Have you ever had a director ask you if you can fix a few bad shots? Have you ever wanted to make a final sequence look better? Have you ever wondered what colors you are really looking at? Do you ever worry about passing Quality Control (QC)? Color grading can be an intricate and complex process. The human visual system constantly adjusts to the surroundings, making it tricky to know what colors you are truly looking at. Combine that with the challenge of ensuring that your technical standards will pass QC. What’s an editor or colorist to do?
Color grading is an important process in creating excellent video productions, and knowing how to use waveform monitors, or “scopes” is critical in doing it right. Whether you think scopes are too technical, or you want to improve your skills, this primer is geared towards editors and will guide you through the process of color correction and using a scope. Whether it’s making a final sequence look better or just fixing a few bad shots, this primer is full of tips and techniques to assist you in honing your craft.
For many film and television editors, color correction can appear to be a skill shrouded in a veil of mystery with only a select few possessing the know-how to unlock the secrets of this sometimes elusive world.
Knowing how to use a great scope makes the job of editing and color grading easier and quicker. Editors can get fast results with less rework by keeping the material within the deliverable specifications. Tektronix has the tools to meet the editing demands of getting the job done in both a fast and accurate manner.
One of the key aspects at the core of doing good color correction for video is making sure that the image can be properly reproduced and delivered to a variety of media and screens. The main technical challenge with maintaining proper color reproduction across a variety of media and broadcast methods is to understand the importance of legal and valid gamut. Tektronix has developed a set of tools that can help editors and colorists easily adjust the color fidelity of the image and maintain the video signal within suitable gamut limits.
By learning how to use these simple, visually intuitive waveform displays from Tektronix, you will improve your color correction skills. For anyone who desires to create beautiful images using color grading software, Tektronix waveform displays are a critical artistic tool, allowing the efficient analysis and manipulation of your images.
This primer is crammed with tips and information on developing color grading as a skillset which can advance your career and set your professional skills apart from other colleagues and competitors. Each section is filled with essentail information on developing color-grading techniques as a creative skill set as opposed to just another step in the post process. By using these tools, post-production professionals can gain an invaluable education, expediting future career advancement in their field.
Section 1: Basic Color Theory and Color Gamut Monitoring
There are two things that are at the core of doing good color grading for video:
- Insuring that the image on the screen looks great and is graded to best tell the story
- Making sure that the image can be properly reproduced and delivered to a variety of media and screens
In Section 1, we’ll concentrate on making sure that the image can be properly reproduced and delivered to a variety of media and screens. The main technical challenge with maintaining proper color reproduction across a variety of media and broadcast methods is to understand the importance of legal and valid gamut. Maintaining proper gamut is crucial in today’s world of repurposed content and cross-platform redistribution of broadcast originated content. Without the ability to accurately monitor gamut issues, there can be problems with broadcast transmissions, recording devices and sponsors and clients whose images are compromised when viewed by certain audiences. Before we examine the concepts of “legal” and “valid”, let’s quickly review the concepts of color space.
What is the HSL Color space?
Video is comprised of three color components Red, Green and Blue (RGB), and various combinations of these colors make up the colors we see. One way to understand the Hue Saturation and Luma (HSL), or RGB color space is to imagine it as two cones joined at their widest point (Figure 1-1).
This is what it would look like in three dimensions - the top of the joined cones is white. At the bottom is black. And as the joined cones widen in the middle, that is the representation of saturation. The points or angle around the width of the joined cones represent the hue.
If you look down from the top of the cones, you see a circle with white at the center, and black is directly under it (Figure 1-2). The chroma strength of the signal is indicated by its distance from the center. The closer the trace is to the outer edge, the greater the chrominance or the more vivid the color. The hue of the image is indicated by its rotational position around the circle.
In order to put these concepts into practice, we need to examine how color is measured, in both amplitude and the limits of amplitude.
Defining legal and valid gamut
The simple definition is that “gamut” is simply a range. Applied to broadcast television, gamut is a range of reproducible colors defined by R'G'B' (red, green and blue) signal values. All colors within the gamut of reproducible colors are possible by independently adjusting the values of the R'G'B' signals. Figure 1-3 shows the color coordinates used by NTSC, SMPTE and Rec. 709.
