[ Note: This article first appeared in the Feb. 2015 edition of KitPlus – TV-Bay Magazine. It has been re-written for this version. ]
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.
NOTE: Scopes display the same values for any video editing software. I’m using screens from Final Cut Pro X, but the information here is true regardless of software used. Also, while gray-scale values are the same for all video formats, color targets change depending upon video format: SD, HD and 4K.
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:
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:
In general, the Waveform Monitor tells us everything we need to know about the gray-scale (black and white) values of an image, but nothing about color. The Vectorscope tells us everything about color, but nothing about gray-scale, while the Histogram shows the distribution of pixels from black to white.
NOTE: The Histogram scope is similar to the histogram in Photoshop. While it is a useful scope, the Waveform Monitor and Vectorscope are much more significant.
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.
(Click any of these scope images to see a larger image.)
Here’s an image showing every gray-scale value from white to black; notice the smooth white curve in the scope. The Waveform Monitor divides gray scale values into seven ranges. From brightest to darkest (top to bottom in the image above) these ranges are:
However, while this image represents every shade of gray, on the Vectorscope, every shade of gray is a single dot in the center of the scope. The Vectorscope is solely dedicated to measure color values.
Here’s an example of an image 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. Look on the left side of the scope; see the highlights from the bright clouds pushing values up to 100%? Yet, on the right side, the sky is darker, with highlights reaching only 90%.
These gray-scale ranges are made even clearer in this waterfall image. Notice the dark areas – around 5% – on the left and right sides of the image, while the bright waterfalls are all highlights with no dark values.
NOTE: In looking at this image on the scope, I can’t say “Oh! This is a twin waterfall.” I’ve been reading scopes for 40 years and you can’t tell the content by looking at the scope. What I can say is: “Hmmm… There are dark elements on the left and right edges, with something really bright in the center.” But whether this is a glowing light, falling water, or a sunset, the Waveform Monitor doesn’t say.
Compare the waterfall image with this snowy forest shot. 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, gray and dead. The “notches” in the display on the right side are caused by the dark tree trunks contrasting against the brighter sky.
(Remember, click any scope image to see a larger image with more detail.)
Let’s compare these same three 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 in the image, simply that the colors exist somewhere within the image
The Vectorscope displays two values for each color:
In this first example, there are two clumps of colors, one heading toward yellow (YL, on the left) and the other halfway between blue and cyan (B and CY on the right). The clouds, which are varying shades of gray, are all near the center of the Vectorscope, along with all the other shades of gray. Only as colors become saturated do they move out toward the edges.
In this second example, colors are less saturated than the green field image, with all the colors contained in a range from brown through yellow and heading toward green. There is no blue or magenta anywhere in this image.
Compare the waterfall picture with this mountain picture. Look at how concentrated toward blue the mountain image is. This is almost a monochrome image, except instead of black and white, there are shades of blue.
Now compare the earlier pictures with the cold winter forest. There is the barest shift toward blue. But, essentially, this is an almost totally desaturated image.
NOTE: A cool tip that helps with color correction is that every shade of gray (including black and white) forms a single dot in the center of the vectorscope. By definition, gray is totally unsaturated, while a color in the middle of one of the small “target” boxes next to a letter is defined as totally saturated.
This leads us to the central tenet of color correction: If something is supposed to be gray, it MUST be a single dot in the center of the Vectorscope. Anything broader than a single dot represents colors that are adding a color cast to your image.
THE MAGIC OF SKIN TONES
The Vectorscope has a magical property that still awes me to this day. It’s the line that goes up to the left, called the “skin tone line.” This 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 value, but blood determines our color. The “skin tone line” represents the color of “red-blood-under-skin.”
(Remember, click any scope image to see a larger image with more detail.)
Look at this example. Her skin color is parked right on the skin tone line.
Or, an Asian dancer. The yellow pillars are heavily saturated and leaning toward yellow. But look closely at the skin tone line and you’ll see that her skin values are right on the line.
This also illustrates how small differences in degree make huge differences in color. There’s about an 8-degree difference between the color of her skin and the color of the yellow pillars. Yet the two colors are clearly different.
Or a black woman.
Or these two children. In all these examples, skin color is parked right on the skin tone line. Their skin 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.
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 and what it can tell us; then use them to help us figure out what’s wrong with our images so we can make them look great.
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