HD Monitor Calibration - White Balance and Color Bars

HD Monitor Calibration - White Balance and Color Bars

February 12, 2012

This post is in regards to HD Rec. 709 monitor calibration only. There are several issues relating to standard definition video and monitoring that do not apply to HD.

1. NTSC Setup, or 7.5 IRE (%) Black Level. Setup is for standard definition only. The black portion of HD test signals hit 0 IRE (%) on the waveform. 

2. Phase: There is no Phase control for digital HD monitoring. Only CHROMA (saturation) affects picture as Phase relating to monitoring is an analog issue only. 

COLOR BAR TEST SIGNALS:

Many cameras and recording decks generate color bars - either the HD SMPTE (Society of Motion Picture and Television Engineers) version, which is this:

SMPTE HD

Or the newer HD specific version, ARIB (Association of Radio Industries and Businesses):

ARIB

Correct use of these test signals will help you properly set your HD monitor's Brightness (black level), Contrast (white level), and Chroma (color saturation level). The process of using either of these color fields is virtually identical. While color bars will help you to setup aspects of your monitor, they do nothing to reveal whether or not it's calibrated, meaning it's accurately reproducing colors and a neutral chroma-free gray scale. This is the most critical component of monitoring and is accomplished through a White Balance Adjustment.

Part 1: Monitor White Balance

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 / color space

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. As is exemplified in the graphic above, this is represented by a two dimensional chart with x and y coordinates. For LCD's, CRT's, and other legacy displays the coordinates for white within Rec. 709 are x .313 y .329

These adjustments are made using a spectrophotometer or colorimeter aka "probe". The process of white balance adjustment with a probe is similar from monitor to monitor. Some can use a probe to do this process automatically whereas others must be done manually.

Abel Cine has a great article on how to use this hardware / software combination to White Balance your monitor >>>  

An overview of the process, first input a 100% white test signal into the monitor and then use the probe to objectively measure the screen. It will tell where this white image is hitting in the gamut by way of x and y coordinates. From here, RGB gains are adjusted until the probe verifies that white is hitting the correct coordinates, x .313 y .329. If you're using an OLED monitor, you will use different coordinates than x .313 y .329 and you may need to adjust Bias as well to compensate for chroma cast in the dark tones of the picture. This process is very similar to adjusting White Balance, just using 20% neutral gray instead of 100% white. 

The most inexpensive way to do a White Balance Adjustment on your monitor is with a free software from Sony used with the i1 Pro and i1 Pro 2 probes. These aren't the most accurate probes available but they are well priced and I've had very good results aligning and matching displays with them. 

Use the probe / software and adjust RGB Gains and Bias to hit the correct targets for both 100% White and 20% Gray. Color temperature is denoted by "x" and "y". Luminance level is "Y". 

100% White (Gains)

x .313

y .329

Y 100 (studio level)

20% Gray (Bias)

x .313

y .329

Y 2.7 (gamma 2.2)

The x and y points will always be the same for both Gain and Bias. The Y level for Bias will change with the gamma setting be it 2.2, 2.4, or 2.6. Consult your manufacturer!  

AN IMPORTANT NOTE ON MONITOR GAMMA:

Rec. 709 doesn't actually specify a gamma but the de facto standard is 2.2. The newer Rec. 1886 specs gamma at 2.4. This topic is beyond the scope of this post so I've written a separate article on it. Rec.ommendations for Display Gamma >>> For simplicities sake, this article assumes we are working at a gamma of 2.2

Once we're successfully white balanced and selected gamma, we can now use color bars to finish the calibration process.

Part 2: Setting Brightness, Contrast, and Chroma Level with Color Bars

AN ANALYSIS OF THE SMPTE HD COLOR FIELD:

The components of this test signal are 75% Contrast Color Bars (Yellow, Cyan, Green, Magenta, Red, Blue), 20% Blue Chip, 10% Purple Chip, 75% Contrast White Chip, 100% Contrast White Chip, 0% Black chips, and The Pluge. When using this test signal to set Brightness, Contrast, and Chroma, all you really need to concern yourself with are the 75% Color Bars, 100% White Chip, and Pluge. The 20% Blue Chip and 10% Purple Chip fall along the IQ Line on the Vectorscope for verification that the color information in the test signal is accurately centered on the scope. 