In broadcast video production, R'G'B' values are defined in terms of voltage ranges. R'G'B' signals extending outside the specified voltage range, or gamut, may become clipped or compressed in subsequent signal processing, distorting the color when displayed on another picture monitor. R'G'B' systems have an upper gamut limit of 700 mV and a lower gamut limit of 0 mV. Think of these values as 100% and 0% respectively. Legal broadcast video signals are signals that do not violate the signal voltage limits for a particular format. If any channel of an R'G'B' signal exceeds either the upper or lower limit, it is out of gamut, or out of range. That violation of gamut limits makes the signal “illegal”.
Another type of violation that is possible is called an invalid signal when a signal is converted from one format to another for instance from RGB to YPbPr. In color-difference formats like Y'P'bP'r, it is possible for the current format to be legal within the voltage limits of 700-0mv for Y and +/-350mv for color difference Pb/Pr, but for the gamut to be “invalid” when the current format is translated, transcoded or broadcast in a different format and exceeds the limits for that format and is illegal within this format.
Gamut and Amplitude Monitoring
Maintaining proper “gamut” is crucial in today’s world of repurposed content and cross-platform re-distribution of broadcast originated content.
Colorist and editors use their eyes to assess the color and quality of the image. However what your eyes see is interpreted by the brain and this processing can sometimes deceive the viewer. For instance, look at the two simple images below. Which one of the center gray boxes is brighter?
This is a trick question - the gray boxes are actually the same color but the surrounding object influences the brain decision on the color and luma level of the object. Therefore to gauge what you actually see, a waveform monitor (scope) is essential in order to interpret the image information.
The waveform monitor, or rasterizer (scope) is a key tool to help you in providing a legal output of your creative product. Part of the responsibility of the colorist is to ensure that the luma and chroma signals are within certain technical limits. Being a brilliant editor and colorist doesn’t mean much if no one will air your product or if the dubs are not viewable. Legal limits are set by broadcasters and cable networks and vary from one to the next. But even if your product isn’t being broadcasted, legal levels affect the proper duplication of your project and the way it will look on a regular TV monitor.
Let’s take a look at how the waveform and vectorscope can help us as we analyze the images we are attempting to correct. There are probably two groups who are about to roll their eyes and flip ahead a few pages.
- The first group is experienced online videotape editors: veterans who’ve been using scopes forever. They know all about sync pulse, blanking, back porches, and breezeways, and they aren’t interested in sitting through the basics.
- The second group is the new wave of digital whiz kids who have no interest in all the hard-core video engineering stuff that is largely obsolete since the death of the quad machine.
Well, we’ re not going to be discussing all that video engineering stuff, with the exception of a brief discussion on keeping levels legal. We want to show how these two pieces of engineering equipment can be put to good use as creative tools.
Introducing the Waveform Monitor and Vectorscope
First, let’s explore the waveform monitor. To many editors and colorists, the waveform monitor is simply a way to look at the luma or brightness of the video signal, but it can also display the chrominance levels of the signal as well. This chrominance information can be used to minimize color casts in your images. One of the most basic uses of the waveform monitor is to allow you to see that your luma and setup levels are legal. This means that the brightest part of the luma signal does not extend beyond the 100% mark — with occasional specular highlights allowed to reach 105%, more or less, depending on the individual broadcaster’s specs — and that the darkest part of the picture does not drop below 0%
For composite NTSC video, the 100% level is equivalent to 100IRE with black level at 7.5IRE mark for NTSC video with setup or 0IRE for NTSC video without setup. So how do you know if your video should be with or without setup? Well, with virtually all analog NTSC video, you need to have setup with blacks at 7.5IRE. However, no digital format has setup. From DV to HDCAM, setup is not used anywhere in the world. In analog formats, only the United States still uses setup (broadcast specification RS170A). NTSC in other parts of the world may or may not have setup. PAL and SECAM do not use setup. Component analog video has different standards also. The SMPTE/EBU (N10) standard does not have setup and typical voltage levels range from 700mv for 100% to 0mv for 0%.. So what do you do if you have a digital VTR and are using the analog in/out connectors? You need to know what standards are being used by the other analog machines. Then match the In/Out settings for Add/Remove Setup to match the rest of the system. Allowing your video to extend outside of the range defined by whatever form of video you are using can cause broadcasters to refuse to air your finished tape until corrections are made or for dubs to have quality issues, like sparkling, bleeding, bearding, and buzzing. Sometimes, if video levels are far enough astray, they can even cause buzzing in the audio channels. There are also legal chrominance levels and other technical specifications regarding timing of the signals and other signal amplitudes and relationships. These all vary slightly from broadcaster to broadcaster. But let’s leave this technical stuff aside and concentrate on how to use the waveform monitor for the fun stuff.
