How to read Video Scopes


Larry Jordan# TV-Bay Magazine
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by Larry Jordan
Issue 98 - February 2015

Understanding what video scopes tell us about our images is essential to creating great looking images; regardless of which video editing software you are using. In this article, I want to explain the basics of video scopes and how to read them.

A QUICK BACKGROUND


Each video image is composed of pixels, small squares formed into rows and columns where each pixel has exactly one color. We measure pixels using three color values:
- Luma, or Luminance. The amount of gray in each pixel from very light to very dark.
- Chroma, or Chrominance. The hue or color of each pixel.
- Saturation. The amount of color in each pixel.

These measurements are displayed on video scopes, where each scope shows the contents of a complete frame of video. While there are many variations, there are three basic video scopes:
- Waveform Monitor and its cousin the RGB Parade
- Vectorscope
- Histogram

In general, the Waveform Monitor tells us everything we need to know about the black and white values of an image, but nothing about color. The Vectorscope tells us everything about color, but nothing about black and white. And the Histogram shows us the distribution of pixels from black to white.

Since no single scope displays all the values we need, in order to understand our images we need to use these scopes in concert. And, when you are getting started reading scopes, the Waveform Monitor and Vectorscope are the most helpful.

WAVEFORM MONITOR
(Fig.1)

The Waveform Monitor divides gray scale values into seven ranges. From brightest to darkest these are:
- Super-white. This is an "illegal" value of gray-scale levels over 100%. Values in this range are safe for the web, unsafe for anything else. Most digital cameras allow gray-scale levels in this region.
- White. Gray-scale values exactly at 100%.
- Highlights. Gray-scale values between 100% and 66%. Highlights provide an image with energy, punch and "life." These values are also commonly, but incorrectly, called "white levels."
- Mid-tones, Mid-grays, or Mids. Gray-scale values between 66% and 33%. These values provide an image with emotion and often reflect the time of day.
- Shadows. Gray-scale values between 33% and 0%. These values provide an image with richness. These values are also commonly, but incorrectly, called "black levels."
- Black. Gray-scale values exactly at 0%.
- Super-black. This is an "illegal" value of gray-scale levels below 0%. Values in this range are safe for the web, unsafe for anything else. Some software, for example, Final Cut Pro 7, traps these values and sets them to 0%.

Here's an example of an image (Fig. 2) with a full range of pixels from light to dark. (This is called a "high contrast" image; because it contains lots of pixels at different values.) Looking at the Waveform Monitor from left to right mirrors looking at the image from left to right. It is easy to see the near-white values of the two waterfalls reflected in highlights between 90 - 100%; while the dark rocks on each side of the image are reflected with very low shadow values from 5- 15%.

Compare the waterfall image with this snowy forest shot (Fig. 3). This called a "low-contrast" image because there isn't a lot difference between the pixels. Nothing is very dark or very light. Gray-scale values range from 20 - 70%. The image lacks energy and richness. In fact, it feels cold and dead.

Let's compare these same two scenes on the Vectorscope. Unlike the Waveform Monitor, which allows us to say things like: "the left side of the image is darker than the center," the Vectorscope has no ability to display where colors are located, simply that the colors exist.

The Vectorscope displays two values for each "2" style="">

Compare the waterfall picture with this mountain picture (Fig. 5). Look at how saturated and blue the image is. So much so that there's virtually no other color in the image.

THE MAGIC OF SKIN TONES

The Vectorscope has a magical property that still amazes me to this day. It's the line that goes up to the left. That's called the "skin tone line" and it is essential for color correction and skin tone matching.

Here's the secret: skin doesn't have a color! As you know from getting cleaned up in the morning, dead skin is gray. What gives us color is not our skin, but the red blood under our skin. Skin determines our gray-scale, but blood determines our color. And that line represents the color of "red-blood-under-skin."

Look at this example (Fig. 6). Her skin color is parked right on the skin tone line.

In this example (Fig. 7), her skin color is also parked right on the skin tone line. Their color is identical, but the gray-scale values are not.

The skin tone line enables easily spotting skin color problems as well as matching skin tones between actors.

SUMMARY

Video scopes enable us to analyze our images, fix problems, match colors and enhance our scenes. The key is to know the strengths of each scope, then use them to help you figure out what's wrong with your images so you can make them look great.


Tags: iss098 | video scopes | waveform monitor | histogram | vectorscope | Larry Jordan#
Contributing Author Larry Jordan#

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