The Pluge will help you set your Brightness (Black Level). It consists of a -3.5% (IRE) chip on the left, 0% chip in the middle, and 3% chip on the right.

Here are SMPTE Color Bars with lifted blacks so you can see the pluge better:

Here's the same signal's luma waveform. This helps to see where everything in the field is hitting in terms of level, particularly the pluge. 

And Vectorscope. You can see that each of the color bars lines up perfectly with thier targets indicating that these are pure, undiluted primary and secondary video colors. If this was a 100% contrast color field, the vectors would land perfectly in their little targets; R = Red, Mg = Magneta, B = Blue, Cy = Cyan, G = Green, Yl = Yellow. 

SETTING BRIGHTNESS, CONTRAST, AND COLOR WITH HD SMPTE BARS:

1. Set your gamma to 2.2 and White Balance to D65 (or User if you have done a custom White Balance Adjustment). Set your Brightness, Contrast, and Chroma to their default levels. If you have a display that can be custom white balanced, make sure that you're aligned before starting this. If you're working with a facility on a project, they can send a technician with a probe to you and he can do it for you. Or you can talk to you manufacturer and get a list of recommended probes for use with your display along with x,y (Color Temperature), and Y (Luminance) targets so you can do it yourself. Once you're there, send the HD SMPTE test signal to your monitor via HD-SDI. This is best done in a dark environment so if you're outside or in an unshielded location, try and keep as much ambient light and direct light off the display as you can. 

2. Everyone has their own way of doing this. I start with Contrast which is the most subjective. Looking at the 100% white chip, turn the contrast up until it visually stops getting any brighter. Now back it off a little bit. This will be different on every display and really the smart way to do is with a probe that reads Luminance level. 

3. By default, increasing Contrast will also somewhat raise the black level. Now use the pluge and set your Brightness so that the left (-3%) pluge chip disappears into the surrounding 0% black field. The right chip which reads 3.5% on the waveform should be just barely visible. 

4. Now check your contrast again. Is the 100% white chip still hitting peak white? If you need to adjust, make sure to go back and check your pluge again. By going back and forth between these 2 adjustments, you should be able to arrive at a satisfactory black and white level. 

5. Now check Chroma. Your monitor most likely has some sort of Blue Only feature. This is used to help you correctly set your color saturation level with Chroma. Turn it on and have a look. 

If it looks something like this, you're in good shape. 

If you're looking at something like this, then the Chroma level is incorrect.

Adjust the Chroma level on the display until you're looking at solid, alternating bars of equal value. The larger top portion of each individual bar needs to blend into the smaller section beneath it. 

6. If you find that you have to make some adjustments to Chroma, this could up slightly changing your overall contrast so turn Blue Only off and check the pluge and 100% white chip again. By tweaking back and forth between all of these adjustments, you will be able to find the most accurate settings your monitor can produce. Please note that some monitors don't have Blue Only but can display in Monochrome. The Chroma calibration process with Monochrome is identical to Blue Only. 

At the end of the process you should be looking at something like this:

AN ANALYSIS OF THE ARIB COLOR FIELD:

This test signal has a greater variety of components than it's SMPTE relative. It contains the same 75% Contrast Color Bars, with the addition of neutral gray chips at various luma levels, and pluge with more steps (-2%, 2%, and 4%) that I suppose offer a bit more finesse for setting black level with Brightness. 

Here's a handy diagram detailing what's what in the ARIB:

Here's the luma waveform of the signal. Note the 0%-100% gradient that ramps through the middle of the field and the pluge at -2, 2, and 4%.

SETTING BRIGHTNESS, CONTRAST, AND COLOR WITH ARIB BARS:

Though it looks radically different, on principle it's the same as the SMPTE. Follow the exact same steps outlined above when using this signal to arrive at correct Brightness, Contrast and Chroma levels. 