Using a Waveform Monitor to Determine Color Balance
Color balance is indicated by the relative strength of each color channel. With neutral (pure black, white and gray) colors, the strength of each color channel should, technically, be equal. The usual goal of color balancing is to achieve an image where neutral colors are represented without one channel being stronger than another. When color neutrality is achieved, the signal is said to be “balanced.”
The most common reason for unbalanced colors is usually related to how the camera is white balanced on location. For example, if a camera is set up to record in tungsten light when it is actually capturing a scene lit with daylight, the blue channel will be stronger than the red and green channels. Also, some camera sensors have a natural tendency to be more sensitive to certain colors in certain tonal ranges.
These errors, in sensitivity or white balance, can be corrected by monitoring the image with a waveform monitor and making adjustments to the signal until the signal strength of all three channels (red, green and blue) is equal when displaying a neutral color, like pure black, white or gray.
Color balance can also be intentionally misbalanced for artistic purposes, for example, increasing the strength of the red channel, or red and yellow channels to create the impression of a warm, “golden hour” scene.
Two types of waveform displays to look at are both defined as “parade” displays because they show you different channels of information in a “parade” one after the next from left to right. The most common method is the RGB Parade, which shows the red, green, and blue channels of color information horizontally across the display (see Figure 1-4). The reference marks are referred to as the graticule. On a waveform monitor, these are most obviously the horizontal lines describing the millivolts, IRE or percentages from black to full power (white). Component video levels are represented in terms of millivolts with black being set at 0mV and white at 700mv. This range of video levels is also represented in terms of a percentage scale with 0% equal to 0mv and 100% equal to 700mv. The scale on the waveform display typically has a millivolt scale on the left hand side and a percentage scale on the right-hand side. Within the Tektronix scopes, the user can also select a percentage scale within the Config menu under Graticules & Readouts. If you chose SDI Waveform Graticule to be Percent, then the left hand side of the graticule will show a percentage scale from 0 to 100 graduated in 10% steps.
You can also look at a YPbPr parade waveform, which shows the luma information in the first “cell” followed by the blue difference channel and the red difference channel (Figure 1-5).
This is the way component video is often encoded for video: The luma is sampled, then the color information is created by sampling the difference between the blue signal and the luma (Pb) and the red signal and luma (Pr). It is fairly difficult to ascertain what is wrong with the picture from this type of parade. This display is also sometimes called a YCbCr display. The first cell shows the luma, the second shows that the blue channel is higher than the center line, which indicates the difference in strength from the luma channel. The third cell shows that the red difference channel is lower than the center line, indicating that red is lower than luma. If you want to use this display to balance the blue cast, the goal would be to bring the Pb channel down
Note: For component video you may sometime see YPbPr and other times see YCbCr in reference to component video. Typically YPbPr denotes the analog component signal and YCbCr represents the same signal within the digital domain.
Whereas a waveform monitor normally displays a plot of signal vs. time, a vectorscope (Figure 1-6) is an XY plot of color (hue) as an angular component of a polar display, much like some familiar color wheels used in color graphics, with the signal amplitude represented by the distance from the center (black).
On a vectorscope graticule there are color targets and other markings that provide a reference as to which vector, or position, a specific color is in.
The signal being monitored generates the trace on the waveform, vectorscope or gamut display. This trace is the representation of the signal itself, often seen in live video as moving, squiggly green lines, though many colorists prefer to change the settings in the waveform monitor so that the trace is viewed as white lines instead.
In color grading applications, the vectorscope helps analyze hue and chroma levels, keeping colors legal and helping to eliminate unwanted color casts. With the gain, setup, and gamma corrections done while monitoring primarily the waveform monitor, the colorist’s attention focuses more on the vectorscope for the hue and chroma work. The chroma strength of the signal is indicated by its distance from the center of the vectorscope. The closer the trace is to the outer edge of the vectorscope, the greater the chrominance or the more vivid the color. The hue of the image is indicated by its rotational position around the circle. The easiest way to imagine these relationships is to picture a color wheel superimposed over the face of the vectorscope as in Figure 1-7.
