The Goals of Color Correction
There are two primary objectives in basic color correction: spread your tonal range and balance the colors. There are a lot of other important goals, including matching shots from one to another, helping to tell the story, and making the images "pop" or "read." But we'll focus on spreading the tonal range and balancing colors in this initial chapter.
Spreading the tonal range means that you take full advantage of the tonal range of your display medium. In most cases this means maximizing the number of levels of gray between the deepest black and the brightest white that your display or broad-cast specifications can reproduce. For most people reading this book, that means a TV screen or maybe a computer display. It can also mean prepping the image for a digital intermediate transfer to film. Tonal range corrections do not always have to spread completely from 0% black to 100% white, but oftentimes they do.
The tonal range is the difference between the brightest and darkest areas of the image. The tonal range of the image -- and how those tones are spread throughout the tonal range -- defines its contrast. For some applications of this phrase, tonal range indicates the actual number of levels of tones that a recording medium can record (256 per channel in the case of RGB 8 bit, or 1025 per channel in the case of RGB 10 bit). For our purposes, we will refer to tonal range (singular) as the range of tones between brightest and darkest. Ansel Adams and other proponents of the Zone System break the tonal range of an image into 11 distinct tonal ranges.
Balancing the colors means that any unwanted color casts are eliminated. The reason for the term "balancing" will become more obvious as we start to examine and analyze our images with a number of different tools. Color casts in images are sometimes desired, for example, the warm, red tones of a sunset or the sad, blue, coolness of a rainy day. These are color casts that often serve the story, so we need to be careful not to eliminate them. Examples of color casts that are undesirable are usually caused by video cameras that haven't been white balanced properly or film footage shot with the wrong filter for the combination of film stock and light temperature.
Much of the information that viewers use to understand and interpret the image is based on the tonal range or contrast between brights and darks. In most cases we want to give viewers as much information as possible, but sometimes you don't, like the case of a thriller or horror movie where you may be trying to hide things in the shadows.
The first step in determining the proper tonal range where your image should "live" is setting the proper level for blacks or shadows.
The first step in determining the proper tonal range where your "live" is setting the proper level for blacks or shadows. Setting the black level is almost always the place where any experienced colorist starts a correction. So the question for a beginner is, "Where do I set the proper black level, and how do I know what I should set it to?"
This book is product and platform agnostic. In other words, this book is less about what buttons to push on specific pieces of hardware or software and more about understanding the process so that you can feel comfortable color correcting on almost any application that exists now or in the future. Because of that, I'll show you the right buttons in a few applications. Hopefully you're using one of the apps that I used as an example. Otherwise, look for similar parameters in the software or hardware that you use.
Blacks are also known as shadows, pedestal, set-up, or lift, depending on the application. Setting the black level usually involves adjusting a slider called either "blacks," "shadows," "set-up," "lift," or "pedestal," such as the controls in the screen shots that are shown here from several of the most widely distributed applications with color correction capabilities.
A waveform monitor displays the amplitude level -- brightness and darkness -- along the vertical axis with the dark parts of the image near the bottom and the brighter parts of the signal near the top. Technically, the horizontal axis of the waveform displays time, but practically speaking, the horizontal axis of the waveform corresponds to the horizontal placement of picture elements across the image with no regard to the vertical placement of elements in the image. The waveform monitor also displays chroma levels, but these are easier judged on a vectorscope.
How to determine the Proper Black level
When I started to learn color correction, the biggest mystery to me was simply "How do you know what's right?" To set black levels you need some tools for proper analysis of the black level. Basically that means a waveform monitor, though there are other tools that would work. We'll get into the full range of analytical tools later in the book. For now, a simple waveform monitor will do. Don't panic. You don't need to be an A/V geek or slide rule engineer to understand this display. We just need to know how low we can go legally, and that's pretty simple.
Firstly, you need to know what "legal black" is on your system or waveform monitor. For most waveform monitors, black is at 0 IRE or 0% or 0 millivolts. In the United States, which defines black as 7.5 IRE for composite analog NTSC signals, black can mean 7.5 IRE or 0 IRE depending on the type of video signal. The easiest way to tell which is right for your system is to feed "filler" (the black signal your system generates whenever it doesn't have real video to send) to the waveform monitor.
The easiest way to tell which is right for your system is to feed "filler" (the black signal your system generates when-ever it doesn't have real video to send) to the waveform monitor.
