HOW-TO: Calibrating Display to Match HTPC Output - Page 2 - AVS Forum
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post #31 of 486 Old 03-25-2005, 04:49 PM - Thread Starter
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Quote:


Originally posted by therealgeno
Now there are two conflicting posts in this thread - one states that vmr9 displays 0-255 (PC levels).

This quote states that vmr9 displays Studio levels (16-235). This is what I thought as well - and that overlay uses 0-255.

Which is it


Here is the simplest way I can think of to explain this.

The usage and interpretation of the word "uses" is what is confusing the matter.

VMR9 carries the full 0-255 range of values. So when you view something with btb/wtw data, such as the THX dropshadow screen attached to this message, on a display calibrated to show 0-255, you see the btb information (values at 7 and 10). Although VMR9 carries the 0-255 range of data, you must confine that to 16-235 (or slightly above 235) to insure reference black/white are at the correct levels, or shown as black and white rather than dark grey and light grey. And to do this, you lower brightness (black levle) and contrast (white level / picture) to do this, using contorls on the display itself. This gives you correct blacks and whites, and leaves the btb/wtw data intact so it can be used by the display to create a more accurate picture.


However, when you use Overlay, it "uses" PC levels, with "uses" meaning that you use PC levels of 0-255 to view it properly. But it, IINM, caps/crushes btb/wtw and shows Video levels (16-235) as they should appear, while maintaining a 0-255 calibration on the display.

I do not know the technical aspect behind this, but that is where the confusion over "uses" is coming from.

So, VMR9 "uses" (displays/carries) PC levels of 0-255, but you have to adjust your display to get correct reference white/black levels. Overlay "uses" PC levels in that it can be viewed with blacks/whites at the proper level using PC levels of 0-255.


In looking at the THX dropshadow screen on this post, with PC levels 0-255 shown, "reference black" for video/DVD, level 16, will appear as dark grey. This is how the screen would look using VMR9 on a display at PC levels of 0-255. On the laptop I am typing this on, I can see btb, etc. The dropshadow and the darkest box is at level 7. But on my HTPC display, calibrated for reference black/white, this screen appears as it should - with no btb visible, etc. But because VMR9 passes the full 0-255 range of information, I could turn up the brightness/black level on my display and make the reference black areas dark grey and be able to see the btb boxes. Using overlay, this btb/wtw information may or may not be passed to the display, depending on the video card and Overlay adjustments. I know my video card has default color saturation for Overlay at 114% rather than 100%, and the Overlay brightness/contrast adjustments seem to be off from a straight diagonal gamma line, perhaps making it possible to show video correctly through Overlay on a PC 0-255 levels screen.
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post #32 of 486 Old 03-25-2005, 04:51 PM - Thread Starter
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Quote:


Originally posted by ChrisWiggles
Geno, again, this is why terminology can get confusing. I actually addressed this in a roundabout way in my post.

VMR9 *Maintains* Video levels, while Overlay *expands* the video range to PC levels. BOTH use the 0-255 range, but they use it differently. VMR9 *maintains* Video levels properly, and maintains the data on the disc, which is (quickly for simplicity) 0-255, with the nominal reference range of 16-235. If you use overlay, or otherwise perform an expansion to PC levels, that range will be expanded such that 16-235 is now mapped to 0-255 and values outside the 16-235 range will be clipped.

Very well said, Chris.

I just posted a reply to that post as well. I'll take part of what I wrote, and borrow some wording from you, and update the top post.
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post #33 of 486 Old 03-25-2005, 04:59 PM
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There are patterns on Avia PRO that contain codes 0 and 255 as image content.

Are you sure about that? How did you measure it? According to the Avia Pro Guide, Levels 0 and 255 are reseverd, and according to Poynton Interface codes 0 and 255 are reserved for synchronization purposes, and are prohibited from appearing in video or ancillary data.

I don't have Avia Pro but IIRC, I think the DVE ramps only go 1-254. Perhaps I should revisit.

