Painting HD Cameras - Basic Colorimetry

Painting HD Cameras - Basic Colorimetry

© 2009 NegativeSpaces (revised January, 2014)

Technical Notes:

The images used in this article were created with a Panasonic HDX900 and the stills / vectorscope information was captured with a Leader LV 5330 Multi-SDI Monitor. Because a Panasonic camera was used, the workflow presented and menu features explained are those found on Panasonic cameras. The menu set on Sony cameras is similar enough though that I feel that if you know one system, you should be able to apply the same concepts to the other. The chip chart used was a DSC Labs Chroma Du Monde 28R CamAlign Chart. These charts are the standard for video engineering and camera alignment. Because the colors and values are so uniformly printed and tested, they can be measured electronically with repeatable results. Correct use of DSC Labs equipment can not only be used to calibrate and match equipment but to paint custom looks in the controlled environment of a studio. The GAIN on the Vectorscope images was set to x1. When doing critical camera matching or alignment with the Chroma Du Monde, it is recommended that you set your Vectorscope Gain to x2. For the purposes of this tutorial, I felt that keeping the Gain at it's default value of x1 best illustrated how to read the scope in the field and objectively evaluate color saturation.  

Part 1: Overview 

In this article we are going to use the video camera's Linear Matrix (also known as Matrix Table or User Matrix) and Multi Matrix (also known as Color Correction) menus to modify its colorimetry, which is the way the camera renders specific colors within its video color space which for HDTV is ITU-Rec. 709.

ITU-Rec.709 Video Color Space (from Wikipedia):

Please refer to the Wikipedia article for a detailed account of the above graphic. 

On naming conventions:

In most Panasonic cameras, the Linear Matrix is referred to as User Matrix and the Multi Matrix is referred to as Color Correction. In Sony cameras, Linear Matrix is referred to as Matrix Linear and the Multi Matrix is referred to Matrix (Multi). As this is a Panasonic oriented article, from here on out I'll be using the Panasonic nomenclature. 

The combined use of both the Chroma toolset (explained here), the Tonal Response Controls, i.e., Gamma, Knee, Pedestal, and Detail Circuitry is necessary to really "paint" a HD camera. There are a ton of tools at your disposal in a video camera's Paint menu and only through lots of trial and error and the use of a calibrated reference grade monitor can one learn to use them correctly. 

In order for any of this to make sense one needs to know the nomenclature. Colors are referred to as Hues in engineering lingo and Phase refers to the relative position of the color as plotted by a line on the vectorscope. For simplicity, in this tutorial I'll be referring to colors as "colors" and not "hues". For describing a video camera's colorimetry, Japanese engineers have come up with a way of describing the six specific color phase adjustments we can make and those are G-B, G-R, B-G, B-R, R-G, and R-B.

“B-R” is read as: Blue ADDED TO Red which affects all colors MOST NOTICEABLY Blue, LEAST NOTICEABLY Red. A positive value increases the Saturation of Blue added to Red. A negative value decreases the Saturation of Blue added to Red. 

“G-R”, is Green into Red. “B-G”, is Blue into Green, etc. With B-R, you are adding or subtracting Blue into or from the Red channel. G-R, adds/subtracts Green to the Red Channel. You get the idea.

The two colors that form the pair, i.e., B-R, are linked and therefore these adjustments will always change these two colors and often a third or fourth color as well. Generally, all colors are affected slightly, some radically.

These adjustments can be used to make colors punchier or more exaggerated, more saturated or de-saturated, create unique looks, or match one camera to another. 

There is a finer point though –

One might think at first that by adding Blue to the Red channel, Red is going to get Bluer when in fact it has the opposite effect – Adding Blue to Red pushes it closer to its neighbor, Yellow, which makes all Red colors in your image become more Orange because that's what you get when you add Red and Yellow. It's actually basic color theory.

The way changing the Linear Matrix works on HD video cameras is you affect a color by pushing or pulling it into an adjacent color space and all colors are linked to the one opposite them. That said, you can make Red's look more Magenta or more Yellow but you can't turn Red into Green. It just doesn't work like that. 

