sRGB vs REC.709: color shift, or identical?

ALO wrote on 2/22/2020, 1:26 PM

Been engaging in some light Color Management reading for sport. My monitor has calibrated presets for sRGB and REC.709. Let's just assume they're perfect.

Are these functionally identical previews?

1)

Monitor --> sRGB

Secondary Display --> "adjust levels from studio RGB to computer RGB" checked

2)

Monitor --> REC.709

Secondary Display --> "adjust levels from studio RGB to computer RGB" *unchecked*

...or is there a very slight color shift between the two color spaces (in addition to the levels difference)?

 

Comments

Musicvid wrote on 2/22/2020, 2:33 PM

sRGB is a universal ICC Profile for matching monitors and printers.

REC 709 (=Studio RGB) is a universal video production standard used for almost all mainstream media delivery.

The two are conjoined by design and functionally identical for our purposes, and are not user-discretionary.

There are so many variations and unknown variables in connecting your secondary display monitor that predicting the correct setting is anybody's guess.

First, turn off dynamic contrast on both displays. Second, calibrate your monitors to give identical display with identical reference. Third, use the secondary monitor display setting in Vegas that will give the same picture as your preview or output, depending on your need. There is a dual-range calibration grayscale for this purpose linked in my signature.

 

ALO wrote on 2/22/2020, 2:50 PM

Thank you -- I think you're answering my question but let me just double check: assuming my hypothetical secondary monitors are behaving as expected (let's just call them perfectly calibrated), the setups in examples (1) and (2) are going to show a functionally identical picture: blacks and whites are correctly mapped, and color values should match. Right?

Musicvid wrote on 2/22/2020, 3:02 PM

As I said, way too many untrapped variables. Use the grayscale.

In 22 years of production, I have never seen or used a "hypothetical secondary monitor behaving as expected." The defaults are different with every single make and manufacturer and user, often to their own detriment.

And you must first ask the question, "Do I want the secondary monitor to look like the Vegas Preview, or the Video Output?"

For further humorous enlightenment about the first 32 "possible" spurious outcomes to your predictions, see my article. Spock would say, "Speculation is futile."

https://www.vegascreativesoftware.info/us/forum/pc-to-tv-levels-a-comedy-of-errors--107325/

ALO wrote on 2/22/2020, 3:53 PM

For my purposes, the question of what the monitor is actual doing (vs what it should be doing) is downstream. I'm just trying to make sure I understand the signal path first--what's being sent to the monitor. :)

Musicvid wrote on 2/22/2020, 5:03 PM

Your question suggests that you may not understand that the Vegas Preview buss and compliant video Output use different color spaces. So you are really asking two questions, not one. Which do you want to see? Do you want to see Vegas Preview RGB levels, or Video Output YUV (709) levels on your secondary monitor? That's what the switch is there for. There is no wrong answer!

Neither is the "signal path" a constant. Some hardware. outputs wont even pass RGB. Do let us know what you come up with for your own workflow, and good luck!

 

marc-s wrote on 2/22/2020, 5:07 PM

ALO,

I've always checked adjust levels from studio RGB to computer RGB as that sets the computer display to preview source video footage (16-235 black/white levels) properly on a computer monitor. A computer monitor by its nature is (0-255 black/white levels) so this solves the inaccurate levels issue that you experience when just using the video preview (assuming you have a secondary monitor properly calibrated to SRGB. In addition my understanding is that Rec 709 is basically SRGB with a gamma difference so I would not uncheck the option since it affects the mapping of the black/white levels not the gamma. Hope this helps as this practice has given me consistent a color workflow output. Whenever possible I also us a Blackmagic design output card feeding a plasma (and eventually an OLED) for stuff destined for television or DVD but SRGB is usually close enough for most stuff.

Oh... and scopes are your best friend. Learn to use the waveform monitor if not already using it.

Kinvermark wrote on 2/22/2020, 5:53 PM

+1. Video levels project plus preview device checkbox = on. You can check by putting the Smpte NTSC generator on the timeline and look at the pluge (google it, but basically one bar barely visible is correct.)