One of the important relationships to understand is the position of the various colors around the periphery of the vectorscope (see Figure 1-7). The targets for red, blue, and green form a triangle. In between each of these primary colors are the colors formed by mixing those primaries. So the color between red and blue is magenta. The color between blue and green is cyan, and the color between red and green is yellow. These secondary colors form another triangle. The other interesting relationship that is formed on the vectorscope is that complementary colors are directly opposite each other. Red is opposite cyan, magenta is opposite green, and yellow is opposite blue. These relationships will play a pivotal role as you begin to manipulate colors. For example, if you are trying to eliminate a magenta cast in an image, a glance at the vectorscope will tell you that you need to add green, which is opposite magenta. Or you could reduce red and blue in equal amounts (the two colors that make magenta). If an image has yellows that are too cyan, then adding red will begin to solve the problem. Eventually, you should not even need the graticule (the graphic part of the vectorscope that identifies color targets) to know where the colors lie on the face of the vectorscope.
The chroma information presented on the vectorscope is instrumental in trying to eliminate color casts in images. As stated earlier, chroma strength is represented by its distance from the center of the vectorscope. Because white, black, and pure grays are devoid of chroma information, they all should sit neatly in the center of the vectorscope. While most video images will have a range of colors, they also usually have some amount of whites, blacks, and neutral grays. The key is to be able to see where these parts of the picture sit on the vectorscope and then use the color correction tools at your disposal to move them toward the center of the vectorscope.
Apply a color bar signal to the vector display with White, Yellow, Cyan, Green, Magenta, Red, Blue and Black bars. You can follow this pattern within the vector display (Figure 1-8) starting from the center trace the display to yellow vector (YL) then Cyan (CY) and follow on to Green (G), Magenta (MG), Red (R), Blue (B) and then back to the center of the display.
For nearly all professional colorists, the various waveform displays — Flat, Low Pass, Luma only, RGB Parade, and YCbCr Parade — plus the vectorscope are the main methods for analyzing your image. While experienced colorists often rely on their eyes, they use these scopes to provide an unchanging reference to guide them as they spend hours color correcting. Without them, their eyes and grades would eventually drift off course. Spend time becoming comfortable with these scopes, and knowing what part of the video image corresponds to the images on the scopes.
Section 2 Overview of Tektronix Displays for Color Grading
Tektronix has developed several monitoring displays to help identify and resolve gamut violations and to assist with artistic color grading. Some of these displays are relatively new and may appear complex, but they are all designed to provide specific visual representations of color gamut that give easy, immediate visual cues when errors are present once you gain understanding of the concepts and techniques behind these displays that are discussed in this next section. These displays are the Diamond Display (Figure 2-1), Split Diamond (Figure 2-2), Arrowhead (Figure 2-3), Spearhead (Figure 2-4), and Lightning Display (Figure 2-5).
Each of these displays is simply a different interpretation or presentation of the color space. Determining which of these presentations is most helpful is sometimes simply a matter of personal preference and sometimes a matter of knowing the intended use and signal path of the video.
For example, Tektronix’s Arrowhead display was developed because component digital signals may still be transcoded and transmitted as a composite signal in local broadcast markets. This conversion to composite could require monitoring the transcoded signal with a separate composite analog waveform monitor, but Tektronix created the Arrowhead display to allow engineers and operators to easily see out-of-gamut conditions in composite color space without requiring a composite encoder and separate composite waveform monitor.
Color Grading and Gamut Monitoring with the Diamond Display
The Diamond and Split Diamond displays were developed to assist in visualizing the interaction of RGB signals (Figure 2-1 and 2-2 respectively) and are intuitive indicators of both R'G'B' gamut violations and the “balance” of the signal, providing immediate feedback when an image has a color cast.
The Diamond display is the most reliable and useful R'G'B' gamut violation indicator available for several reasons. Because the top diamond in the Diamond display indicates levels of blue and green signal components, while the bottom diamond displays only red and green, it is easy to identify which channel or channels are in error when manipulating the R'G'B' signal (Figure 2-6 and 2-7). And, with the green signal indicated on the left side of both diamonds, the ability to identify errors in specific channels is made even easier.