In video colorimetry, each color has a relationship with both its neighbors and its compliment on the other side of the scope. Really not much different than a color wheel used to teach art foundations. 

If the naming conventions seem complicated, fret not because fortunately this tool set is essentially the same on every HD video camera. They make the visible colors in their color gamut from six primary video colors: Red, Blue, Green, Cyan, Magenta, and Yellow. This differs from traditional color theory somewhat in that there are three primary colors and three secondaries which are made from them. 

As you can see there are six colors represented. In the center of the scale is the “chroma free” zone. This area is neutral and colors close to it are very de-saturated and pastel. You can also tell if an image has been correctly white balanced when the concentration of this neutral picture information is centered on the scale. Colors increase in saturation as the move away from it towards the outer edge of the circle. Colors that leave the circle are “out of gamut” and aren’t broadcast legal. White Balance is an extremely important factor in digital imaging. For colors to appear "correct", meaning colors that look natural to your eye, your camera must be white balanced. White balance is a global control and moves your entire chroma information in between the red/yellow - blue/cyan axis. Creative use of white balance is an important tool at your disposal in artistic camera painting.

You can tell these images are properly white balanced because the neutral information is exactly in the center of the scope. On the left is an image with enhanced color saturation. The vectors are approaching the edge of the circle and the colors are extremely saturated, our Yellow has actually wandered "out of gamut" and is no longer broadcast legal. On the right is a very de-saturated image whose color information is close to the neutral center of the circle.

This is great because if you learn how to paint one camera you can basically paint any camera that has a User Matrix and Color Correction menu. That’s the nice thing about digital HD video – despite its many different flavors it’s all essentially the same. 

All User Matrix menus have the same six adjustable attributes: B-G, B-R, R-B, R-G, G-R, G-B. You can use these menu adjustments to subtly or radically alter the color characteristics of your image. The User Matrix and Color Correction does NOT affect the camera's Tonal Response which is White, Gamma, or Black levels.

I've heard a video camera’s Linear Matrix likened to a film stock. I like this because just as film stocks have very specific responses to individual colors, so does a video camera’s colorimetry. That said you ordinarily wouldn’t change film stocks in the middle of a scene so a similar attitude toward the matrix is generally recommended. You can quickly end up with mismatched images if you start painting away like crazy so a good eye for continuity can’t be overstated. That isn’t to say you can’t do it but proceed with a light touch because matrix adjustments are baked into the video image and are often times irreversible. 

As you’ll see in the examples below, there is no way to single out one individual color with the User Matrix adjustment tools. However, you can use the Color Correction menu toolset to more closely isolate and adjust individual colors within the Linear Matrix. You can't single out a specific color though as any other color that contains this color will also be affected. Always check the entire scene to gauge the effect. 

In my experience, I use the User Matrix tools for the following:

-Camera matching or emulating the color response qualities of another imager such as a film stock or a CCD chipset from another manufacturer. 

-Emulating a specific look such as Bleach Bypass, Day for Night, or some sort of Color Filter effect.

-Creating a base colorimetry or color response for a specific scene or project.

I use the Color Correction menu tools for: 

- Adjusting the Phase and Saturation in the Yl-R Color Correction attribute is a great way to make fine adjustments to skin tone hue and saturation. 

- Working on a specific color that needs modification. Often times with product shots, there is a very specific hue and saturation that must be followed.

- Fine and subtle color correction. As the name implies, the Color Correction menu can in fact be used this way however it can be an intense process and there is rarely the time on-set to spend on it so you're better off painting various looks in pre-production or at the checkout and modifying those as you go. Just be sure to keep everything within the limits of the legal gamut and don't throw colors completely out of phase and won't make things any worse. 

In a nutshell, the User Matrix is more for broad strokes whereas Color Correction is for small. Ultimately though, you're always going to be using them together to arrive at the desired effect. 

Part 2: Getting into the User Matrix menu

Within the “Paint” menu on the HDX900 you will find a menu called “Matrix”. Within it you will find the six attributes I mentioned that are adjustable in both positive and negative increments. On the HDX900 these increments max out at 63. On Sony cameras, I believe they max out at 99. At any rate, in practice it’s all the same. The comparison for each attribute is a value of 0 versus a maximum value of either +63 or -63.

