Survey: What min/max levels does your cam shoot?

Comments

GlennChan wrote on 6/27/2011, 8:25 PM
There are different issues going on.

First issue: (This may be esoteric and not really matter.)

There are three colorspaces you need to know about.

Y'CbCr... as is commonly used and described in Rec. 601 and 709. (We'll assume that 601 and 709 are the same because it doesn't matter for this discussion, even though they aren't.)

studio RGB, which is RGB with black at 16 and white at 235.
computer RGB, which is RGB with black at 0 and white at 255.

2- Y'CbCr has black at Y'=16 and white at Y'=235. Suppose you are capturing from an analog tape and the calibration is a little off or there is edge sharpening going on. The code values above 235 (Y') will record those values. Later on you could potentially bring those values back into legal range. Had the standard been white at Y'=255, then you can easily get clipping and you'd never be able to bring that information back.

3- Vegas converts Y'CbCr to studio RGB for DV and HDV (in the later versions... I think 6 and above). Sometimes/oftentimes there is useful information above 235 Y'. Mapping 235Y' to 255RGB would clip all the information above 235Y'.
Also, almost all video cameras have useful information above 235Y'.

4- However, this does not mean that all information is necessarily preserved. Yes, 255 Y' will get mapped to 255 RGB'. However, Y'CbCr color space still far exceeds computer RGB. Y'CbCr can represent a lot of crazy colors, some of which can't exist because it would require the existence of negative light. In practice, some cameras will record Y'=255 while the chroma is not neutral, so the equivalent RGB value will have one channel that is above 255 RGB.

Clipping can still occur when converting from Y'CbCr to computer RGB. I believe that some cameras actually do record colors that fall outside computer RGB (though I could be wrong here).

--------------------------------

Second issue:

The values in Vegas depends on what codecs you are using and how they are configured.

For example, if you set Vegas to use the Microsoft DV codec (not recommended), DV footage will decode to computer RGB.... 0-255. And you will get clipping.

If you set Vegas to use 32-bit in project properties and use the full range setting, then HDV will decode to computer RGB. You will have illegal values above 255 RGB, though you can bring this information back since 32-bit floating point numbers are being used.

It may be more useful to say what default codec Vegas uses to decode footage from that camera.

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Third issue:
Some/most cameras record values outside the legal range, and/or do not follow standards at all.

Almost all video cameras will record information above 235 Y'.

I can't remember if some of the dSLRs will put black level at Y'=0 and white at Y'=255.
GlennChan wrote on 6/27/2011, 8:38 PM
On the other hand though what Fuchs is saying is quite correct, the histogram can much more easily give the wrong impression when looking at unknown footage. Looking at that GH2 footage's waveform and peering very closely I can see the very tiny amounts of the image going below 16. I'd actually hazard a guess myself that they might be the result of some edge enhancement, you get the same thing with Vegas using the unsharpen mask.

I think the dominating factor is noise in the sensor. Suppose that the sensor has no light hitting it but there is noise. While the signal is supposed to be zero, noise will always push the signal above zero.

If you apply some offset to them or subtract a value from the signal, then you will get values below black level.

2- Some cameras also have borders as others have pointed out.

---
Here's my easy practical solution: You simply adjust the blacks until they kiss legal black level. You may need to do this anyways as flaring in the lens depends on the shot and may change in the middle of a shot.

There may be rare situations in which there aren't actually any black details. But even in those situations the picture will probably look ok. And if you are using an external monitor, you can adjust the picture to whatever looks right.

Tutorial here:
http://www.sundancemediagroup.com/articles/glennchan/levels_in_sony_vegas_part_one.htm
Note that I suggest you setup Vegas' scopes "incorrectly". For practical purposes, that is what you should do.
NickHope wrote on 6/27/2011, 8:44 PM
Thanks Glenn.

How do you feel about the following media fx preset as a general recommendation for legalizing levels and rescuing some highlight detail in footage that we are seeing decoded to approx.16-255 (HDV, AVCHD etc.)?

GlennChan wrote on 6/27/2011, 8:56 PM
Yes that would work.

I have a more neutral preset where the slope is constant and there is no rolloff for superwhites (in my color correction tutorial in the .veg). The minor rolloff that you have would look good too. It comes down to taste right?

2- If you have two tangent points in the bottom left area, then you can select both of them and tap left/right on your keyboard to adjust black level. And still have illegal blacks clipped to 16.

-------------
To go back to the whole uploading to Youtube discussion. I think that applying that preset all the time would be a better idea than applying a Levels FX "computer RGB to studio RGB" preset.

