Rec.ommendations for Display Gamma

Rec.ommendations for Display Gamma

May 12, 2013

Old news really as this white paper from the International Telecommunications Union (ITU) standardizing gamma for reference monitors in HDTV was released in March 2011! But the topic of Gamma Correction as it pertains to Display Gamma is one that comes up constantly in conversation with colleagues in both post and acquisition. So much so that as I was combing through old NegativeSpaces articles detailing monitor calibration, I felt that a post on Gamma for broadcast monitors is one well warranted.

The ITU, which is located in Geneva, Switzerland, is the inernational governing body for broadcast, transmission, and television system standards. And like most international bodies, the ITU has no real authority and can only make rec.ommendations, such as Rec. 601 for digitally encoding interlaced analog video signals, and Rec. 709 for HDTV broadcast which after only a few years of widespread implementation is about to be superceded by Rec. 2020, a guideline for UHDTV, higher than HD resolution television systems (read 4k and beyond).  

So soon? Yep.

Anyone involved with the creation of motion picture content should be aware of Display Gamma as a factor that has a profound effect on the way an image is perceived by the end user. And one that cannot be controlled by the content creator!

No point in reinventing the wheel here as a wonderful knowledge base is readily available at the excellent site, Cambridge in Colour, a learning community for photographers. This snippet from their article provides the best explanation and examples of Display Gamma I've been able to find. 

Here's a link to the full article, Cambridge in Colour - Understanding Gamma Correction. Additonally, their article on Input and Output Tone Curves is an excellent companion to this topic. 

Gamma is an inevitable part of the digital imaging process and can mean different things depending on where in the chain we're looking - file gamma, camera, display, etc. But as we see in the above example, what gamma effects is the midtones. That is - grayscale which is neither black nor white but somewhere in-between. In all optical perception, the separation between midtones and black or white is ultimately what defines the contrast. And as we can see in the most simple terms in the examples above, the exact same image seen with various display gammas will be profoundly different. 

This topic is a doozy and one I have no intention of delving too deeply into. To any academic reading this, they will no doubt find what I've written to be overly simplistic and only glossing the surface of a very complex topic. That's ok. There are plenty of great scientific and very thorough bodies of work already out there on the topic. Being a technician for motion pictures that are primarily broadcast bound, my concern is Display Gamma for HD Television Systems. The problem is the current "rulebook" for HDTV, that is Rec. 709, actually makes no mention of gamma. Because of legacy technologies - NTSC, sRGB, and others - we've arrived at a defacto "average" display gamma of 2.2. Despite this number not ever being specified by the ITU in Rec. 709, it is the display gamma you are likely to find on virtually any HDTV found in the home, on laptop and LCD computer displays, and many tablets and smart phones. And again, this number of 2.2 is a ballpark figure with many devices measuring at odd numbers such as 2.15 for the iPad Mini for example. The long and short of it is Display Gamma is one of the most critical components in digital imaging as it pertains to the end viewer experience and one that's in need of a universal standard. 

Enter ITU-R BT. 1886

(Another way these recommendations are listed is ITU-R BT."#", "BT" denoting "Broadcast Television". "ITU-R BT.709", reads "Radiocommunications Sector of International Telecommunications Union Recommendation for Broadcast Television #1886", the truncated version being BT. 1886 or simply Rec. 1886.)

Why this new recommendation? (from the white paper)

"This Recommendation specifies the reference electro-optical transfer function (EOTF) that the 

displays used in HDTV programme production should follow in order to facilitate consistent picture 

presentation. The reference EOTF is specified as a simple equation, with exponent function, based 

on measured characteristics of the Cathode Ray Tube (CRT)."