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post #34 of 486 Old 03-25-2005, 05:01 PM
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Dave, BTW thanks for the sRGB link. It is interesting, though it's all the more reason not to use sRGB as a term to mean Studio RGB, as it's very obviously PC Levels and Graphics-defined colorspace, not for video applications.

Agreed. That was my point.

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post #35 of 486 Old 03-25-2005, 05:22 PM
 
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So, VMR9 "uses" (displays/carries) PC levels of 0-255, but you have to adjust your display to get correct reference white/black levels. Overlay "uses" PC levels in that it can be viewed with blacks/whites at the proper level using PC levels of 0-255.

Don't say this, this is totally confusing.

VMR9, if done correctly, should maintain Video levels just fine. It will not perform an expansion to PC Levels, meaning that Video black, 16, and video white, 235, will be maintained at these levels, and data outside these bounds will also be maintained within the full 0-255 range.

Overlay will perform an expansion, and re-map 16-235 to the range 0-255, thus clipping all values in the original material that were outside the bounds of 16-235. This expansion will also introduce banding artifacts.

Note that if you properly preserve Video Levels from the source, and maintain reference black and nominal reference white at 16 and 235 ,respectively, you will need to calibrate your display to these values. This calibration will not align with graphics that place black and white at 0 and 255 respectively, or with video that may be expanded to that range.
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post #36 of 486 Old 03-25-2005, 05:25 PM
 
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I don't have Avia Pro but IIRC, I think the DVE ramps only go 1-254.

I think that is the case too. There are only like two patterns on Avia PRO that contain 0 and 255 codes I believe. I assume DVE is 1-254. I don't know what's on DVE Pro, haven't explored it at all yet, nor seen it. Hopefully soon, perhaps! Cause I can always use more patterns to look at
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post #37 of 486 Old 03-25-2005, 05:45 PM - Thread Starter
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I updated/rewrote the top message. Re-read it if you like, and if you have any comments/suggestions, please let me know. I really appreciate all the feedback that has been provided so far.

Hopefully the mods will think this thread is useful enough to make into a sticky. Personally I think this is a very important issue for HTPC users.
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post #38 of 486 Old 03-25-2005, 06:04 PM
 
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My criticisms are as follows:

Quote:


Overlay...
This will mask 0-15 and 236-255, showing video levels (16-235) as they should appear, by expanding the 16-235 range to 0-255.

Mask is a misleading word, clip is more clear. Depending on how the expansion is done, usually the expansion will clip at 16 and 235 and map these to 0-255 as you have described. I do note in my Guide, however, that other ways of doing this exist, such as not providing a re-mapping, and moving the range down 16 steps. This prevents banding problems, but causes a severe mismatch in how white is defined between video white which would now be 235-16 or 219, and graphics white at 255. Other expansions may expand part of the way towards 0-255, thus clipping only some values outside the 16-235 range.

This clipping, is not how video "should appear" or should be handled. It is a devation from video standards, and should be avoided for the reasons that have been argued at length before. It is a change which degrades video performance, but does serve, in some fashion, to attempt to converge the difference between Video and Graphics applications. Whether this degradation to video is acceptable for the sake of aligning Video and Graphics is a user choice, but at AVS I assume that users are striving for home-theater use, or otherwise idealized video, in which case expansion to PC levels should be avoided.

Quote:


This will show or "pass through" the full 0-255 range

To be more clear, perhaps you could say that it "maintains" the full range of values that are encoded in the source, as they are in the source, and does not re-map them, something like that.

My other note, is that while I did note that Avia and Avia PRO have moving patterns, there is an ambiguity in your post since these are mentioned in the Note, though you discuss DVE in your post. DVE contains patterns that you cite as being useful for observing the full source range from 1-254. However, consumer Avia does not. You must use the deep ramps on Avia PRO, or the patterns mentioned on DVE. Avia's patterns are limited to 16-235, so if these is clipping of data outside these bounds in your system, no pattern elements in Avia will allow you to observe this clipping. You must use Avia PRO instead.