Here is a waveform of our basic, uncorrected Chroma Du Monde chart:

A correct exposure for the Chroma du Monde chart is indicated by exposing the 11 step grayscale so that is crosses at 60 IRE.

The importance of good exposure to camera painting can't be understated. Without good exposure, you don't have much to work with so start by ensuring your image is properly exposed. 

USER MATRIX ADJUSTMENT EXAMPLES:

1. B-G, BLUE INTO GREEN: 

On the left - Positive Value, In the middle - Default Value, On the right - Negative Value

B-G +63 (increase in value)

B-G –63 (decrease in value)

2. B-R, BLUE INTO RED:

On the left - Positive Value, In the middle - Default Value, On the right - Negative Value

B-R +63 (increase in value)

B-R–63 (decrease in value)

3. G-B, GREEN INTO BLUE:

On the left - Positive Value, In the middle - Default Value, On the right - Negative Value

G-B +63 (increase in value)

G-B –63 (decrease in value)

4. G-R, GREEN INTO RED:

On the left - Positive Value, In the middle - Default Value, On the right - Negative Value

G-R +63 (increase in value)

G-R –63 (decrease in value)

5. R-B, RED INTO BLUE:

On the left - Positive Value, In the middle - Default Value, On the right - Negative Value

R-B +63 (increase in value)

R-B –63 (decrease in value)

6. R-G, RED INTO GREEN:

On the left - Positive Value, In the middle - Default Value, On the right - Negative Value

R-G +63 (increase in value)

R-G –63 (decrease in value)

Part 3: Getting into the Color Correction menu

All Color Correction menus allow the same six components of the Linear Matrix plus six additional in-between components to be individually adjusted. They are: 6 primary video colors - Red (R), Yellow (Yl), Green (G), Cyan (Cy), Blue (B), and Magenta (Mg) and the 6 colors in between the primaries - Red-Magenta (R-Mg), Magenta-Blue (Mg-B), Blue-Cyan (B-Cy), Cyan-Green (Cy-G), Green-Yellow (G-Yl), and Yellow-Red (Yl-R)

As exemplified in the above graphic, the colors in and around these areas will be affected by their corresponding adjustments. To modify the Hue or Saturation of Red, use the "R" Color Correction attribute, for the colors in-between Yellow and Red, use "Yl-R", etc. 

These Color Correction attributes are modified with a Phase and Saturation control. A negative Phase value (-) will move the color to the left on the vectorscope, a positive Phase value (+) will move it to the right. A negative (-) Saturation value will move the color closer to the center of the vectorscope, decreasing saturation and a positive (+) value will move it closer to the edge of the circle, increasing saturation. By altering the Phase on an individual color you are moving it out of alignment with other colors and reducing the amount of shades the camera can reproduce. Using these controls you can work on individual colors (such as skin tones) and subtly alter their hue and saturation but you still will affect any other color that contains the color you are modifying. The effect is far more subtle than the Linear Matrix adjustments, however often necessary to arrive at a very specific hue or color saturation. Color correction in post production allows for a much finer degree of control so some cases, it's best left to them. 

For the sake of simplicity, the goal of this first Colorimetry tutorial is to show how these controls work. It's rare that you'll only use one attribute at a time to color correct a scene. The hard part is knowing how these controls can be used together to create custom looks. But if you know what each one does, you can predict how they will work together and a skillful well practiced hand at the paint box can create some remarkable images.

If there are any errors or omissions, please bring them to my attention. These tutorials need reader feedback to be effective learning tools so if you have something to add, please don't be shy.

Sony OLED Calibration part 2

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Sony OLED Calibration part 2

January 4, 2014

After updating Part 1 of this article, I felt an independent followup was in order. I would say that monitor calibration is at the center of the ever evolving craft that has come to be known as "DIT". This being the relatively new space we occupy, somewhere amorphously between production and post but never completely one or the other. This short article is intended to expand on and clarify some of the points of the previous one. 