Because...

a) that reference displays play a crucial role in television programme production as they are used as reference for picture presentation; 

b) that the characteristics of reference displays should be unified to ensure consistent picture presentation of programmes produced for use in broadcasting; 

c) that historically, picture presentation characteristics were determined based on Cathode Ray Tube (CRT) characteristics, and the opto-electronic transfer characteristics were implicitly based on the CRT physical characteristics; 

e) that CRT reference displays are no longer available; 

f) that the electro-optical transfer function (EOTF) of CRT displays differs amongst manufacturers, amongst models, and amongst regions, as well as varying with the settings of contrast and brightness; 

g) that for the consistency of picture presentation, it is desirable that newly introduced display technologies have an EOTF that closely matches that of the CRT; 

h) that the reference EOTF for a non-CRT display has not been defined in any ITU-R Recommendation; 

j) that Recommendation ITU-R BT.709, provides specifications for the opto-electronic transfer characteristics at the source, and a common electro-optical transfer function should be employed to display signals mastered to this format

Buried in Annex 1 (page 2) is this very important bit of business -

γ (Gamma): Exponent of power function, γ = 2.40

As of Rec. 1886, the ITU is now recommending a Display Gamma of 2.4 for reference monitors in HDTV Systems.

But what does this mean for acquistion and post production where our work (for now) will almost universally be viewed on 2.2 displays?

The reality is, if you're monitoring and mastering at 2.4, contrast in these images will always seem slightly lifted on 2.2 displays. So for example, if the room we're coloring in has a Sony OLED using a gamma of 2.4, then our deliverables will look quite a bit different when seen on our client's 2.2 gamma iPad or MacBook Air. Rec. 1886 was needed but is far from universal implementation. Ask many a technician in both the field or in post if they had even heard of it and you would likely hear "no" more often than not. My own personal rule of thumb is that in situations where I'm handling the deliverables myself and those deliverables are for 2.2 displays, I set my monitors to 2.2. When I'm working with a facility and a colorist whose monitor is set to 2.4, then my monitors are 2.4. It's very important to be on the same page with whoever gets the stuff next and it will be different from project to project. The Sony OLED's fortunately have several choices for User Display Gamma which have made these monitors indispensable tools in today's digital imaging workflow chaos. 

At the end of the day, Rec. 1886 is just that, another rec.ommendation, but a step in the right direction.

UPDATES:

My Wireless HD Video article (Cutting the Cord) has been updated with several more systems as per reader suggestions. I got great feedback on this post and the content was improved because of it. Thanks for that. I truly appreciate it and makes the hours I spend researching and writing a blog post totally worth it. 

The new site is coming along great albeit terribly slow. Working TV hours, I have to be realisitic about just how long these projects take. That said, If I can go live with the new version before 2014 shows up, I'll be feeling great. I've been writing this site in the form of HD Cinema on Blogger since 2007 and on this domain since 2010. For better or for worse, I can't shake this project and it's something I'll probably continue to do for as long as I'm involved in this business. Looking forward to getting its next iteration out there. 

Sony OLED Calibration part 1

Sony OLED Calibration part 1

April 22, 2013

Some time ago I wrote a blog post on how to correctly white balance your monitor and how essential this is to a calibrated viewing environment. I would go on to say that this is the most important component in any set to post workflow as color correction data generated on monitors that aren't displaying 100% chroma free white, is in fact for practical purposes, useless. 

So how does one determine if their monitor is displaying white that is truly white?

First a recap -

When we are calibrating a HD monitor, we are adjusting it so that 100% white is reproduced as completely neutral and chroma free within the Rec. 709 color gamut. 

View fullsize

Rec. 709 Gamut

Rec. 709 is the standard color space for HD images. It specifies a white point at D65, 6500 degrees kelvin. If this white point is placed correctly, it should ensure that all colors and grayscale within the gamut are accurately reproduced. If it does not, then there are calibration issues that cannot be resolve through a white balance adjustment alone. When we white balance a monitor we start at D65 and then adjust Red, Green, and Blue gains to push 100% into the correct target white point in the gamut. As is exemplified in the above graphic, this is represented by a two dimensional chart with coordinates. For LCD's, CRT's, and other legacy displays the coordinates for white within Rec. 709 are x .313 y .329. It turns out however, these targets aren't well suited for today's OLED monitors.