I don't recommend THX patterns, as they have been known to vary.
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post #39 of 486 Old 03-25-2005, 06:05 PM
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Quote:


Personally I think this is a very important issue for HTPC users.

I agree it certainly has a lot of debate and confusion associated with it, but in all honesty, whether I output and calibrate for PC levels or output and calibrate for StudioRGB levels makes little if any difference (it escapes my perception) to the images I see on my displays be it CRT, LCD, or DLP.

Again, where are the A/B comparisions or even titles / timestamps of all these DVD's that have significant BTB and WTW image information encoded on them that PC levels are clamping? Isn't that what makes the headroom and footroom of StudioRGB so magical? And, if the headroom and footroom are so magical and necessary for digital images why isn't it used in all other digital imagery? Are our cameres, printers and non-video graphics missing the magic too?

Dave
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post #40 of 486 Old 03-25-2005, 06:13 PM - Thread Starter
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Quote:


Originally posted by ChrisWiggles
My criticisms are as follows:


Will fix now.
These are hardly criticisms, btw.
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post #41 of 486 Old 03-25-2005, 06:15 PM
 
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ok fine. If you really want some criticism, I don't think anyone is gonna read this borign crap because there aren't any picture.

Please find a way to include scantily clad attractive women to describe this, and I think it will be far more effective.

Pshhh, long techy text FAQs, pffft!
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post #42 of 486 Old 03-25-2005, 06:28 PM
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Quote:


ok fine. If you really want some criticism, I don't think anyone is gonna read this borign crap because there aren't any picture.

Why can one tell us about all the disadvantages one sees when clamping BTB/WTW that may be encoded on a DVD yet can't seem to show us? Even a title / timestamp? Seriously.

cyberbri,

You seem to have the ability and the appreciation of an A/B frame capture for documenting such things. You got any examples of the differences you are seeing?

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post #43 of 486 Old 03-25-2005, 07:15 PM
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One potential add to the discussion for the "which is better" section:

"In deciding between Overlay or VMR, many people are concerned that they lose color information by changing reference black from 0 (in PC Video) to 16 (in Studio Video) and by changing reference white from 255 to 235. Since commercial video is mastered using Studio Video levels, there is no color information to be lost when choosing to use VMR9. In fact, the opposite is typically the case and is why hard-core HTPC users are excited about using VMR."

I'm not sure whether this is too pedantic or redundant, but it is one of the key decision hurdles that people face when they are first starting out. On a related (being pedantic) note, do you want to include in the discussion the differences between VMR7 and VMR9? Either VMR version will use Studio levels, the question is merely what's the underlying technology and how much is offloaded to hardware. One final point about VMR9 is that for best performance, a user should have a graphics card that is capable of accelerating DirectX9 functions in silicon. Lack of DX9 capability is probably why most people would use VMR7 at this point.

Dave - Take a look at Chris' settings thread. In it, he links to a discussion on this very issue where Don Munsil shows where BTB and WTW data occurs with screen captures.

Later,
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post #44 of 486 Old 03-25-2005, 08:02 PM
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Dave - Take a look at Chris' settings thread. In it, he links to a discussion on this very issue where Don Munsil shows where BTB and WTW data occurs with screen captures.

Correct me if I'm wrong, but not in a full-frame, A/B type manner where meaningful clamped/ unclamped comparisons can be made. Also, I'll again note that many of those highlighted BTB pixels are in the black bars of 2.35:1 movies. I'd hope we could all agree that there isn't significant image information hidden in the black bars.
FWIW, I have done many A/B comparisons and posted some of them. I have many more un-posted but they just don't show any perceptible difference IMO.

Dave
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post #45 of 486 Old 03-25-2005, 08:54 PM - Thread Starter
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Ursa,

I have added your paragraph.