If the camera doesn't actually shoot superwhites... it's not that bad because whites will be pretty white to the human eye. And if the area surrounding the youtube player isn't white but black then it's really, really hard to see anything wrong.

Where I see it failing is if somebody encodes to a codec that expects computer RGB levels instead of studio RGB. But then they wouldn't be following instructions... so I'm not sure if I would worry about that.
NickHope wrote on 6/27/2011, 9:50 PM
>> 2- If you have two tangent points in the bottom left area, then you can select both of them and tap left/right on your keyboard to adjust black level. And still have illegal blacks clipped to 16. <<

Glenn, can you possibly give me a bit of a walkthrough on this because I can't get it to work? I'm attempting to do this on the curves in your "curves-and-secondary-presets2.veg" and it seems there are duplicate points on top of each other. Is that intentional? If I click one of them and move it left or right, the tangents go haywire and another point appears underneath the one I selected?

EDIT: OK I get it now. We have to select the 2 superimposed points together by using a marquee, and then shift them left or right with the keyboard arrows.
farss wrote on 6/27/2011, 11:14 PM
Glenn said:
Here's my easy practical solution: You simply adjust the blacks until they kiss legal black level. You may need to do this anyways as flaring in the lens depends on the shot and may change in the middle of a shot

That's what I do, watch the scopes while tweaking a curve like this:



Now my EX1 just plain would never go to black, even with the iris and lens hood closed. Turns out it and many other Sony cameras have around 5% Setup on black. Since dialing that out my blacks read better.


There's one trap with all this that's caught me out several times, compositing. Get all your black legal at 16 and Vegas can add black to black and give you grey i.e. 16 + 16 = 32...ooops.

Bob.
GlennChan wrote on 6/27/2011, 11:45 PM
There's one trap with all this that's caught me out several times, compositing. Get all your black legal at 16 and Vegas can add black to black and give you grey i.e. 16 + 16 = 32...ooops.
That's brutal and something I wouldn't think about. :/

This is why I'd like to see future versions of Vegas handle all this stuff better. File a feature request if you'd like to see it happen.
NickHope wrote on 6/28/2011, 1:43 AM
Glenn, in your Color Correction Tutorial you talk about legalizing the levels first by using color curves. Then you go on to talk about fixing white balance afterwards with the Color Corrector. If I do it that way round, the color corrector takes the levels back outside legal limits. Wouldn't it be better to do that the other way around?

Further to this, do DVD players/TVs in general clip individual R/G/B channel values outside of 16-235 or just the overall luminance?
GlennChan wrote on 6/28/2011, 9:11 AM
Wouldn't it be better to do that the other way around?
Yes.

Further to this, do DVD players/TVs in general clip individual R/G/B channel values outside of 16-235 or just the overall luminance?
I believe that some do and some don't. At least that is the assumption that I would make. (Maybe I'm wrong?)
NickHope wrote on 6/28/2011, 11:57 PM
How much of a problem is it to let superblacks pass (for DVD)? What's the danger?

I'm remastering an old DVD here and I've been clipping everything at 16 in Vegas, which I didn't do before, and somehow it looks like it's missing some "depth" on TV. Difficult to put my finger it but perhaps the MPEG-2 compressor is rolling off the values around 16-20, or perhaps the compressor is making a worse job of the values around 16-20 because it can't use the sub-16 values on some channels that it had before. Or perhaps the DVD player is doing one of those 2 things.
musicvid10 wrote on 6/29/2011, 12:02 AM
I've got five DVD players and the only thing I'm sure of is that they all do it differently.
farss wrote on 6/29/2011, 1:32 AM
"How much of a problem is it to let superblacks pass (for DVD)? What's the danger?"

None, the player will clamp the blacks at 16 and the whites at 235.
I've only tested this from the composite output, simply fed that back into an A>D converter. I have no way of actually checking what they do on the component or RGB outputs.

The reason they do this is because anything seriously below 16 can cause a TV / monitor to loose sync and you'll get a roll.

The one format you really needed to be careful with was VHS.

Bob.

Marco. wrote on 6/29/2011, 3:05 AM
I think "illegal" should be kept as a broadcast thing, not bond to BT.601/BT.709 where RGB ranges from 1-15 and 236-254 are well expected as headroom for under- and overexposure.

-----------------------------------------

From BT.709:

Quantization levels

-----------------------------------------

Cameras (and in bit different terms even other devices like players and displays) which use this headroom still work according to BT.601/BT.709 as long as them keep RGB 0 and 255 reserved for sync encoding. It's more a matter of HOW this headrooms are used rather than them ARE used.