Sony's Trimaster Series OLED's, both BVM and PVM, have proven to be excellent displays. The BVM E and F panels are reference grade, facility-level monitors that also perform very well in the field. The PVM's, with some calibration, are also quite accurate and for many users have proven to be adequate. Please note that the gain and bias adjustments are much more coarse in the PVM monitors than in the BVM's which is to be expected for a product that is half the price. You can get these monitors very close to their calibration target but if 100% is required, then the BVM monitors are a better tool.

When these monitors first became available there was a lot of misinformation swirling around about how to best calibrate them. They were supposedly "perfect" right out of the box. Later it was recommended that they be aligned to the traditional targets for CRT's and LCD's of x .313 y .329. Turns out neither of these were correct. 

Two points - I always felt there was a strong yellowish green cast in all of these Sony OLED displays right out the box. When aligning them to the targets of x .313 y .329, resulted in a cleaner white but one that was still just a touch warm / yellow-green. Apparently Sony came to similar conclusions so they are now recommending the following targets for all of their OLED monitors:

x .307

y .318

Y 100 

These numbers are based on the 1951 Judd Modification Color Matching Function aka "Judd-Vos CMF". Sony has released an informative White Paper on the topic >>>

This modification is to address the persistent problem of Metamerism Failure. Flanders Scientific Instruments produced this excellent video explaining it.

These Judd modified targets of x .307 y .318 really helped me in aligning my BVM monitor to facility displays and then closely matching PVM and now PVM-A monitors to the BVM as well. There are so many of these OLED's in use now, it's with Sony's permission I'm making this information public to the benefit of anyone doing color critical work with them. 

USING SONY'S AUTOMATIC WHITE BALANCE SOFTWARE FOR OLED MONITOR CALIBRATION:

Using the i1 Probe or one of several other spectrophotometers, Automatic White Balance software, and the Judd targets; you can get your OLED monitor reproducing an extremely neutral gray scale. This will make anyone on the set who is referencing it confident in what they're seeing and will ensure that any color correction data made with it will be useful. Is a calibration using the i1 probe as accurate as one done with a facility level colorimeter? The answer is no but you may be surprised just how close you can get with this relatively inexpensive tool. 

Download the software for free >>>

You will need two test signals to do this, 100% white and 20% gray. These can be made in Final Cut Pro easily. To create 100% white, generate a white field and looking at the RGB Parade, all 3 channels should be hitting 100 IRE exactly. If they are not in equilibrium, there is chroma contamination in the video. The process is the same for 20% Gray, generate a gray field and adjust the Luminance level until the red, green, and blue channels in the parade hit 20 IRE perfectly. 

Abel Cine has a great primer on how to do this process automatically but it can also be done manually, with the following steps.

1. With USB, plug the probe into your Windows machine and run the Sony software.

2. With the calibration cap (solid, light-tight black cap) firmly on the probe, select the X-Rite i1 from the probe choices in the software. If the probe and computer are communicating, in a moment the CALIBRATE button will become available. Hit it and your probe is now calibrated and ready to read. 

3. Remove the cap and put the probe in the cradle that allows it to hang flush on the surface of the monitor. Leaning the monitor slightly back will help gravity hold the probe in place. 

4. Send 100% white test signal into the monitor via HD-SDI. In the software, you will get a reading for x and y, which is color temperature and Y which is Luminance, or white level measured in cd/m2 (candela per meter squared) aka "nits". All of these variables are intrinsically related so affecting one will likely affect the other. Hitting all targets requires a good deal of back and forth adjustment.

5. Using your monitor's CONTRAST control, adjust until Y hits 100 nits or as close as possible. 

6. Using RGB Gains, you will now affect the x and y point. You shouldn't need to adjust Blue unless your reading is way off from your target. Red gain adjustment largely affects x and Green gain adjustment largely affects y. Adjust your gains until you hit the recommended targets of x .307 y .318. It's likely that doing so will lower your Y reading so adjust Contrast again to hit Y 100. If this affects your x y reading, adjust Red and Green gains accordingly. Rinse. Repeat.