This little bit of nomenclature alone I find interesting as many production people I work with seem to be under the impression that my cart is the "DIT" and that I'm the "DIT Tech" or the "DIT Operator". I've even occasionally found this is how I'm listed on the call sheet. Most of the readers of this site I would assume are aware that this acronym refers to the person working the cart and not the cart itself.  D.I.T. = Digital Imaging Technician, a human being, not an unwieldy pile of video and computer gack on wheels. However, these loosely thrown around three letters have come to mean something completely different depending on who you're talking to. 

So in DIT, or for DIT's, or however you prefer to frame it; being able to trust what you're seeing on your monitor is one of the most important components of our craft. You have to be confident that the digital images you're working with are being faithfully represented. This is accomplished through calibration.

As in the previous article, to calibrate the Sony Trimaster EL OLED monitors I'm using the X-Rite i1 Pro or Pro 2 Spectrophotometer and Sony Automatic White Balance Software. There are of course many other spectrophotometer options available but this is one that's affordable and that I've come to trust. 

Sony Automatic White Balance Software

X-Rite i1 Spectrophotometers

WHITE BALANCE ADJUSTMENT:

100% White Test Video

The first thing I'd point out when calibrating the Sony PVM series OLED's is that they are far trickier than their big brothers the BVM's. The RGB Gain and Bias as well as the Brightness, Contrast, and Chroma adjustments are much coarser than those found in the more expensive monitors. Because of this lack of subtlety, it's difficult if not impossible to perfectly hit our OLED calibration targets of x .307 and y .318 on the PVM's.

While I think the PVM's are excellent displays and priced appropriately, I have found small differences and inconsistencies from panel to panel. Let's be realistic, if these monitors were intended to be used as a true reference, the purpose of the higher end BVM's would be displaced and sales cannibalized. The PVM's when calibrated can come very close in terms of color and tonality but are not a true reference and I don't believe were ever intended to be.

While I think the quality control at Sony is good, in the case of the PVM which is not the premium line, uniformity can't be expected to be as stringent. Even on monitors that may not be able to hit x .307 and y .318 perfectly, you will always be able to get very close, for example x .308 y .318 or x .307 y .319. This will yield an "acceptable" result and most users will be hard pressed to see much, if any difference between a monitor reading these numbers and one coming up perfect. I realize that this information may not please Sony but after measuring scores of these monitors, this is the conclusion I've come to. 

BIAS ADJUSTMENT:

20% Gray Test Video

The issue stated above is even more problematic with Bias calibration on the Sony PVM's. For whatever reason, it's difficult if not impossible to get a straight reading for Bias using a 20% neutral gray video signal. At least not using the X Rite probes with the Sony White Balance software. The numbers of the probe reads tend to jump around between 2 to 3 decimal places. Because of this, it's difficult to tell if you're actually aligning. I've found that if you can get the numbers within a decimal point or two of the targets, the gray video will look very neutral. Between x .306-.308 and y .316-319 has proven to be acceptable. And once again, I've encountered certain monitors that will hit the targets pretty much spot on whereas others will not. Why this specific instability? Looking for answers.

Neither of these issues are as apparent in the Sony BVM OLED monitors. These OLED panels are of the highest quality and extremely stable. If nothing less than total accuracy is required then stick with BVM monitors and a probe like the Klein K-10. If "close enough" works for you and your clients, the PVM's and X-Rite have proven to be sufficient. And by "close enough" I mean "entirely acceptable" for most us working in the field. Beyond the calibration and panel quality differences between the two lines, be aware there are also video signal processing differences that are most evident in the way motion is rendered. The BVM's employ a more sophisticated Interlace to Progressive (I/P) Conversion with less delay and smoother motion. 

BEST CALIBRATION PRACTICE FOR SONY OLED MONITORS:

1. Follow the instructions in this article to automatically white balance your Sony monitor >>>

2. Input 100% white test video into the monitor after the Automatic White Balance is complete. If it is not reading the targets x .307, y .318, manually adjust RGB gains until they are. 

3. Input 20% gray test video into the monitor and do the same process for Bias if necessary. 

RELATED ARTICLES:

HD Monitor Calibration - White Balance and Color Bars

Sony OLED Calibration part 1

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. 

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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