Dave,

I have not done any screen caps of clipped versus unclipped. I wouldn't clip the values on purpose in VMR9, and I won't be going back to Overlay anyway. So to me there are more important things I want to focus on than that.
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post #46 of 486 Old 03-25-2005, 09:12 PM
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Quote:
Originally posted by ChrisWiggles
Overlay will perform an expansion, and re-map 16-235 to the range 0-255, thus clipping all values in the original material that were outside the bounds of 16-235. This expansion will also introduce banding artifacts.

Ahhhh, this explains it perfectly to me now. IMHO, and I will be the first to admit I'm not very smart, but this is/was totally confusing to me.

cyberbri, I also wanted to thank you for taking the time not only to start/edit this thread, but also to explain things further.

And now there is a thread dedicated to explaining this!! I wish this were here two months ago - I've spent the last two months figuring this stuff out. Half my posts are questions about this very thing!!


I do have one more question though - do most displays offer the choice of PC and Video defaults?

On my 4805, it is set for PC defaults - so Krasmuzik has spent the last month helping a few of us calibrate the 4805 to Video levels in the RGB controls. I now have proper Video Levels and NO banding - it took me awhile though .

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post #47 of 486 Old 03-25-2005, 10:52 PM
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Dave, if someone finds a scene with significant BTB and/or WTW information, I don't think it would convince you one way or another if maintaining Studio Video RGB levels (sidenote: I've heard that terms "Studio RGB" and "Studio Video RGB" as being described as different from each other - is that confusing or what?) is worthwhile to you or not - you would just claim it was an improperly mastered scene anyways. So, I don't really see the point of discussing it in this thread?

Isn't this thread about keeping your HTPC as close to reference as possible? If the studio engineers agree that using Studio Video RGB levels is the correct way, then we should focus on that here, and worry about what looks better elsewhere.

cyberbri, I wonder if you should mention that adjusting contrast and brightness and saturation with the VMR9 Procamp controls (as found in TheaterTek, Zoomplayer, and other VMR9 players) can adversely affect the picture if they are not to the neutral (reference) values? This is why I consider the Print-Screen method extremely important - upgrading to newer drivers can and will cause the RGB grayscale values to track differently. For example, on my PC using the NVIDIA 67.66 drivers, I can keep all the VMR9 controls set at 0 and everything is perfect. But if I use the 71.84 drivers, I have to use brightness at 10 and contrast at -10 (or maybe I got that backwards). If I leave them at 0, it expands almost to PC levels, which is very bad. To make matters worse, some users use a brightness of 2 and a contrast of -2 with the same drivers and videocard!

I think it is imperative that the DVE and AVIA screenshots, as shown on my mistermax.smugmug.com website, display properly down to the RGB values first, before you touch the controls on your display. With all the different HTPC software and hardware out there, this is proving to be a more difficult task than ever before.
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post #48 of 486 Old 03-26-2005, 01:25 AM
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Quote:
Originally posted by maxleung
I think it is imperative that the DVE and AVIA screenshots, as shown on my mistermax.smugmug.com website, display properly down to the RGB values first, before you touch the controls on your display. With all the different HTPC software and hardware out there, this is proving to be a more difficult task than ever before.

Indeed. When running DVE test pattern with Zoomplayer in windowed mode reference black is being displayed at 16 and reference white at 235. When going fullscreen with Zoomplayer the picture gets brighter. Reference black is now at 33 and reference white at 241. Same result with both Nvidia and DScaler decoder, removing ffdshow doesn't have any effect. Same result with VMR9 windowless/windowed and VMR7.

System: Radeon 9550, Catalyst 5.3, default settings.

Anyone else experiencing the same or got a clue how to do something about it? As far as I remember it's always been like this on my system; the picture getting brighter when going fullscreen in VMR9.
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post #49 of 486 Old 03-26-2005, 03:44 AM
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Dave, if someone finds a scene with significant BTB and/or WTW information, I don't think it would convince you one way or another if maintaining Studio Video RGB levels is worthwhile to you or not

Yes, it would. Particularly when it's been encoded on all our DVDs all along. It would make me question all my past calibrations. I would finally see all these details' that I've been supposedly missing all the years using HTPCs and PC levels and this upside to StudioRGB levels. NTIM, but I didn't just fall off the calibration turnip truck. I've been responsible for the material, setup, characterization and calibration of my employers technology demonstrations at most of the CES and SID shows for the past 5-6 years. To rave reviews.