BIAS:

Bias affects the dark tones of the picture. Just as the case with white, we want dark gray and black to be reproduced completely neutral and chroma-free. Conveniently, our x and y targets for bias are the same as for gain, x .307 y .318. Y however is obviously a different level and and one that is dependent on what gamma we're using. In my experience with Sony BVM OLED's, once you get the white point aligned, usually Bias adjustments will be minimal. The PVM monitors are a bit trickier and it is difficult if not impossible to totally hit x .307 y .318 on them. You can dance between gain and bias adjustments and get your targets very close but because the adjustments are coarser than on the BVM displays, 100% alignment does not seem to be possible on them.

1. Send 20% gray test signal to the monitor via HD-SDI. 

2. If you're using gamma 2.2, your Luminance target is 2.7 cd/m2. For 2.4 gamma, use  2.4 cd/m2. And for 2.6, 2.1 cd/m2. If you're not hitting your target, adjust BRIGHTNESS until you're there. Then if necessary, adjust Red and Green Bias until you hit x .307 y .318.

3. If you had to do a lot of adjustment here, go back and double check white. If you start swinging things too far in Bias, you'll likely affect Gain and visa versa. On a BVM, the adjustments should be pretty minimal. On a PVM, there may be more of a dance to find a satisfactory level of calibration. 

FOOTNOTE:

Sony PVM's ship overly saturated and with a white point around 170 nits which is substantially brighter than what you would typically find on a monitor used for critical evaluation. I've found these displays require a good deal of calibration but using all of the available controls - chroma, brightness, contrast, RGB Gain, RGB Bias - and hitting as close as possible to Sony's new recommended targets, you can produce an image that's remarkably similar to what you find in the BVM series monitors. Not 100% but dare I say, "good enough". 

THE CASE OF "A".

There is a big problem with these OLED panels and that is they have a brutally unforgiving viewing angle. There is so much color temperature shift when you're off axis that the only way to safely view is to sit right on top of them. It's a real problem when you have three or four people crowded around one monitor as each viewer will see something different. About 18 months after the initial release of the Trimaster OLED series, we now have the updated "A" line which greatly improves this viewing angle problem.

RELATED ARTICLES:

HD Monitor Calibration - White Balance and Color Bars

Sony OLED Calibration part 2

DDC?

DDC?

Digital Dailies Colorist. 

In the January issue of ICG Magazine, this short statement by cinematographer Paul Cameron, ASC caught my eye..

Soon after I began hearing rumors of movement towards the establishment of a new union classification, something like an "On-Set Colorist", to distinguish the task of creating color corrected dailies from the traditional role of the Digital Imaging Technician (DIT). 

Here we are in April now with IATSE Local 600 elections around the corner and I was very surprised to read this mailer in email tonight - 

To CAMERA GUILD/ Local 600

Brothers and Sisters,

My name is Paul Cameron. I am a Western Region Candidate for NEB Director of Photography & Visual Effects Supervisor.  I have been a Director of Photography and member of the Camera Guild for over 20 years.  Recently I had the opportunity to testify on behalf of Local 600 in a closed deposition aimed at reducing rates for First Assistant Cameramen.  It reminded me that decisions are made every day to challenge our Union and its Members.  That now is a time to be focused and aware of our future. That now is the time for all members including myself to participate in our Union .   A union that provides the Film Industry highly talented and qualified craftsmen and women.   A union that secures our rights in the work force.  Secures our rates as Technicians.  Provides its members options for Health benefits and secures a plan for Retirement funds. 

It became apparent to me at the last Board Meeting that all the issues that came to the floor were vital.  Being a board member means looking at the issues we face as a union in an environment that wants organized labor to vanish.   It's also a time of great change with the advent of Digital Capture.  A time where technology is changing exponentially.

