Time to define an alternative to ANSI contrast? - Page 2

Doesn't a Kodak grey card which is 18% represent the average light and color an average image returns to a camera? Would that be a good starting point for an APL ANSI test?

Gary

No, an 18% luminance card is half gray. It's 18% luminance, or 50% gray. It's actually 50% APL because it's a full field of 50% gray. I think most scenes are a lot lower than 50% APL.

If I go from a Ruby with DI to a native 15,000:1 pj (RS1), it seems that light reflection control in the room would become even more important with the RS1. I think this would be the case because the dynamic range will be much greater.

For instance, with a star field and a bright object - before the the Ruby the bright object may have only been half as bright as it should have been. Now with the RS1 the bright object will be much brighter, but the darks will be similar as with the Ruby. So in that case it seems logically then that there is much more light causing reflections in medium to low APL scenes and therefore would have a greater impact on the ANSI CR.

Is this line of thinking correct?

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L*50 = 18%

L* 20 = 2.9%

To be nit-picky, an 18% card shouldn't be called "50% gray" because that's a mathmatical expression. To the average eye, a gray swatch that reflects 18% of the light incident upon it is considered "medium gray", but that is purely a psychological response. (Yeah I know, another word for "medium" is "50%", but "medium" indicates a lack a precision that is more indicative of our less than perfect understanding of "typical human perception".)

When you video record a 18% card, you will get an APL of approximately 42% depending on the gamma of the video camera (the 2.2 gamma of REC709 actually works out to almost 2.0 exactly when the specified tail is taken into effect.)

And finally when you show the 42% APL signal on a monitor it will likely output a luminous of less than 18%. (Typically 11% or so for a CRT due to the higher gamma of the display)

All of this is why I strongly recommend that any ANSI-esque pattern developed should be a mixture of white and black to eliminate the role of display gamma in the measurement.

-Mr. Wigggles

Sure, I probably should have said 50% lightness=18% luminance. I was loosely assuming that when I said "50% gray" that it was perceptual lightness rather than physical luminance. Sorry if that was unclear.

http://www.brucelindbloom.com/index....dCalcHelp.html

FWIW, I have done research in regard to the limits of the CIE perceptual model, i.e. LDR 8-bit images, and I've found evidence presented in the form of a master thesis that states " L* value of '8' is the effective break point where this model breaks down". Corresponding to max contrast ratio of 112 or a density 2.05.

Let's try another example, but taking the room into account this time. Let's suppose the room limits ANSI CR on the screen to about 200:1, so the reflectivity value is 0.01.

According to the calculator, a projector with 4000:1 on/off CR and 250:1 ANSI CR would produce 884:1 CR for that pattern.

If we boost the on/off CR to 8000:1 but keep the 250:1 ANSI CR, the pattern's CR would be 982:1, which is an increase of 11%.

Or, instead, if we boost the ANSI CR to 850:1 but keep the 4000:1 on/off CR, the pattern's CR would be 1183:1, which is an increase of 34%.

The 34% increase is about 3 times as much as the 11% increase. Basically, the ANSI CR boost would improve the pattern's CR more than the on/off CR boost would, even though the pattern has a low APL, and even though the room is reflective.

Just for grins, if we boost both at the same time (on/off CR to 8000:1 and ANSI CR to 850:1), the pattern's CR would be 1366:1, which is an increase of 55%.

In addition, even if we boost the on/off CR to infinity but keep the 250:1 ANSI CR, the pattern's CR would be 1105:1, which is still less than what the ANSI CR boost produces.

What are you concluding from this?

It shows that ANSI CR matters, possibly more than on/off CR matters. Even if the scene has a low APL, ANSI CR matters, possibly more than on/off CR matters. Even if the room is reflective, ANSI CR matters, possibly more than on/off matters.

I don't think that conclusion follows from your example. The choice to go to an ANSI increase more than 2x to an unrealistic ANSI CR measure of 850:1 is dubious.

Instead, if you double ANSI (as you doubled on/off CR previously) in this particular example, you end up with a CR of 1077, which is a 21% increase which is still twice the 11% with a coublind of on/off CR in this particular example.

Greg Rogers recently measured 845:1 modified ANSI CR on the Sharp 20K. If 845:1 is realistic, then isn't 850:1 realistic also?

Anyway, I'm not quite sure what you are taking issue with. Are you saying it is wrong to conclude that ANSI CR can matter more than on/off CR in my example?