Another important advantage of the Diamond display is its ability to be used as a subjective measure of the severity of a gamut violation. Instead of the simple “on/off” or “yes/no” violation displays of some gamut indicators, the Diamond Display presents the user with a simple visual display that shows not only the specific location of an error, but the severity of that error. The distance that the trace extends beyond the diamonds provides great visual feedback about the importance or severity of the violation.
The Diamond Display indicates pure black where the two diamonds join in the center of the display. If the black is too light (milky) then the trace starts above the bottom of the top Diamond and below the top of the bottom diamond. If there is a color cast to the black, that cast is represented with the trace swinging to the left or right of the diamonds.
A perfectly monochromatic (i.e. black and white) signal of the full contrast range or voltage range of video will display as a perfect vertical line. This line extends from the absolute center of the two diamonds, vertically to the top of the top diamond and the bottom of the bottom diamond, forming a perfect straight line up the middle of the diamonds.
Monochromatic images are rare in modern broadcasting, but even full color images that are properly balanced usually show a good balance through the middle of the two diamonds, with nearly equal amounts of the trace extending away in all four quadrants of the diamonds.
As for gamut, the Diamond Display clearly indicates any gamut violations by any portion of the trace that extends outside the boundaries of one or both of the two diamonds. Again, to see gamut violations in the black areas, it is helpful to switch over to the Split Diamond Display. But it is easier to see the “balance” of the image in the standard Diamond Display.
If blacks are too low, it is difficult to see on the Diamond Display, which is why the Split Diamond was designed. The Split Diamond is essentially the same display as the Diamond, but the two diamonds are off-set slightly, left and right, so that the exact position of the trace in the black areas is easier to see (Figure 2-8).
In the Diamond and Split Diamond Display, pure white is indicated by a trace at the center top of the top diamond and the center bottom of the bottom diamond (Figure 2-11). If white is not at full voltage (power or brightness), then the trace will fall short of reaching the top of the top diamond and the bottom of the bottom diamond (Figure 2-10).
If there is a color cast to the “white” signal, the trace will skew off to left or the right at the top of the top diamond and the bottom of the bottom diamond.
Pure gray is indicated in the center of the broadest part of both diamonds. If there is a color cast in the mid-tones or gammas, the trace will skew off to the left or the right across the breadth of either or both of the diamonds (Figure 2-12).
In creative and operational environments where monitoring R'G'B' gamut limits is critical, the Diamond display provides the greatest information in the most intuitive display. Conventional waveform monitor display modes, such as parade and overlay, are great for measuring signal levels, but gamut limit violations don’t stand out like they do on a Diamond display. The specific component causing the error can be determined with the Diamond display (Figure 2-13).
Signal monitoring in the R'G'B' color space does have several very important applications. Even though signal transmission in R'G'B' is rare, many creative and operational controls continue to bear the labels R', G' and B'. Computer graphics workstations, camera painting systems and color grading applications operate in the R'G'B' color space and all of them can easily be adjusted to create illegal gamut. Monitoring these types of systems is an obvious application for the Diamond display. Diamond excels in several areas for R'G'B' gamut, however it is not the answer to every measurement question. Other color spaces, such as composite, require different displays and measurement techniques.
Color Grading with the Spearhead Display
The newest gamut display from Tektronix is the Spearhead display, which shows the artistic metrics of color saturation and color value or lightness combined with RGB gamut limits. To fit Spearhead into the concept of color space, let’s return to the color cones. If you collapse the diamond on its vertical access to be a triangle, then that view of the color space is represented by the Spearhead (Figure 2-14).
The Spearhead is essentially a compressed, “folded” view of the Diamond display. If you take the bottom green/red diamond and fold it up vertically over the green/blue diamond, you have a single diamond with green on the left and red/blue on the right, with black at the bottom and white at the top. Then fold the green side of the diamond horizontally over the red/blue side, with the fold along the black to white axis of the diamond. Now you have a triangle with black at the bottom, white at the top and saturation on the horizontal axis (Figure 2-15).
This allows a colorist to adjust live video signals in the HSV (Hue, Saturation, Value) space within the valid signal gamut range. The Spearhead display is constructed by plotting the maximum of the R’, G’, and B’ color values for each sample versus the minimum of the three values. The resulting area is a triangle that represents the full RGB color gamut. This triangle represents Lightness with black at the bottom left corner of the triangle and white at the top left corner. The horizontal axis represents chroma increasing from left to right. Value increases up from the bottom, extending towards the right and Saturation extends from the top towards the right.
