AVS APL Study - Adjunct to AVS Contrast Project - Page 3 - AVS Forum
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post #61 of 65 Old 07-21-2008, 01:49 PM - Thread Starter
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Quote:
Originally Posted by tstites View Post

Seems to me that trying to arrive at some sort of function to evaluate how a unit will perform in a real environment is not going to be practical without modeling the entire visual environment.

FWIW, that wasn't my intent with this (and I doubt it was Mark's with his thread). The purpose was to gain a greater understanding of where real world content falls in terms of ADL. Since we've been gaining a greater understanding of how projectors perform across the Luminance range (thanks to Mark's efforts), it seemed appropriate that we should try to better understand where real world content falls on that "curve".

Oh, and I'll be off for a drive as soon as I tweak my front fender so I can turn again

See what an anamorphoscopic lens can do, see movies the way they were meant to be seen
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post #62 of 65 Old 07-22-2008, 09:10 AM
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Originally Posted by tstites View Post

Mark,

I also do a lot of Dual Sport riding...have an '06 DRZ400S...some great trails and forest service roads all over N.Ga, E.Tn and W.Nc.

Fun Most of the riding out this way is desert and mountains. If you're out this way in the winter time drop me a line and I can show you some really interesting desert rides.

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No, I don't think it's that far out there, just that with front projectors, they are part of a "system" that consists of the projector, screen and the room. Unless you are in a totally light absorbtive room, every room will give you a different response in terms measureable or perceived CR vs APL. The point at which a higher ANSI CR rating becomes imperceptible, and therefore irrelevant, in various rooms is going to be all over the map.

Seems to me that trying to arrive at some sort of function to evaluate how a unit will perform in a real environment is not going to be practical without modeling the entire visual environment. Not at all unlike the complexity of the acoustic modeling I used to do when designing commercial sound reinforcement systems for theaters and such.

It's very true that room effects play a huge role. When I first started the AVS contrast project the idea was to have two parts. The first part would measure projector performance and the second part would use screen measurements to see how various rooms also played a role. Ric (Lovingdvd) had started a thread along those lines where many people measured ANSI at the screen and some experimented with different room treatments to see how it affected their ANSI readings. Unfortunately the whole idea was a little too grandiose and it ended up being about a full time job just to capture some insights into projector performance.

Even still though I think both of these threads (the original thread and this adjunct) provide some useful information. As an example, most people agree that the room plays a huge role in final (screen) ANSI performance and less of a role in on/off. But what does this mean to the majority of scenes that people watch in their HT? Is the luminance generated from a ANSI pattern typical of most scenes or is it rare? What's the average luminance for typical movies and in dark scenes what is the norm? That's what this thread discussion is about. Armed with that information it's easier to relate projector performance and also room effects.

It's an interesting state of affairs that we have two measures of contrast (on/off and ANSI) and neither are particularly good at measuring contrast in the range of luminance for the vast majority of images that most people use. As stranger89 has mentioned these two threads help supply some information about the in between range. I realize that this sort of discussion isn't for everyone though
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post #63 of 65 Old 08-21-2008, 09:23 AM
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Isn't there a big fat problem in your AVIsynth script?

Shouldn't you be using the Decomb filter (telecide and decimate) to turn the interlaced footage back into the progressive frames with a real 24FPS frame rate? Also, why are you sampling only once per second, and cropping of the black bars? Unless you're doing this for a CIH setup the black bars are part of the image that the projectors use for DI calculations.
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post #64 of 65 Old 08-17-2009, 10:50 AM
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Ok, this is a few years late, but ...

APL, as the term is used by a video engineer, stands for average picture level, and refers to the mean of the Y' component (luma) component across the image area of a frame (or sequence, or movie, or whatever).

Average luminance - more correctly, average relative luminance - is the mean of the true CIE linear-light luminance across the image area of a frame or sequence, relative to reference white luminance.

The math that relates the two isn't simple, and it can't be done at all absent all three components R', G', and B' or Y', CB, and CR. Apart from the special case where R', G', and B' are all equal to either zero or one - notably, 100% colour bars - APL and average luminance differ, and sometimes dramatically.

At a display, video R', G', and B' signals (for our purposes, scaled to 0=reference black and 1=reference white) are each raised to a power of between 2.0 and 2.5 to yield linear-light RGB components that are directly related to luminance. That act maps 0.5 on the video scale to about 0.18 in relative luminance.

So: Don't mix up the two!

The APL of broadcast video is very roughly 50%; its average luminance is very roughly 18%.

The average luminance of a movie as presented in the cinema - and not necessarily a dark movie - is roughly 10%. I have this on extremely good authority, from a studio/DI guy who has access to DCI movie data and ran some perl scripts. (Sorry, I know that's gross, but he's an old awk/sed/grep kind of guy.)

The average luminance of a movie as transferred to DVD or Blu-ray is up for discussion, but I'd make a guess somewhere between 10% and 18%, say 14%.

- Charles

Charles Poynton
www.poynton.com
[author of "Digital Video and HD Algorithms and Interfaces," Second edition (2012) - if that sounds scary to you, don't read it!]

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post #65 of 65 Old 08-17-2009, 11:19 AM
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Hi Charles,

Just want to say thanks for the information and nice to see you post. I talked to you for a while a couple of months ago about some different things (like how things are setup for mastering) and appreciate your time.

As far as this:
Quote:
Originally Posted by poynton View Post

The average luminance of a movie as transferred to DVD or Blu-ray is up for discussion, but I'd make a guess somewhere between 10% and 18%, say 14%.

I've been wondering whether any of the stuff that is ending up on Blu-ray has had all of its gamma related stuff (not counting bit depth changes or color gamut type changes) done for DCI and then not get adjusted for Blu-ray. That is, aimed more for 2.6 display gamma (you can correct me if I have that wrong for DCI), and this could impact the average luminance. It seems to me that if DCI content is converted to Blu-ray in a way that takes the different gammas into account then the average luminances should end up in the same ballpark on playback, although I haven't thought about it too deeply.

--Darin

This is the AV Science Forum. Please don't be gullible and please do remember the saying, "Fool me once, shame on you. Fool me twice, shame on me."
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