Also, I don't think that I'm alone in being skeptical of the assertion that there is significant BTB/WTW image information encoded on all DVD's.
Quote:
you would just claim it was an improperly mastered scene anyways.

From page 13 of the Avia Pro Guide. A well respected author and credited contributor to Chris' guide.
Quote:
The above white bar can clip without endangering details of properly mastered material.

Because of the occasional poorly mastered recording, an ability to process signals in the below back and the above white regions can be useful.

I know Chris disagrees with this.

I do find it interesting that many of you have interest and see the value in direct A/B image comparison for many other claims, settings and evaluations but not for the BTB/WTW image information that has been encoded on all our DVDs all this time. It's an easy test.
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So, I don't really see the point of discussing it in this thread?

Drats. OK then. I thought this might finally be the thread to discuss it.

Dave.
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post #50 of 486 Old 03-26-2005, 03:54 AM
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Oh yea, an example of PC image details that can't be resolved when calibrated at StudioRGB levels.
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post #51 of 486 Old 03-26-2005, 10:18 AM
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Uh, where did that come from? (looks like it's from DVE) If it was video then then that whole image would be clipped if you expanded to your beloved PC video levels.

Besides, you've been shown examples, plenty of them, and if you're not convinced it's important then go on happilly calibrating to PC video levels, it won't bother any of us.

See what an anamorphoscopic lens can do, see movies the way they were meant to be seen
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post #52 of 486 Old 03-26-2005, 10:27 AM
 
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Why would that not be resolve Dave? It should be, that's the point.

This has been my point all along, that you have never gotten in your head, and I'm not going to debate on again here:

1) Video has defined standards, video engineers have created space in digital video standards for data beyond reference black and white. Your claims that this data is not important or seen would very clearly insinuate that this data space in unecessary (which is clearly what you are arguing) and can be fully clipped off anywhere in the video chain, including at the display). This clearly says that you believe that video engineers should not have designed the system with this headroom and footroom, and that they are thus bonkers to do so. This is quite a stretch.

2) You seem very adamant that for some reason about this topic, yet you think this data does not matter, and you can't see the banding artifacts. So my question is, why do you enter all these threads? Go watch Overlay, clip what you want, and enjoy. If you can't see any difference, go watch. It is not *my* job to prove to you why years of video engineering is correct. If you don't think it's better to handle video correctly, that's fine, go do your thing. The only thing that you could say is that clipping to PC levels in what you've seen is not a discernable degradation. But there is not performance *benefit* that you can claim for moving video to PC Levels. So then if you think both are equal, why do you care?
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post #53 of 486 Old 03-26-2005, 10:43 AM
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Quote:
Originally posted by ChrisWiggles
1) Video has defined standards, video engineers have created space in digital video standards for data beyond reference black and white. Your claims that this data is not important or seen would very clearly insinuate that this data space in unecessary (which is clearly what you are arguing) and can be fully clipped off anywhere in the video chain, including at the display). This clearly says that you believe that video engineers should not have designed the system with this headroom and footroom, and that they are thus bonkers to do so. This is quite a stretch.

Not trying to start another "war" or make anyone mad - I just had another question:

Are we supposed to see this headroom/footroom? Why is it there? BTB we clip - Chris, I think you recommend this as well, right? We are not supposed to see BTB info. WTW is different.

Is this something to simply leave room for, say, errors? Is it correct to say that info below reference black (<16) are errors?

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post #54 of 486 Old 03-26-2005, 10:47 AM - Thread Starter
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You don't "clip" (remove) btb data. You just don't show it on the display. You adjust brightness/black level so that the black of the display is at reference black of 16. Values below that are left intact all the way to the display, and just blend in with 16 black.