A computer monitor won't be broadcast accurate, but that probably isn't an issue for you. Otherwise you need a flanders scientific or Sony monitor. Some guys are now compromising with LG Oled TV's which they calim are very accurate for non HDR grading.

 

 

 

ALO wrote on 2/22/2020, 7:04 PM

Thanks for the comments. Let me just note that if you use Vegas with the studio-to-computer box checked and no further workarounds, some of your FX won't behave properly (especially curves!) because they always assume your source is in the computer space.

I'm definitely in the "good enough" camp when it comes to color management. If I can reasonably match what I see on my screen to what my viewers see on theirs, I'm happy.

I'm aware there is a whole other dimension of complexity when it comes to broadcast. Just trying to make sure I understand the difference, if any, between what Vegas calls "studio levels" and the REC.709 spec.

Musicvid wrote on 2/22/2020, 7:13 PM

Just trying to make sure I understand the difference, if any, between what Vegas calls "studio levels" and the REC.709 spec.

The REC 709 and Studio RGB Luminance levels are one and the same. For the record, those levels are 16-235 RGB (Y').

Studio RGB = REC 709 = B.T.709 = ISEC 601 (SD) = REC 2020 (8 bit) = YUV = Y'CbCr = Apple Limited Range = yuv420p (vui) = NTSC Broadcast Luminance = 100 IRE = 740 mv p-p (analog voltage)

Here is the answer I respectfully gave you in my first reply. Have a Happy Learning Curve.

REC 709 (=Studio RGB) is a universal video production standard used for almost all mainstream media delivery.

 

Grazie wrote on 2/22/2020, 10:12 PM

@Musicvid - Don't you ever, ever contemplate vanishing from our Forum!! We, and MAGIX, need you here. As a side-bar, might I add that the issues and confusions about Monitoring Video almost hold true for Audio too. I know I've been culpable in making mistakes when appreciating Audio Headroom, Clipping and so on.

Musicvid wrote on 2/22/2020, 10:51 PM

I know I've been culpable in making mistakes when appreciating Audio Headroom, Clipping and so on.

And intuitively, as with your video color grading, you have made some audio events worthy of my highlight reel, rather than the cutting room floor. You know which ones.

If I had your affinities, I wouldn't need all this math bullshit.

@Grazie

Kinvermark wrote on 2/23/2020, 12:19 AM

For the record, those levels are 16-235

+1. That is in the rec 709 definition AFAIK.

The gamma function is a different matter as it's not clear to me if 2.2, 2.4, or btu 1881 is recommendation or requirement, but my feeling is that it doesn't really matter unless you're broadcast bound.

RogerS wrote on 2/23/2020, 1:34 AM

Fascinating discussion, and let me see if I'm getting this right. Until now I've been just doing the levels conversion upon output, but I want to try the device preview "adjust levels from studio RGB to computer RGB" instead.
My goal is to preview what the video will look like upon output for web display.

My external preview device is a hardware calibrated NEC with internal LUT. I use it extensively in my work as a photo editor.

I loaded up a few test images with grayscale ramps and step wedges and one with skin tones, as well as a video file. What I want to test is if the "adjust levels" on the secondary monitor is checked, will Vegas correctly adjust levels from both photos and videos?

I thought the scopes would help answer that but I am confused as to settings- I have had Studio RGB checked, but if that is checked, the still image step wedges go from -10 to 110. Unchecking StudioRGB makes it go from 0-100. That also affects the readings for the video clip, though.

And the answer is... no. This does not seem to be a smart conversion, but just dumbly applies a correction regardless of the nature of the source material

Looking at the step wedge photos on the external monitor, there's no real detail from 0-5% or 95-100%. Whereas Photoshop and the preview window show each step clearly. The media generator "pluge" only visually shows one of the dark bars in the secondary window while the preview window shows 4.

I then moved the photos to their own track and added a track Fx of studio to computer. I put one of the wedges and the pluge on both tracks, so we can easily see the difference.

I rendered a video that is a combination of the still test files and a video with Colorchecker Passport. No levels conversion upon output. Bringing it back into the Vegas Pro timeline, levels appear identical visually (and on the non-studio RGB scopes) between the rendered file and the individual clips and photos on the timeline.

