January 05, 2014

Painting HD Cameras - Skin Tones

January 05, 2014/ Bennett Cain

Painting HD Cameras - Skin Tones

In my experience color correcting video cameras in the field, 9 times out of 10 I’m trying to resolve some sort of skin tone issue – taking green out, bringing overly magenta skin back within a normal range, or sometimes just injecting a little bit of warmth and saturation. Knowing how to correctly use a video camera’s User Matrix menu and Color Correction menu as well as the Tonal Control menu is the key to working through these inevitable problems. In building upon my previous article on in-camera color correction for HDTV, this next article will specifically address how to use the various matrix menu attributes to affect skin tones.

This article builds off what was established in Painting HD Cameras - Basic Colorimetry.

Technical Notes:

The images used in this article were created with a Panasonic HDX900 and the stills and vectorscope information were captured from 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 feature 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 CamBelles 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 your studio.

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.

Part 1: Overview

First to re-hash, there are six attributes that affect a video camera’s Linear Matrix: B-G, B-R, G-B, G-R, R-B, R-G. Those are read “Blue into Green, Blue into Red, etc.” Additionally, there are twelve Color Correction attributes we can modify: R, Mg, B, Cy, G, Yl, Yl-R, R-Mg, Mg-B, B-Cy, Cy-G, and G-Yl. For an in depth account of how these attributes work by pushing and pulling colors around the vectorscope, please refer to the previous tutorial. Using the handy DSC Labs Chroma Du Monde Chart with its 4 "generic skin tone" swatches, let's have a look at our camera's "out of the box", default colorimetry:

Interestingly enough, virtually all human skin regardless of its hue or saturation resides somewhere within or nearby this red circle which for simplicity we'll call the "Skin Tone Region". The area resides along the I line on the Vectorscope and above the Q Line (see the intersecting lines on the graphic below). Where the Q Line crosses the I Line, skin tone saturation is at zero. The closer the skin tone information is to the boundary of the circle, the greater its saturation. Smart camera software such as Skin Tone Detail Circuitry knows to look within the Skin Tone Region and is thus able to isolate the information there to make independent adjustments. This is very helpful because it becomes easier to predict how the values are going to move around on the vectorscope as adjustments are made to the camera.

Now before we start playing, let's get a better idea of how these variables will affect actual human skin by using the DSC Labs CamBelles chart. Obviously sitting models would be better but for what it is, this chart is incredibly precise and I've used it to paint looks in the studio that have worked perfectly well in the field.

The lovely ladies of DSC:

There is a good variety of skin tones here and the light in the scene is modeled enough that you can examine a good range of values. Also the fact that they're wearing bright clothes and are on a blue background helps to isolate the skin tones on the vectorscope.

Here's what they look like on the Vectorscope:

This isn't a tutorial on tonality but part of getting good colors means getting a good exposure. This is what my properly exposed and properly white balanced CamBelles look like on the waveform.

And if you have False Color on your monitor, you can use it to confirm your exposure:

Usually you want to keep it in the green-yellow zone for light skin tones and green-blue for dark. Orange is 80% which is where skin starts to break up so you definitely don't want your key light hitting that hard.

Skin tones can also be affected globally with Master Saturation Controls. Increased Saturation on the left and decreased Saturation on the right:

Part 2: User Matrix menu and skin tones

Typically you wouldn't use matrix adjustment to specifically affect skin tones as these are more global adjustments but it's good to see what the effect is. You're also hardly ever going to only use one of these adjustments. When creating a custom look, you'll most likely be pushing values around in all six menu options.

For example, let’s look at a side by side of the Cambelles when you put the G-B (Green into Blue) attribute at its maximum value, +63 on the left and its minimum value, -63 on the right:

As you can see, you’re never only affecting the skin tones. In your quest to render the perfect skin you’re also affecting plenty of other colors. It’s very easy to get caught in an endless cycle of color correction where you fix one thing only to create a new problem with another color. Only through trial and error and understanding the basic principles behind how in-camera color correction works will you be able to quickly execute the best solution.

Now let's have a look at both what happens to our skin tones when we adjust each of the user matrix variables:

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)

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)

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)

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)

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)

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: Color Correction menu and skin tones

Unfortunately I don't have CamBelles examples for working with the Color Correction menus. The attributes you'll be working with the most in regards to skin tones are the following three video colors: Red-Yellow (Yl-R), Red (R), and Yellow (Yl).

In the Color Correction menu set, we can isolate and modify the following twelve individual vectors: six primary video colors - Red (R), Yellow (Yl), Green (G), Cyan (Cy), Blue (B), and Magenta (Mg) and the six 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 is 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 in some cases, it's best left to them.

As mentioned in the previous article, you're very rarely only going to work with one attribute at a time. It's really understanding how they're all used together that's the key to good camera painting. Every task is different and there is no "one size fits all" approach. However I will Yl-R in Color Correction is often where I start when trying to inject some warmth and life into dull looking skin. Please support this blog by leaving comments and feedback. It's really only through user support and feedback that content can be fine tuned so I always appreciate hearing from you.

January 05, 2014

Painting HD Cameras - Basic Colorimetry

January 05, 2014/ Bennett Cain

Painting HD Cameras - Basic Colorimetry

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.

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)

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.

January 04, 2014

Sony OLED Calibration part 2

January 04, 2014/ Bennett Cain

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.

WHITE BALANCE ADJUSTMENT:

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:

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

Blog

Part 1: Overview

Part 2: User Matrix menu and skin tones

Part 3: Color Correction menu and skin tones

Part 1: Overview

Part 2: Getting into the User Matrix menu

Part 3: Getting into the Color Correction menu