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Re: HDR question

Re: HDR question


JD Houston
 

On Mar 17, 2018, at 3:59 AM, alister@... wrote:

So if a monitor can manage 8 stops and the scene is 8 stops and if the brightness of the monitor can match the brightness of the scene, in the same environment both will look the same. So to say that monitor contrast and scene contrast are different things is not correct. It is the viewing environments that are different and it is the difference in the viewing environment that changes the perception of the image, not differences in the contrast ratios of monitors. Unless I’m greatly mistaken and my whole knowledge of contrast and brightness is flawed.
 
The first line is correct.  If you have a monitor that can show up to 10,000 nits (the brightness of a piece of paper in the Sun) so that you can match the brightness of daylight
and you are also outside looking at a monitor, it will look the same up to the point that the monitor can show  (you would need a monitor of 1000000 nits to match the Sun and
direct Sun glints.   Vision is a log thing)
 
But I would dare to say that ALL monitor viewing is currently done in environments that are not a direct match to the original scene.
This is as true in dark scenes where the eye is dark adapted and the surround is also dark.
 
So in practical terms,  the scene contrast and the display contrast are almost never the same. Yes, you are correct that
the difference is because of viewing environment.
 
In most video and film, the contrast build-up in a display is systematically designed into the system.  This is what makes the ratio’s 
different.
 
In cameras, the ‘taking curve’ has an assumption (say Rec709 curve) and the output curve of a display
has a buildup assumption (Rec.1886’s gamma 2.4 for example),  this gives the video system an effective
contrast boost of about 9%  (2.4/2.2)
 
So by design, comparing scene contrast ratios and output display contrast ratios will not give you the same
number.  To simplify the example,  if you have a scene contrast of 100:1, and you want to show it in a 
dark environment, you need a 150:1 to accurately show it.  If you are showing it in a video dim environment,
you would need an output contrast of 109:1 to show it.  (As an aside, the overall gain of 1.2 that is sometimes 
used brings in a complicating factor of audience preference for a certain style of reproduction.  Audiences prefer
boosted contrast looks over reality matches)
 
There is one other consideration.  Most scene contrasts are described in terms of simultaneous contrast — the dark to light ratio
including everything in the scene at the same time (fill, flare, etc).  Most display contrasts are described as 
sequential contrasts — the ratio of a full on white to a full-on black.  This makes the displays seem to have a higher contrast.
But it is not true.    It is especially a problem with projectors because the projection lens can have a drastic effect
on the display contrast.   But even OLED monitors can have systematic issues in displaying images with the original scene contrast.
This is all a reason to never use display contrast as a way to evaluate scenes.
 
From an operator standpoint,  the ratios to use are based on the light in the scene.  So targeting key to fill ratios
of a certain number is the right way to do it.  You are mixing light and light is linear but it can be expressed as
ratios — photographically almost everything is a ratio (e.g.  this light is twice as bright as that light, this exposure is
half as much as the previous one )
 
Why does an operator need to know about contrast ratios and the effects of picture rendering.   Because in the world of HDR, it now 
matters which target you are going for.  If you are never going into a movie theater, then you don’t have to maintain the same limits
for where you would want Zone 1 or Zone 10 detail (to use that as a metaphor).
 
For a project going onto full range PQ, you may want tonal separations of as much as 1.5 million (20.55 stops)  Of course current cameras
aren’t really there yet, so you might have a little time before you have to worry about that.  But even today’s cameras with about 14+ stops
requires decisions about where to place highlight and shadow details that are going to reproduce cleanly on certain types of monitors.
For most productions, the least common denominator applies (which is LCDs).  
 
Because of that, it is important to know that Cinema Output Ratios for a Dark Surround, are different that Video Output Ratios for a Dim or Average Surround).
Dark needs a 1.5 boost.  Dim needs a 1.2 Boost.  So when you consider what you are trying to achieve, it helps if you know
what your audience will be looking at.
 
Yes, it is a colorist problem to fix in the end — it is what you see is what you get.  But knowing the choices that will be faced
improves the usefulness of the source material, so is good for DPs to know.
 
 

Jim Houston
Consultant, Starwatcher Digital, Pasadena, CA

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