Looking at the file in Media Player Classic Black Edition, it looks as expected.
Ditto on YouTube:

Conclusion: the external preview "adjust levels from studio to computer" works fine if you are dealing with standard video source material, but not a mix of video and stills. In that case you need to make them match each other, and applying a computer to studio levels change to only photos appears to do that. Output is WYSWYG- you can see the shadows being blocked up on the external display.

(Side note: is it okay that I am not doing anything to limit the video levels upon output? YouTube and the media player seem fine with this.)

ALO wrote on 2/23/2020, 9:27 AM

(Side note: is it okay that I am not doing anything to limit the video levels upon output? YouTube and the media player seem fine with this.)

This is I think a really good question and I also think it doesn't have a single answer. I worry that if I send superblacks/superwhites as spurious values in my final render, something (algorithm) or someone (a "helpful" editor) will assume I made a mistake and "fix" the render.

That said, YT seems to get the question right -- for now -- and ignores those values. Then again, I haven't tested that lately...

Conclusion: the external preview "adjust levels from studio to computer" works fine if you are dealing with standard video source material, but not a mix of video and stills. In that case you need to make them match each other, and applying a computer to studio levels change to only photos appears to do that. Output is WYSWYG- you can see the shadows being blocked up on the external display.

Yes -- but beware your FX behavior. In this workflow, for example, curves will not map black to black, as you'd expect, because black = 16, and any applied curve will push that value higher or lower accordingly (I think this is in part why so many people using Vegas find themselves having midtone contrast issues).

ALO wrote on 2/23/2020, 9:47 AM

Just trying to make sure I understand the difference, if any, between what Vegas calls "studio levels" and the REC.709 spec.

The REC 709 and Studio RGB Luminance levels are one and the same. For the record, those levels are 16-235 RGB (Y').

Studio RGB = REC 709 = B.T.709 = ISEC 601 (SD) = REC 2020 (8 bit) = YUV = Y'CbCr = Apple Limited Range = yuv420p (vui) = NTSC Broadcast Luminance = 100 IRE = 740 mv p-p (analog voltage)

Here is the answer I respectfully gave you in my first reply. Have a Happy Learning Curve.

REC 709 (=Studio RGB) is a universal video production standard used for almost all mainstream media delivery.

 

Yes, luminance is the same. But I'm guessing there's a reason Vegas calls it "studio levels" rather than REC 709. Maybe the gamma or the color space don't exactly match? I assume studio is sRGB (or maybe full range RGB with no associated color space -- who knows??) with a levels correction only, but as far as I know, the two look identical.

Musicvid wrote on 2/23/2020, 10:16 AM

You seem to be asking the same questions and expecting different answers.

Welcome to the forum.

 

fr0sty wrote on 2/23/2020, 10:32 AM

I believe the answer phrasing ALO seeks is:

The color spaces are identical.

The transfer function (gamma) differs slightly.

Going by Musicvid saying the luminance levels are the same, and knowing the colors are the same as well, including the white point, it can be safely assumed that srgb = rec709.

More reading: https://www.w3.org/Graphics/Color/sRGB.html

Musicvid wrote on 2/23/2020, 11:32 AM

The Levels fx, as it's name implies, affects only the luminance and chroma in video processing. Things like gamma, matrix and primaries are not directly addressed.

sRGB is an ICC Profile, mostly used for matching monitor displays, printers, and scanners. It goes beyond a video color space because it deals with reflective, as well as transmission light energy. It does not talk directly to your video encoder, but may be accessed when decoding stills, I'm not sure.

REC 709 is a Color Space standard for everything from DVD to the internet to broadcast. It includes luminance and chroma endpoints identical to Studio Levels, and also information about color primaries, reference points, matrices, other standards, and a whole host of other stuff.

Three different jobs, three co-independent team players, all working from the same manual. Like having a good lawn crew --

if it works, don't mess with it.

