of the rays of light lose texture detail, even washing out some of the facial detail of the character in the background.
2001 - A Space Odyssey: 1320 nits frame peak
500 target nits
175 target nits
The second example shows some lost texture detail in a very bright specular reflection. At first glance, the clipping of detail caused by the tone mapping roll-off is more subtle in this example. The frame peaks at 1320 nits, which is far brighter than either of the targets used. If you look at the reflection on the top and bottom of the astronaut's helmet, you can see that it becomes more reflective at 500 nits, like someone applied a buffing cloth to the surface of the helmet. The added depth and contrast provided by the higher target nits allows the small specular reflections to appear more three-dimensional so they appear brighter than the rest of the helmet.
The other hint of clipping caused by tone mapping compression is the bright green hallway light in the background that is blown out at 175 nits and restored at 500 nits.
Mission Impossible - Ghost Protocol: 914 nits frame peak
450 target nits
175 target nits
The third example shows how changing the display target nits can help to shift the image from a flat SDR presentation to more of a high dynamic range presentation. This scene in a dimly-lit garage features a single light source (a beam of bright sunlight) focused on a small portion of the image that peaks at 914 nits. The 1:1 representation of this scene at 175 nits with a neutral reference white makes the scene appear somewhat lifeless and SDR due to some obvious clipping of the specular highlights and the lack of contrast between the bright sunbeams and reference white. This makes the "bad guy" look somewhat less menacing and thoughtful.
Increasing the display target nits to 500 nits provides some noticeable improvement in the visible detail of the background elements, especially the foreign text on the crate in the back of the frame and the cardboard box placed to the left of the crate. But the higher target nits also noticeabley changes the contrast of the image similar to how changing the exposure on a camera is used to capture additional dynamic range.
The 2.86:1 ratio of display target nits to real display peak nits lowers reference white from 100 nits to approximately 35 nits, which causes some shadows to creep in on the face of the bad guy that are likely not inherent to the source footage. But the perceived increase in dynamic range better recreates the intended contrast of the scene by tightly focusing the rays of sunlight on the nearby canvas with less clipping and adding some contrast between the sunbeams and dimly-lit garage. This makes the bad guy appear a little more menacing and better uses the specular highlights as the director intended them to be used.
What Happens If the Source Has a Low Dynamic Range?
Blade Runner 2049: 116 nits frame peak
175 target nits
425 target nits
The intent of the target display value is to preserve HDR contrast. If the source happens to have a low dynamic range, this contrast can be unnecessary and will only serve to create a dark image.
Blade Runner 2049 is a classic example of an HDR movie with a low dynamic range. The majority of the presentation stays well under 200 nits. Here, high target nits simply cause the image to lose its vibrancy and colorfulness with no gain in true contrast. In this example, 425 target nits at 175 actual display nits appears as a mismatch in gamma that leaves the presentation looking dull and dark.
Closely mapping the target nits to the display will better preserve the brightness and color vibrancy of the source. You don't ever want to set this value below the peak brightness of your display because the image will be mapped with incorrect brightness and end up too bright and washed out.
This example also highlights how color accuracy can be distorted by using a high target nits. Tone mapping is designed to desaturate all colors as they lose luminance. The greater the mismatch between the target brightness and actual display brightness, the more oversaturated and dark color tones will appear, as all colors are displayed at a darker luminance than calculated by the tone mapping algorithm.
Summary
Because HDR content is about high dynamic range, it makes the most sense to preserve the HDR contrast of the source as much as possible. Sources with a lower dynamic range can use lower target nits to maintain the source brightness without having a negative impact on perceived contrast or detail. However, sources with a higher dynamic range require a choice between lowering the brightness of the source to represent its high contrast at a lower displayed brightness, or more accurately mapping the source brightness at the expense of clipping some bright detail through range compression.
madVR makes this decision intelligently by changing the display target nits to match the dynamic range of each movie scene. This alters the display target nits as the movie plays to always provide the ideal display target nits for each detected scene change. This eliminates the compromise of catering a curve to a low or high dynamic range by always selecting a display target nits that keeps the display curve in the "sweet spot" where no scene is ever compressed to the point of becoming too dark or too bright where not enough specular highlight detail is retained.
Part III: Dynamic Display Target Nits
What Does Changing the Target Nits Look Like in Action?
I can show you some examples of what changing the display target nits looks like visually in terms of changing the shape of the highlight roll-off and how it alters the contrast of the image.
The sample display is an LED monitor set to SDR output that reaches a peak of approximately 175 real display peak nits.