If "filler" is 0 IRE, then your goal in most color correction shots is to get at least some portion of the picture down to 0 IRE. If you get black lower than 0 IRE, your program may not pass quality control (QC) if you are sending it for broadcast or mass duplication.
I always suggest that color correction be done with external scopes, such as those by tektronix, leader, videotek, omnitek, and others. These scopes generally have more power and information than internal scopes and give you a true sense of the levels coming out of your system because they are downstream from the video card. All internal monitoring, such as the scopes in Color or final Cut or avid, do not represent the actual video signal as it has left your system, and software scopes usually do not represent the entire signal. Due to limitations in the amount of computational power required for these displays, software companies often only display every other line or only every fourth line of video. Sometimes, having real time scopes enabled on your chosen Nle (non linear editing system) can actually impede the performance of the Nle itself as it tries to do the important tasks that it is actually designed for. So you can make your editor or compositing software run better by turning off the internal scopes and running with a good external scope.
A vectorscope displays chrominance and hue. The saturation (or gain) of the chroma (or color) is measured by how far it extends from the center of the scope. Neutral images (black, white, and all levels of gray) register as a dot in the middle of the vectorscope. Hue is indicated by the position of the trace around the perimeter of the circle. Vectorscopes have graticules that show each of six different colors (red, green, blue, magenta, cyan, and yellow) in a different, fixed vector (position) around the vectorscope. Color includes a cool 3D vectorscope that allows you to rotate the vectorscope in 3D space to see luminance displayed as well. 2D vectorscopes cannot display luminance information.
Watch the video tutorial "Spreading the tonal range" on the DVD.
The Tektronix WFM7120 that I use is configurable, and I have it set to display blacks at 0 IRE. This is how I will display black throughout the book unless otherwise noted.
If your black levels are generally higher than 0 IRE, then your picture may look washed out and flat. We'll get more into the complexities of the waveform monitor later in the book.
Using your waveform monitor in luma only mode, and knowing where your black level should be by looking at filler, you can use one of the tools in Figures 1-1 to 1-5 to set your black level to the proper position on some real video.
You can use your own video footage for this exercise, or load one of the video examples from the DVD. For the purpose of this tutorial, we'll use the FCP Color Corrector filter and the file from the DVD called "ChromaDuMonde_overbal.mov."
I suggest you drag all of the tutorial video files to your internal hard drive or a media drive from the DVD now.
1. Launch Final Cut Pro (or whatever application you want to use to follow along with the tutorial)
2. Import the "ChromaDuMonde_overbal.mov" file, dropping it into a sequence.
3. Then go to Effects>Video Filters>Color Correction>Color Corrector.
4. Go to Tools>VideoScopes to call up your internal scopes if you do not have external scopes.
5. In the Viewer, switch to the Filters tab to see the controls for the effect. You can view this filter in either Numeric mode or Visual mode.
There are "legal" limits for black levels, white levels, and the strength of colors (saturation). These limits are determined by international committees and national governing bodies, such as the FCC. They can be further stipulated by specific broadcasters, such as PBS or the Discovery Channel. If you are creating a program for broadcast, check with your outlet for specifics. Some replicators or duplicators also have legal levels that videotape masters must adhere to.
This is a unit of measure for waveform monitors. It is named for the Institute of Radio Engineers, which defined the unit. The scale starts in negative numbers and goes beyond 100 IRE.
Millivolts
This is another unit of measure for waveform monitors. It is not as easy to use as the simple IRE scale or a simple percentage scale, so we will not use this scale to describe video levels.
Trace
The trace is the portion of the waveform or vectorscope that indicates the levels of the video signal. It is the portion that responds to level adjustments. On most scopes, the trace tends to be green or sometimes white. Sometimes the trace is actually referred to as the "waveform," while the actual device that displays it is referred to as the "waveform monitor." The traditional color for the trace is green. More colorists don't like the green trace, so on many monitors the color of the trace is user selectable. Many colorists choose a neutral gray or white color for the trace.
The graticule is the fixed lines on the waveform and vectorscope that provide scale and positioning information, for example, the IRE lines on the waveform monitor or the color targets on the vectorscope. The graticule is often orange as a contrast to the green of the trace. This word is not limited to video. Graticule also defines the network of longitude and latitude lines on a map.
6. Adjust the "Blacks" slider down until the shape at the bottom of the center of the waveform monitor reaches the bottom orange line labeled "Black" or 0%.
Figure 1-16 this figure represents the image when the black level has been brought down too low. the flattening of the shape at the arrow indicates clipping or crushing of the signal, which results in a lack of detail in the shadows.