The biggest difference is with the pluge. When using this signal, the middle (2%) and right (4%) pluge chips should be barely visible with the the -2% chip blending into the surrounding 0% black field. 

When setting Chroma with Blue Only, this is what you should be looking at with the correct level:

And when you're all done, this is what you should be looking at:

SMPTE 100% COLOR FIELD:

Some recording decks will output these but very few cameras will. I don't think this is as useful a calibration signal as there's no pluge to help you set black level. It's good for checking saturation on displays or if for some reason you needed to check a 100% signal in a video system. This field contains the 3 primary video colors (Red, Green, and Blue) and 3 secondary colors (Yellow, Cyan, and Magenta) at 100% contrast along with a 0% Black Chip and 100% White Chip. 

On the vectorscope at Gain x2, you can see the colors hit their 100% targets spot on.

When looking at this color field with Blue Only and if your Chroma level is set correctly, you should be seeing something like this - bars of equal and alternating value:

RELATED ARTICLES:

Sony OLED Calibration part 1

Sony OLED Calibration part 2

Rec.ommendations for Display Gamma

Great Explanation of Color Matrices

Great Explanation of Color Matrices

May 11, 2011

I don't usually re-post the content of others on this site but occassionally I come across a gem that's so in the nerd spirit of Negative Spaces that I feel obliged. Art Adams, author of the supremely useful technical blog, Stunning Good Looks, shares some thoughts on how a DSC labs Chroma du Monde chart works with your video camera's color matrix. Probably the most succinct and understandable explanation of video color response yet. 

Art Adams from ProVideo Coalition on Vimeo.

Alexa Log C - Workflow Options

Alexa Log C - Workflow Options

This post is intended to be a work in progress so I'll be adding more information over time as workflows evolve. I'm intentionally leaving some of this open ended because I'd like anyone reading this who can speak to the topic to help me flesh this out. If you have something to add, please shoot me an email. 

These past few months, it's been all about the Alexa. Now that there's a healthy amount of them in the rental market, they're popping up all over the place. I can't speak for other people but for me, all of the jobs that would have been RED three months ago are now Alexa. What happened? 

First of all, the images are just stunning. In my opinion, for now, the Alexa is the digital option that really stands up to motion picture film. Just like color negative, there's no harsh and sudden clip in the white, instead the highlights gently and elegantly roll off. There's tons of picture information in the shadows and the noise signature looks more like pleasing film grain than crummy video gain. The color response is faithful and natural. And it's unbelievably simple to work with. Arri really has done an excellent job with this camera and commercial production has been quick to embrace it. 

There's a lot of discussion out there as to the best way to work with it - Log C, Rec709, and as of a few days ago, Arri Raw. So what's to be gained by shooting in Log C over Rec709? Really the answer to that question very much depends on how much time and money is available for post production. The out of the box Rec709 video on the Alexa actually looks very good and has ample dynamic range so if there's no time/money for a grade, then the WYSIWYG video is certainly a viable option. The fact is though that Log images encoded to a high quality RGB recorder are always going to give you far more flexibility in the online suite. And now with 2k Arri Raw there's even more flexibility and more resolution than the current 1920x1080 raster. As this newest option is only now available and I haven't worked with it yet, I can't speak to it in this blog post. Instead I'll just be listing a few options for live color correction of the camera's Log C video output. 

Unless you have a lot of experience with Log space, it can be difficult to evaluate exposure because there is very little contrast in these images. The nice thing about Log though is that it's a conservative format and bad exposures are actually pretty forgiving. Within reason (more on that later). But you still need to ensure an overall baseline for consistency and continuity. To do this, savvy productions would use some form of viewing LUT (Lookup Table) or in other words, a color space transformation of the Log stream to non-destructively put it into a more normal range of contrast and color saturation. 

