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Old 15th October 2018, 18:09   #53266  |  Link
Warner306
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Join Date: Dec 2014
Posts: 1,127
Quote:
Originally Posted by mytbyte View Post
I'm not technical enough to understand the BT.2390 paper in full, and can't quite grasp how the rolloff algorithm works (for 200 cd/m2 max brightness on a comp monitor, the 70% I measured as being way too high, 80-100 are flat i.e clipped in the graph (probably depends on MaxL of content and mastering monitor etc). But, everything under 50% (95 nits) is more-less following the PQ curve at all times so there should be no major black crush from MadVR's tone map curve (note: I'm doing HDR->SDR)
Something is lost in translation when going from HDR -> SDR.

If the target nits is higher than the display's actual brightness, you are only getting a ratio of the original curve.

Example: 480 target nits shown at 200 actual nits

480 nits is the lowest value of BT.2390 where the soft knee (where compression begins) is above reference white (100 nits). Lower values will place the soft knee lower and lower and compress at least some of the first 100 nits. The lower the target nits, the more reference white is compressed.

Approximate display brightness for reference white for 480 target nits at 200 actual nits:

4.8 ratio of target nits/reference white — 480/100 = 4.8 — at 200 actual display nits — puts an untouched diffuse white at close to 42 nits — 200/4.8 = 42 nits

In current test builds, if the content is below 480 nits, clipping is selected for as long as possible. This helps improve the brightness of content that is above the soft knee up to the set target nits, and even above the target nits, by eliminating the compression curve and reducing it as much as possible up to the peak brightness of the source.

Approximate brightness of 470 nits actual scene peak at 200 display nits with clipping enabled:

2.4 ratio of target nits/display nits — 480/200 = 2.4 — at 200 actual display nits — puts 470 nits at close to 196 nits — 470/2.4 = 196 nits

Clipping is always predictable because it is the ratio of the target nits/display nits. BT.2390 is only predictable up to the soft knee. After the soft knee, the entire range is compressed and will change depending on the brightness of the scene.

You would actually lose less brightness by outputting in PQ (HDR Output) to an HDR display because the target nits and display nits should match.

The gamma curve mismatch means this scale is not exact, but it is a general guide to actual brightness output.

Last edited by Warner306; 16th October 2018 at 15:48.
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