The Spearhead is a unique display because it represents both luminance and saturation values in a single display. Normally, luminance values are displayed on a waveform monitor and saturation is displayed on a vectorscope. So the vectorscope and Spearhead together show two orthogonal views of threedimensional color space.
YPbPr component monitoring with Lightning Display
The Lightning display (Figure 2-16) facilitates quick alignment of component color bar signals to ensure correct amplitude and timing of the signal. A traditional vector display only shows the two color difference components, whereas the Lightning display shows the luma and color difference components. The Lightning Display is not intuitive for use in creative or artistic uses, but has value to the engineering department when monitoring color difference signals.
Often, operators and engineers want to monitor and adjust levels on the component signal. Many users are familiar with making these adjustments with a composite signal using a waveform monitor and vectorscope. Tektronix developed the Lightning display to simplify the task of making these adjustments within the component domain, using just one display.
The Lightning display also offers inter-channel timing information by looking at the green/magenta transitions. When the green and magenta vector dots are in their boxes, the transition should intercept the center crosshair in the line of nine timing marks. Therefore the Lightning display offers a simple means for checking luma and color difference signal amplitudes and inter-channel timing — all in one display using a standard color bars test signal.
The Lightning display is generated by plotting luma versus P'b in the upper half of the screen, and inverted luma versus P'r in the lower half — like two vector displays sharing the same screen. The bright dot at the center of the screen is blanking level (signal zero). Increasing luma is plotted upward in the upper half of the screen and downward in the lower half of the display. If luma gain is too high, the plot will be stretched vertically. If P'r gain is too high, the bottom half of the plot will be stretched horizontally. If P'b gain is too high, the top half of the display will be stretched horizontally.
If the color-difference signal is not coincident with luma, the transitions between color dots will bend. The amount of bending represents the relative signal delay between the luma and color-difference signals. The upper half of the display measures the P'b to Y' timing, while the bottom half measures the P'r to Y' timing. If the transition bends outward toward white, the color difference signal is leading the luma signal. If the transition bends inward toward the vertical center of the black region, the color-difference signal is delayed with respect to luma.
If color bars are at the header of a tape leader the editor can use the lightning display to ensure the ampltude is at the correct level, or make adjustment of the video and chroma levels until all the dots are in the boxes. Then the editor can ingest the material knowing he has compensated for any variety between tape machines.
Gamut/Amplitude Monitoring with the Arrowhead Display
The Arrowhead display can be used on component signals to determine if the signal is within gamut for a composite PAL or NTSC signal, without the need to process the signal through a composite encoder (Figure 2-17).
Composite Color Space
The greatest concern over modern color gamut is in R'G'B'. That is because most applications edit and create in either R'G'B' or Y’Cb’Cr (color difference) or translate and transcode between the two. However, program material may still be transmitted as a composite signal in a hybrid facility or for distribution in local markets. The requirements are different for keeping this signal legal.
Normally, the component signal is applied to a composite encoder. The composite signal is then measured and monitored using a familiar analog waveform monitor and vectorscope. Tektronix developed the Arrowhead display to allow engineers and operators to easily see out-of-gamut conditions in composite color space without requiring a composite encoder.
The Arrowhead is similar in basic structure to the Spearhead but illustrates composite color gamut. The artistic use of Arrowhead is limited, but it is essential to the technical monitoring of the composite signal.
The Arrowhead display plots luma on the vertical axis, with blanking at the lower left corner of the arrow. The magnitude of the chroma subcarrier at each luma level is plotted on the horizontal axis, with zero subcarrier at the left edge of the arrow. The upper sloping line forms a graticule indicating 100% color bars total luma + subcarrier amplitudes. The lower sloping graticule indicates a luma + subcarrier extending toward sync tip (maximum transmitter power). The electronic graticule provides a reliable reference to measure what the luminance plus color subcarrier will be when the signal is later encoded into NTSC or PAL. An adjustable modulation depth alarm capability is offered to warn the operator that the composite signal may be approaching a limit. The video operator can now see how the component signal will be handled in a composite transmission system and make any needed gradings in production.
Normally for NTSC transmission the threshold is set at 110 IRE because values over this limit can cause problems at the transmitter. The multi-format nature of Tektronix waveform monitors permits the Arrowhead display to be used not only for standard definition, but also for high definition video signals which maybe downconverted to standard definition for broadcast or distribution.