I really like this simple definition of a LUT that my friend Adrian Jebef came up with - 

"..a LUT (Lookup Table), in image processing, is used to transform input data into a more desirable output format. Turn X to Y. Grey to Black. Or Grey to White."

And that's it in a nutshell. To really get into the meat and potatoes of color space transformations is way beyond the scope of this post so if you're looking for more information, Steve Shaw ofLight Illusion published some articles on the topic that answer a lot of questions. 

Using LUT's

1D vs. 3D LUT's

Linear vs. Log

Technical Papers (the DI guide is particularly useful)

Using a LUT on-set not only makes evaluating exposures easier but it also makes the production feel much better about what they're shooting. Who wouldn't rather look at a colorful, contrasty image than a flat, murky one? To me this is kind of a no brainer - which would you rather the agency see -

this?

or this?

Lookup Table's can range from simple "one light" 1D's to complex 3D's involving multiple secondary grades. And they can serve many purposes both on-set and in post but in my experience, there's no such thing as a one size fits all LUT. Every scene and lighting situation is going to be a little different and require some adjustment. After you've established your exposure baseline, you needn't make extreme changes but I've found that most shots require a little LUT tweaking here and there. That means that if you think you can get away with one simple LUT for the whole show, that's not really the case. Even a simple "DeLog" LUT that's intended to mildly normalize contrast and saturation can be too extreme in low light situations and too weak for high contrast exteriors. Doing a live color correct on the Log output has the added advantage of it now being very cheap and easy to record the corrected images to a small solid state device like the Ki Pro or Nano Flash. In this scenario, the production walks away with dailies that reflect the intentions of the DP, can be dropped into their edit without transcoding, and with matching timecode can be used to seamlessly return to the Log images for finishing. Everyone wins with this workflow. 

Here's something else that's suddenly entered the picture even as I'm writing this - Arri's LUT Generator. 

The LUT Gen can pump out LUT's for a variety of uses such as on-set DeLog, LUT's for Dailies, and for multiple devices such as Cine-tal monitors, Truelight box, HDLink Pro, Sony SRW-1 deck, etc. This is very a rudimentary tool though and lacks any ability to fine tune. I used it to generate their basic 1D DeLog LUT for HDLink Pro and it's the exact same look as when you're in Log C on the camera and you switch the Monitor output to Rec709 - a little bump up in contrast and saturation. To be honest, it doesn't look so great. I also tried a 3D LUT for Apple Color and I kind of feel like most colorists would prefer to just do their own work rather than use some cookie cutter preset like this. These "auto LUT's" could be useful for quickly baking a look for dailies but again, I would prefer to use LUT's that were generated on-set for this purpose because those images will be a far better representation of the production's intentions. 

As for the on-set LUT's from the Arri LUT Gen, it can't yet make a file that can be uploaded to the camera. According to Arri's release notes, the camera does not yet support the ability to upload "Looks" to it via SD Card anyways. I believe eventually this functionality will be present and you'll be able to jog through any number of LUT's you've created, one of which will probably get you about where you want to be in terms of luma and color response. Once that's implemented I imagine that will be a far more common way of monitoring Log C on-set.

Until that happens, here are a few other ways of doing it - 

HDLink Pro + LinkColor

Blackmagic Design's HDLink Pro $475, LinkColor software $500. This is by far the most cost effective way to do on-set color correction and LUT generation. All you have to do is intercept the HD SDI signal to the HDLink Pro and then use the software's intuitive sliders to paint the image. Total price $975 excluding the host computer. 

What I Like -

-The ability to load a Pre-LUT or DeLog. With this you create your baseline level of contrast and saturation and from there it's just a matter of making little tweaks as needed. This way you always have a place to start and return to when necessary. I make my DeLog LUT using a DSC Labs Chroma du Monde chart in the light that's being used for the scene. This way you always know where you've placed black, the middle gray crossover point, and white so you have an objective range of values to base exposure on. 

-Global Presets - It's so easy to recall and keep track of your work with these. It's also great for flicking through Looks with the DP. 