Kinvermark wrote on 2/23/2020, 12:08 PM


Conclusion: the external preview "adjust levels from studio to computer" works fine if you are dealing with standard video source material, but not a mix of video and stills. In that case you need to make them match each other, and applying a computer to studio levels change to only photos appears to do that. Output is WYSWYG- you can see the shadows being blocked up on the external display.

(Side note: is it okay that I am not doing anything to limit the video levels upon output? YouTube and the media player seem fine with this.)

@RogerS

Yes. So applying a blanket full range to limited range conversion to all the photos is one valid way of working, but you may still need to make a colour grading pass to make the photos look "good." You could, for example, have two photos at different exposures, and the levels change makes one look good and the other look bad - that's one reason why people get so confused.

The solution, of course, is to grade to your scopes and monitor, both set up for 16-235 range.

Theoretically, you can use either full range or limited , but it is really easy to go wrong, so setting up project, scopes, and monitoring for 16-235 rec709 is the safe, tried and true way as @Musicvid has suggested.

 

 

 

 

marc-s wrote on 2/23/2020, 5:20 PM

"Yes, luminance is the same. But I'm guessing there's a reason Vegas calls it "studio levels" rather than REC 709"

Studio Levels = what most video cameras record at (black is 16 and white is 235)

Computer Levels = Computer monitors, programs etc. (black is 0 and white is 255)

By check marking the convert box in secondary preview you're telling Vegas to view studio level footage properly in a computer level environment.

 

RogerS wrote on 2/23/2020, 10:54 PM

Very interesting thread, thanks for the discussion. If working in computer levels causes some Fx to not work as expected, I think I'll go back to my default way of working (16-235 rec709, and applying a levels correction to video upon export).

I remember struggling with this 10 years ago looking at posts from Glenn Chan and Eugenia. I'm amazed users still have to deal with this in 2020!

marc-s wrote on 2/24/2020, 12:59 AM

That's odd I've never noticed the checkbox causing any issues with certain effects. All it's really doing is applying an output correction to the view mode so you can see how it would look in a proper video pipeline. People that use the video preview screen only (not secondary preview) sometimes temporarily apply a levels effect for the same purpose and then remove it before rendering.

With that said I'm mostly a Resolve user these days so perhaps I have not come across the issue with certain effects causing problems in newer versions.

marc-s wrote on 2/24/2020, 10:57 AM

Note: I remember Vegas sometimes interpreting certain DSLR cameras as computer levels in the past (only program I've experienced this on) which is why you always want to use your waveform monitor to see what's going on with the footage. It does the same thing with still images.

This screenshot shows video (studio levels) and the waveform. Notice black is at the bottom 0 IRE line.

This screen shot shows a still image (computer levels). Notice the waveform bottom going into super black territory and whites going above 100 IRE. Most programs do not do it this way as they convert the still to studio levels. To fix I will put a levels filter and raise the black while lowering the whites.

ALO wrote on 2/24/2020, 1:46 PM

Very interesting thread, thanks for the discussion. If working in computer levels causes some Fx to not work as expected, I think I'll go back to my default way of working (16-235 rec709, and applying a levels correction to video upon export).

I remember struggling with this 10 years ago looking at posts from Glenn Chan and Eugenia. I'm amazed users still have to deal with this in 2020!

Roger I think you read me backwards. I meant to say the FX all work predictably in the 0-255 environment.

As an alternate workflow, if you convert everything on your timeline (video included) to computer levels, preview in computer levels, edit as desired, and then (and only then) apply a master levels correction back to studio immediately before you render out, the curves and levels (gamma) FX behave exactly as you'd expect.

If, on the other hand, you check the options-->preview "adjust levels from studio levels to computer..." box, your secondary display shows you correct levels, but your FX on your timeline are still acting in the computer levels space. This is relevant in the sense that a curves manipulation doesn't remap the levels in the way you'd expect. Black ought to stay black, for example, but instead (if for example you brighten your shadows) black gets remapped to gray.

Probably most people edit, as you do, by checking the box and never realize the curves behavior is wonky; they just boost overall contrast to compensate.

I miss hearing from Glenn. He was one of a very few who seemed to actually understand how Vegas works. :)