One of the issues I would like to address immediately is the re-classification of DIT - Digital Imaging Technician to DDC - DIGITAL DAILIES COLORIST.  The responsibilities of the DIT have changed dramatically over the last 10 years.  It's my firm belief that the position of DIT was created to help bridge the transition and development of new technology in Digital Capture. It was a time when the DIT matched digital camera bodies during the check out and painted the cameras on set while wrangling the endless amounts of new cables and conversion boxes.  The DIT became this hybrid Electronic Engineer/Camera Assistant.  Cinematographers and Camera Assistants were relying on DIT's to handle Camera settings and make decisions regarding how images were captured.  Then almost overnight Cinematographers lost the Off Set Dailies paradigm.   Dailies were no longer sent to be transferred or graded by labs or Telecine facilities.  The DIT became the On Set Colorist.  Now Cinematographers are relying on DIT's to handle all Color Timing and Transcoding of Dailies with those looks applied.   The job of the DIT has shifted.  Producers are aware of the change but confused what a DIT really does.  The DIT color corrects and transcodes all Dailies at this point.  That is certainly the present + future of that position.    Camera Assistants need to stop relying on DITS for Camera Settings and Reloads.  Traditionally the Camera Assistant never let anyone touch much less change a frame rate or shutter angle on a Camera.  It is my belief Camera Assistants need to reclaim that space.  Part of that means additional training for Camera Assistants on current Digital Capture Systems.  Current DITS’s also need further training in Color Correction as well as Data Management.   Establishing the best Digital Workflow is the responsibility of the Cinematographer and the DIT.  The DIT is now responsible for helping establish and securing Cinematographers looks through the Metadata/Digital Pipeline.   Once Producers + Producers understand DDC -Digital Dailies Colorist they will value that position more than ever.  Once there is an honest acknowledgement that DDC's Color Correct all Dailies from Cinematographers there will be a new found respect for that position.  The DDC's are also integral in interfacing with Production and Post Production in the ever-changing world of constantly changing Digital Workflows and needs in Deliverables. There is also the opportunity now to welcome into Local 600 the best Digital/Telecine Colorists from Labs + Post Production facilities and make them union members. The DDC can also be a position in Post Production facilities and Studios can hire to ensure Timed Dailies and Work Flows are being handled correctly in Near + Off Set Labs.  I hereby advocate the transition of DIT to DDC.   Regardless of whether I am asked to serve on the Board or not I plan to push this through.  This is one example of a few changes I will try to bring to the Board.  We need this re-classification as soon as possible.

This would be the first time I serve on the board. It would be an honor to represent and be a voice for each and every member.

Thanks for your consideration. Vote now and send your Ballots in.  Your voice counts.  Every small step or action you take means something.

Paul Cameron, ASC

Candidate -Western Region -  NEB Director of Photography & Visual Effects Supervisor.

To anyone actively working as a Digital Imaging Technician, the constantly evolving nature of our job description is something that keeps the work very interesting but can also be nerve wracking as the future of the position is clearly so tenuous. What I take from this piece is that we need more standardization among our ranks. The reality of so many independent owner/operators offering such a helter skelter variety of services at completely arbitrary rates, has done little more than put a gigantic question mark in the brain of many producers as to what a DIT is, what a DIT does, and most importantly - why do I need one? 

The problem is that two distinct skill sets have emerged - video and computer. There are many talented operators out there who excel at both and can effortlessly jump back and forth between doing a live color correct for four cameras one day and then handling Codex deliverables on a workstation on another. But then there are those who really better suited for either one or the other because of their background, interest, conviction about what the position "is and isn't", etc. The problem is obvious. In the eyes of a producer, we are all the same because we are all DIT's. Is there are a 1st AC out there who doesn't pull focus? Or one who only builds the camera? No matter what your opinion is on this sensitive subject, I think we can all agree we have a "brand" problem as a union classification. 

What do I personally want to happen? It doesn't matter. The market will decide. I, like everyone else, just want to keep busy and for the position to remain viable. Whatever it's called. 

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