-Bypass Button - With the click of a button, you can turn the LUT you've created on and off. Very handy for evaluating the Log image against your correction. 

-User Matrix - This is a great feature that isn't present in other on-set color correction tools. This works just like the linear matrix in a traditional video camera with its 6 variables to modify color response R-B, R-G, G-R, G-B, B-R, and B-G. This ties in nicely to the DeLog functionality because when you have your chart up and setting contrast and overall saturation you can also push the colors around to create the desired response with this matrix toolset. This feature is for 3D LUT output only and is not compliant to the ASC-CDL which only allows RGB Lift, Gamma, Gain, and Saturation. 

-For now, LinkColor can export 3D Cubes for HDLink Pro, Truelight .cub files, ASC-CDL, Apple Color, and a .lcg which seems to be proprietary to LinkColor and can be used to import a previous LUT into the software. I've heard that more export options are on the way. 

I'm a big fan of LinkColor because of its speed, utility, and price. Also because it's built around inexpensive hardware, it's nice that there can be a HDLink Pro on the other end at post and all they have to do is load my LUT's and they're looking at the exact same thing we were on-set. Takes a lot of the variables out of the equation. 

Gamma & Density's 3cP + Matrox MXO2 + HDLink Pro

This is another on-set color correction option I've been using lately. The software is $1500 for the "SF" version which is professional but excluding support for the LUTher box and Panavision GDP. This is the version I have and for my needs, it's worked out nicely. The Matrox MX02 LE In/Out Box is about $1400 and allows for 3cP to ingest the incoming HD SDI video. The $475 HDLink Pro then allows you to upload the LUT's you build in the software to transform the Log signal. Of course a MacBook Pro or Mac Tower is also requisite but the price tag on the components alone is about $3375 so again, a realtively cost effective way of building a workflow. 

Check out this video outlne of the process >>>

What I like -

-3cP is a very powerful tool for color management. It's fully compliant to the ASC-CDL which is the de-facto standard in communicating grading decisions between set and post. The software scopes are extremely accurate and the tool set is easy to use and allows for a fair degree of fine tuning. 

-The ability to interface with so many different hardware devices - Matrox MXO, RED Rocket, HDLink Pro, Truelight, Cine-tal, and with the more expensive version LUTher and Panavision GDP. 

-It can export a good variety of LUT formats for various platforms - Nucoda, Pogle, Truelight, HDLink Pro, Scratch, Apple Color, Technicolor DP Lights, ASC-CDL, etc. 

-The automated data management tools are excellent. 

-Monitor calibration and profile building tools.

-Dailies Generation for FCP, Avid, and Apple devices such as iPhone and iPad.

-Tiff and DPX image sequence generation.

-Great support for RED R3D file import and export. I actually prefer the R3D grading tools in 3cP to RedCine-X.

One of 3cP's big objectives is to close the communication gap between set and post and they've added a lot of functionality to eliminate the potential. It can generate these reports (below) that accompany the original media so that post production knows exactly what the DP intended. The objective watermark stamp that accompanies corrected images adds further assurance that what you're seeing is what he/she had in mind. I'd say they've made a pretty bullet prooof system. 

The only thing I'm not too crazy about is I've found the communication between the software and the external hardware to be a little buggy. Maybe it's just my system though. I'm on a MacBook Pro that's about 3 years old. Long overdue for an upgrade. If you stay in the software you're fine but if you're switching between a lot of different applications I've occasionally lost the link between 3cP and the Matrox. Also, a better set of Look Management tools ala' LinkColor or SpeedGrade would be fantastic. The ability to store and quickly recall looks is incredibly useful and is sorely missing in 3cP.

It's really a fairly expansive software and I'm just glossing the surface of what it can do so for a more in-depth description of functionality, please refer to Gamma & Density's site. 

Cine-Tal Monitor + SpeedGrade OnSet

Cine-tal owners swear by them. I've used them a handful of times as a rental item and it's definitely a fantastic display but I don't have enough experience with it really to speak to this popular workflow option. If anyone reading this would like to share some thoughts on this, please email me. 