For sure if you produce video for broadcasting purpose you are on the safe side to let your final product strictly conform to RGB 16-235. Though the R.103-2000 defines tolerances on "illegal" colors in TV.

-----------------------------------------

Quote of R.103-2000:

Ideally there should be no illegal combinations in a television signal but experience has shown that a certain tolerance can be allowed, based on similar tolerances allowed for RGB signals.Experience has also shown that so-called colour gamut "legalisers" should be used with caution because they may create artefacts in the picture that are more visible than the colour gamut errors.

-----------------------------------------

And the EBU Tech 3320 which defines the user requirements for video monitors in tv production says:

Handling of under- and over-shoots

GlennChan wrote on 6/29/2011, 2:15 PM
None, the player will clamp the blacks at 16 and the whites at 235.
According to some of the calibration DVDs out there, some players do this and some players don't. They get around this problem by having a block of color that is barely above black level. It moves back and forth. You make the image darker until you can barely see the block (or the blocks if there are more than 1) move.
john_dennis wrote on 7/4/2011, 8:17 PM
I'm looking for a camera so I went and shot some vidoe on a couple models in the store.

Sony HDR-CX-700

CanonHF-S30

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NickHope wrote on 7/4/2011, 8:35 PM
Thanks John. I've added the Sony HDR-CX700 to the [16-255] cameras in the first post.

Not sure what to make of the Canon. It goes down to about 9 in your histogram. What scene did you shoot?
Marco. wrote on 7/5/2011, 6:24 AM
Be aware you cannot compare these results with that ones created under the conditions you asked for: Pure white and pure black if possible. If you do right that way, a camera working according to the BT.709 should take the signals as main signal with no under- or overexposure and should display the black shoot at 16 and the white shoot at 235. But if you shoot "anything" then it's no wonder there are super-blacks and super-whites and the histogram isn't the perfect scope tool.

The test conditions you suggested are very good: pure black and pure white and then use the histogram. Results you include for your statistic should not aberrate.
NickHope wrote on 7/5/2011, 8:23 AM
That's a good point Fuchs, and the histogram John posted for the CX-700 is not definitive, particularly in the highlights, where there is a big shoulder at about 230-235. So I've removed it from the results in the first post.
NickHope wrote on 1/12/2012, 1:01 AM
Added the Sony NX70 to the list of cameras that shoot nominally 16-255 (in the first post), based on this.
stevecrye wrote on 1/12/2012, 5:47 PM
Hi Nick;

FYI - I've read in many places, and I believe it to be true, that the "guts" of the NX70 consist of the lenses, sensor and processor circuits used in the CX700. Sony added bells and whistles and rainproofing and called it a NX70. So, I bet the CX700 results, when they arrive, will look very similar.

I wonder if the way the red, blue and green sections of the NX70 histogram in the full-white test are offset has anything to do with the severe chromatic aberration on the NX70?

Steve
amendegw wrote on 1/12/2012, 6:27 PM
Steve,

Glad to see you've posted on this forum. I think I have a pretty good grasp of what's going on in these color space issues (others may disagree - [heh]). However, the are posters here that are much more versed in this subject than I and can explain it better than I can. Furthermore, since my background is in Engineering & IT, sometimes I screw-up in my video terminology.

...Jerry

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amendegw wrote on 1/29/2012, 8:29 AM
Panasonic AC130




And an editorial comment: There are all manner of cameras in this forum and it's a simple 5-10 minute test to get these results and post them back here.

...Jerry

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Udi wrote on 1/30/2012, 2:34 AM
Sony A77, 2012, AVCHD 60p sRGB, black min 10 peak 15, white max 253 (red at 255) mean 251

Note that changing the project to 32bit effects the result. Selecting 32bit and full range - all is 0-255.

Udi
paul_w wrote on 1/30/2012, 7:10 AM
Adding my Sony FS100 to the list, late to the party.

As i dont know if this should be measured using 8 or 32 bit properties, i tested both.

[8bit mode]
BLACK
min = 18
max = 28
average = 22

WHITE
min = 253
max = 254

[32bit / video mode]
BLACK
min = 16
max = 28
average = 23

WHITE
min = 253
max = 255

black; Gain = 0, body cap on, black bag over the front area, shutter 10000th.
white; lens cap off, gain up, shutter 12th, totally over exposed.
Picture profiles set to OFF. With setting the profiles better, i can get blacks down to 16-17.

Paul.