I believe the reasoning for not clipping it is in the Source Settings guide, if you want to read up on it.
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post #55 of 486 Old 03-26-2005, 10:52 AM
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A couple of things come into play with the headroom/footroom. First it aids in any processing/filtering that gets done. Filters work better when they don't run into a hard edge (like 0 RGB).

Next, as was explained by dmunsil in another thread, while (eg) DVDs are mastered such that they are constrained within 16-235, they are stored in component colorspace. It's is very possible, quite likely, and happens quite often that 100% "legal" (meaning between 16 and 235) component values, when converted to RGB fall outside 16,16,16 and 235,235,235 RGB.

Finally with a CRT (or a digital that emulates a CRT) there is no defined max white level, CRTs essentailly go on forever (within reason ) so the result is that you can't clip white on a CRT, when properly calibrated picture elements with levels >235 will still be visible on a CRT. Also CRTs don't have a defined black level, it changes with picture content, therefore when properly calibrated, picture information <16 can be visible on a CRT.

See what an anamorphoscopic lens can do, see movies the way they were meant to be seen
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post #56 of 486 Old 03-26-2005, 10:54 AM
 
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Geno, this is all covered in my writeup.

The "real" level that black lands at will float around as APL changes due to the behavior of CRTs. As black level floats on a CRT, this allows compensation in the mastering so that the actual black level at any instant on a CRT can be compensated for so that the final image is correct, roughly speaking. Peak white will be very easily visible on a CRT. BTB data is also useful in image processing, dithering cycling etc etc.

No, these are all legal values in video. It was mentioned earlier, but there are only two illegal codes in the video range and those are 255 and 0.

The 16 and 235 reference points for black and white are nominal values. As I noted, the "real" value for black will move in reverse-compensation for the moving black on a CRT display, which is the reference.
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post #57 of 486 Old 03-26-2005, 10:56 AM
 
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stranger, just a quibble, but I believe the bounds for YCbCr values are different, the Y is I believe 16-240.
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post #58 of 486 Old 03-26-2005, 12:30 PM
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stranger, just a quibble, but I believe the bounds for YCbCr values are different, the Y is I believe 16-240.

Wrong again Chris.

Rec. 601 specifies eight-bit coding where Y' has an excursion of 219 and an offset of +16. This coding places black at code 16 and white at code 235. CB and CR have excursions of +/-112 and offset of +128, for a range of 16 through 240 inclusive.

The PC image I posted has no data below 237. If you've calibrated your displays (esp a digital) white point for StudioRGB at 235, The image details should be unresolvable.

You guys enjoy the emperors new clothes. Bummer they are so elusive that I seem to be the only one who cares to try to take a picture of them.

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post #59 of 486 Old 03-26-2005, 12:47 PM
 
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The PC image I posted has no data below 237. If you've calibrated your displays (esp a digital) white point for StudioRGB at 235, The image details should be unresolvable.

Are you dense or what!

PC Levels will clip what you posted. Studio levels will PRESERVE all that detail. And it's also why I advocate, though not stringently, that users with digital displays calibrate for a max peak white, instead of at 235. If you use PC levels it doesn't matter what you do at the monitor because those values are clipped at the source/processor, so they can never be recovered.

What new clothes, I've been repeating myself over and over to you, across different threads for months, and across different forums. And you still don't get it.
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post #60 of 486 Old 03-26-2005, 01:12 PM
 
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Bummer they are so elusive that I seem to be the only one who cares to try to take a picture of them.

But you still don't even understand what it is you're talking about. What you took a picture of proves MY point. First, that image looks like it's been expanded to PC levels. And if it hasn't it proves my point about the visibility of peak white detail and preservation of the full video range.

I don't see why this is so difficult for you, or why you keep pursuing it. If you don't see or think there should be a difference, then let it go and watch how you want and stop bothering us all about it.
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