Technicolor's DP Lights System

"DP Lights™ is an exclusive Technicolor system which can be deployed as a pre-visualization, on-location color correction tool, or used in a unique configuration to drive high quality rear screen projection.

In support of 2D or 3D digital capture, the system enables real-time creation of pre-visualization looks during commercial production. Because the system is fully ASC-CDL compliant, looks established on location flow downstream to any Technicolor dailies facility for dailies mastering and use in the DI Suite. For commercial productions ultimately intended for film distribution, the DP Lights™ system includes integration of on-location film print emulation. It is also possible to record a color-corrected feed during digital capture to a format such as XDCAM -- which is supported by Technicolor’s Dailies on Demand playback server -- or to other formats such as HDCAM."

Filmlight's Truelight System

"Truelight is the world’s only complete film colour management system for pre-visualising film images on electronic display devices. Truelight enables artists, colourists, cinematographers and directors to review, accurately, the final look of the film at any point in the digital post-production process – reducing film-outs, matching and rework, and saving valuable time.

Truelight’s success lies in its innovative, measurement-based process to generate 3D transforms (known as cubes or LUTs) that convert specific colour spaces. And, unlike desktop publishing or broadcast video devices, the Truelight Monitor and Projector Probes are purpose-built instruments for characterising high-contrast motion picture displays.

As a result, a correctly calibrated Truelight system is able to compensate for any variable in the post-production workflow, such as differing lab processes, film stocks and displays, and generate a preview that is identical to the final print or Digital Cinema Distribution Master (DCDM)."

Earlier I mentioned that Log exposures are pretty forgiving. Just how forgiving though? I was curious so I did some chart exposures with a skin tone and then DeLogged them into a Rec709 look. 

This was my process:

I put the camera into 709 gamma and exposed the grayscale crossover on the chart at 55 IRE. They say you should do this at 60 but that always looks a little bright to my eye so I usually do it around 50 or 55. The brightest part on my pasty, white hand hit just under 70 IRE which is fine for this exposure. For the purposes of the test we'll call it an "ideal exposure". 

I then switched the gamma mode to Log C and kept the exposure the same noting that the middle gray swatches hit around 40 IRE which is Arri's recommendation in their Log C specs. When going from Rec709 to Log C, I've noticed that the crossover point on the chart always drops about 10 IRE, Black lifts up to 20 IRE and white hits somewhere in between 60 and 70 IRE. This 20-70 zone seems to be the sweet spot for Log images. 

Next I dropped 3 stops on the lens.

After that I opened up 4 stops from my original key light exposure. 

Now in Apple Color I loaded these 4 files - Rec709, Ideal Log C, Underexposed, and Overexposed and matched the waveforms and vectorscope of the Log images to the Rec709 exposure. 

Here are the results. 

Alexa Log C - DeLog Tests - Over and Under Exposure Correction from Ben Cain / Negative Spaces on Vimeo.

Note the Optimum Log C exposure that's been been DeLogged is super clean and with smooth and subtle gradiations. I actually think that Log C images that have been linearized like this look much better than their Rec709 origin counterparts. 

The under and overexposed images are salvageable but notice how much noise there is the dark tones on the underexposed image. Look at the shadows on the skin and you can see that there just isn't enough picture information there to smoothly render the tones. 

Something strange happened in the overexposed image, it looks very thin and stretched. It's also noticeably softer. When you smash pixels too far one way or the other in this format you just don't have as much separation and while it is salvageable, it's certainly less than ideal. 

Keep in mind that these images were recorded in an early build of the camera when ProRes 4444 wasn't yet available so these were 422 HQ. I'm curious as to how much better these DeLogs would have looked if recorded to higher quality encoder. 

The wonderful thing about shrinking video down to web size is that it hides a lot of nasty problems - as is the case with the video above. It's a little hard to evaluate what really happening with this small picture so here is the original full res media file for you to evaluate >>>

If anyone wants to play around with these files, they're available for download here >>>

That's all I've got on this one.