True, and doubling ANSI CR to 500:1 would still be better than going to 37,000:1 on/off CR instead in this example.

I was not aware of that. What I was taking issue with at first was the assertion of the supremacy of ANSI over on/off CR. I was going to write a long response but I got distracted with other things, and I don't really take issue with what you said anyway. I don't really have any issues with what you said, I just am not convinced that the particular example is fully convincing, but I don't necessarily disagree either. Of maybe to put it another way: I don't really know what I meant.

I had appended the following sentence to my post to try to make it more convincing.

In other words, boosting ANSI CR from 250:1 to 850:1 is better than boosting on/off CR from 4000:1 to any amount, even to infinity, in this example. Is that convincing enough? I'm not sure if I can make it more convincing than that.

I think Erik was just saying that it ANSI CR can be more important even in an image that is closer to absolute black than the ANSI CR pattern, but understands that in other cases on/off CR would be more important than ANSI CR. Even with the same pattern. As we both know, either one can be the weak link. Start with very low on/off CR and okay ANSI CR and the on/off CR is that one that basically needs improvement.

And it can be even more complicated than the numbers since improving the places where a projector is weak is usually more important than improving the places where it is strong, even if the percentage improvements are the same. That is one of the things I tried to emphasis in this section of my article on CR:

http://www.hometheaterhifi.com/volum...06-part-5.html

--Darin

Regarding this particular example (the test pattern and the room), we can determine which spec is the weaker link (on/off or ANSI) by comparing them to each other as follows. If the on/off is greater than 9 times the ANSI, then ANSI is the weaker link, in the sense that doubling ANSI would improve the pattern's CR more than doubling on/off would. Or, if the on/off is less than 9 times the ANSI, then on/off is the weaker link, in the sense that doubling on/off would improve the pattern's CR more than doubling ANSI would.

For example, if the projector has 4000:1 on/off and 300:1 ANSI, then ANSI is the weaker link, since 4000 > 9 * 300. Or, if the projector has 3000:1 on/off and 400:1 ANSI, then on/off is the weaker link, since 3000 < 9 * 400.

Furthermore, if the on/off is greater than 18 times the ANSI, then ANSI is so much the weaker link that doubling ANSI would improve the pattern's CR more than going to infinite on/off would. For example, if the projector has 4000:1 on/off and 200:1 ANSI, then doubling ANSI would improve the pattern's CR more than going to infinite on/off, since 4000 > 18 * 200.

You shouldn't limit yourself to thinking only about squares. One of the reasons that the old test patterns were checkerboards was because it was easier to implement the signal generator hardware. But since these patterns are probably going to be photoshopped and then encoded, we are no longer limited to what shapes are easy to implement in hardware.

Not that I have any better ideas than using squares, I just want to make sure we don't limit ourselves unnecessarily.

In other words, don't be a square!

I found an archived thread that discussed it, but it's a short thread.

Instantaneous CR from on/off CR and ANSI CR

Once you have the test patterns perhaps you can hook up with GetGray to have the patterns included on his calibration disc.

Sounds good. One thing that will be interesting is that as we experiment with these patterns we can measure our pjs and share our results here to get a sense for how different pjs are performing. This should also tell us if the patterns seem to be a good test indicative of each pjs capabilities with regards to ANSI CR.

One thing that is very interesting to see using the Contrast Calculator is how much on/off CR plays a role for ANSI CR at lower APLs.

I'd like to make sure I am understanding something correctly about this though...

In the table of results in the calculator it shows the various % of white. Which one of these values is most representative of what we actually see in most typical scenes?

Its been said here that the typical APL in scenes is about 20-ish. Does this mean that the results at 10, 20% and 50% are more important that what it shows at 100%?

Also on somewhat of a different note - is it a safe assumption to say that generally speaking if you have a pj with very good ANSI CR and light/reflection control in the room that you'd want to aim for a gamma of about 1.5 (as compared to a poorer ANSI CR pj and/or poor light conditions where you may want to aim for 2.2)?

In another thread, I calculated the APL of a scene from the movie Gladiator to be around 15%.

To calculate the contrast of this scene approximately, enter these values:
2.5 gamma
5% screen area covered by the checkerboard
90% checkerboard stimulus

Then enter your own values for on/off CR, ANSI CR, and room gain.

1.5 is way too low for a display gamma. Did you mean 2.5?