Note: highlight recovery strength set to high
Avengers - Infinity War: 824 nits frame peak
500 target nits
275 target nits
Here, you can see both the difference in display luminance mapping and the space allocated to the highlight roll-off. The frame is paused so you can clearly see the detail of the bright starburst. A target of 500 display nits visibly lowers the APL of the image, particularly the background elements, but the more gradual roll-off provided by the larger target nits creates additional space to better render the fine detail of the starburst.
At 275 nits, the visible luminance steps are a little too close together at the peak of the roll-off, and as result, some clipping occurs at the display peak where portions of the rays of light lose texture detail, even washing out some of the facial detail of the character in the background.
2001 - A Space Odyssey: 1320 nits frame peak
500 target nits
175 target nits
The second example shows some lost texture detail in a very bright specular reflection. At first glance, the clipping of detail caused by the tone mapping roll-off is more subtle in this example. The frame peaks at 1320 nits, which is far brighter than either of the targets used. If you look at the reflection on the top and bottom of the astronaut's helmet, you can see that it becomes more reflective at 500 nits, like someone applied a buffing cloth to the surface of the helmet. The added depth and contrast provided by the higher target nits allows the small specular reflections to appear more three-dimensional so they appear brighter than the rest of the helmet.
The other hint of clipping caused by tone mapping compression is the bright green hallway light in the background that is blown out at 175 nits and restored at 500 nits.
Mission Impossible - Ghost Protocol: 914 nits frame peak
450 target nits
175 target nits
The third example shows how changing the display target nits can help to shift the image from a flat SDR presentation to more of a high dynamic range presentation. This scene in a dimly-lit garage features a single light source (a beam of bright sunlight) focused on a small portion of the image that peaks at 914 nits. The 1:1 representation of this scene at 175 nits with a neutral reference white makes the scene appear somewhat lifeless and SDR due to some obvious clipping of the specular highlights and the lack of contrast between the bright sunbeams and reference white. This makes the "bad guy" look somewhat less menacing and thoughtful.
Increasing the display target nits to 500 nits provides some noticeable improvement in the visible detail of the background elements, especially the foreign text on the crate in the back of the frame and the cardboard box placed to the left of the crate. But the higher target nits also noticeabley changes the contrast of the image similar to how changing the exposure on a camera is used to capture additional dynamic range.
The 2.86:1 ratio of display target nits to real display peak nits lowers reference white from 100 nits to approximately 35 nits, which causes some shadows to creep in on the face of the bad guy that are likely not inherent to the source footage. But the perceived increase in dynamic range better recreates the intended contrast of the scene by tightly focusing the rays of sunlight on the nearby canvas with less clipping and adding some contrast between the sunbeams and dimly-lit garage. This makes the bad guy appear a little more menacing and better uses the specular highlights as the director intended them to be used.
What Happens If the Source Has a Low Dynamic Range?
Blade Runner 2049: 116 nits frame peak
175 target nits
425 target nits
The intent of the target display value is to preserve HDR contrast. If the source happens to have a low dynamic range, this contrast can be unnecessary and will only serve to create a dark image.
Blade Runner 2049 is a classic example of an HDR movie with a low dynamic range. The majority of the presentation stays well under 200 nits. Here, high target nits simply cause the image to lose its vibrancy and colorfulness with no gain in true contrast. In this example, 425 target nits at 175 actual display nits appears as a mismatch in gamma that leaves the presentation looking dull and dark.
Closely mapping the target nits to the display will better preserve the brightness and color vibrancy of the source. You don't ever want to set this value below the peak brightness of your display because the image will be mapped with incorrect brightness and end up too bright and washed out.
This example also highlights how color accuracy can be distorted by using a high target nits. Tone mapping is designed to desaturate all colors as they lose luminance. The greater the mismatch between the target brightness and actual display brightness, the more oversaturated and dark color tones will appear, as all colors are displayed at a darker luminance than calculated by the tone mapping algorithm.
Summary
Because HDR content is about high dynamic range, it makes the most sense to preserve the HDR contrast of the source as much as possible. Sources with a lower dynamic range can use lower target nits to maintain the source brightness without having a negative impact on perceived contrast or detail. However, sources with a higher dynamic range require a choice between lowering the brightness of the source to represent its high contrast at a lower displayed brightness, or more accurately mapping the source brightness at the expense of clipping some bright detail through range compression.
madVR makes this decision intelligently by changing the display target nits to match the dynamic range of each movie scene. This alters the display target nits as the movie plays to always provide the ideal display target nits for each detected scene change. This eliminates the compromise of catering a curve to a low or high dynamic range by always selecting a display target nits that keeps the display curve in the "sweet spot" where no scene is ever compressed to the point of becoming too dark or too bright where not enough specular highlight detail is retained.
Part III: Dynamic Display Target Nits