Gamut Monitoring Conclusion
While virtually all component video transmission occurs in one of the standard color difference formats today, viewing those signals in the R'G'B' color space remains a necessity. As computers become more widely used in video applications, the possibility of creating illegal video signals increases and the Tektronix-exclusive Diamond display is the best tool for keeping track of gamut violations. Performing color grading adjustments, gray scale tracking adjustments, or black balance adjustments are additional tasks that are simplified by the Diamond and Spearhead displays. The Spearhead and the vectorscope are great complementary displays that provide a full orthogonal view of three-dimensional color space. The Arrowhead display offers a means for the digital hybrid facility to ensure that the R'G'B' signal is valid for transmission in a PAL or NTSC environment, and it keeps the operator’s task within the digital domain. For tape alignment, the Lightning display ensures correct amplitude and inter-channel timing of the signal using the familiar color bars test signal. These exclusive Tektronix displays help simplify the tasks that video professionals face in an ever increasing complex environment.
Introduction to the Luma Qualified Vectorscope
The Luma Qualified Vectorscope (LQV) operates like a regular vectorscope except that the user can filter the signal being displayed on the vectorscope based on a maximum and minimum luma range or, more specifically, voltage.
A traditional vectorscope has no way to display luma values. It simply shows the saturation and hue values. Saturation is represented with less saturation displayed in the center of the vectorscope’s circular display, and greater saturation displayed as the trace extends out towards the edges of the circle. The vectorscope also displays the hue of the signal as angles around the circle. Roughly, the red hue is displayed near the top of the vectorscope, blue to the right and green to the left. Specific hues can be more closely determined by looking at the targets for the primary and secondary colors that are displayed on the vectorscope graticule (Figure 2-18).
However, on a vectorscope, it is impossible to tell white from black. Since both are without saturation, both appear superimposed in the center of the vectorscope along with all levels of pure gray.
The LQV was designed with the colorist in mind. Virtually all color grading toolsets divide the ability to alter hue and saturation based on specific tonal ranges. Most popular of these tools are the three trackballs or color wheels that provide control over the hue and saturation of the shadows, mid-tones and highlights of the signal. On a traditional vectorscope, these three tonal ranges are overlaid on each other with no way to distinguish the hue and saturation levels of one tonal range compared to any other.
But with Luma Qualified Vectorscopes, the colorist or operator can specifically determine what exact tonal range is displayed. This means that control over the signal using the three trackballs, or any tonal range-based controls, is much easier to see.
To understand the practical nature of this display, it must first be set up according to the preference of the user. Tektronix 7000 and 8000 series waveform monitors and rasterizers can display as many as four “trace” displays at once. The 5200 series can only display two “trace” displays. For the 7000 and 8000 series, it is possible to display three separate Luma Qualified Vectorscope displays - one for each tonal range - and a standard vectorscope display all at once. For the 5200, the most practical combination of displays is to have one Luma Qualified Vectorscope set for the shadows and another set for highlights.
Set-Up Of A Luma Qualified Vectorscope
To set up this combination on a 5200:
- Using the thumbnail button to choose one of the four quadrants. Preferably a “trace” display if there are already two “trace displays” on the monitor.
- Press the Vector button to turn that quadrant into a vectorscope display (see Figure 2-21).
- Hold down the same Vector button until a menu appears in the vectorscope quadrant.
- Set the display type to vector. The bar targets can be set to either 75% or 100%. The “center on” should be set to Black. The display mode should be set to Normal. These are all basically the default settings.
- To change the Luma Qualified Vector menu option use the “Adjust” down arrows to highlight the Luma Qualified Vector option.
- Press the SEL button or the right arrow key to toggle between the “Off” status and the “On” status for this option.
- When the status for Luma Qualified Vector is ON, two additional menu choices appear below.
- Press the down arrow to move to the LUMA HIGH option.
- Press the right arrow to select the numeric range choice.
- Use the GENERAL dial to select the top of the range. If you want to have this particular LQV to display highlights, then the default should already be 766.92. If it’s not, then rotate the dial clockwise until the numeric display reaches its maximum.
- Press the left arrow key to move back to the LUMA HIGH option, then press the down arrow key to move down to the LUMA LOW option.
- Press the right arrow to select the numeric range choice.
- Use the GENERAL dial to select the bottom of the range. In this case we’ve set the top to the upper limit in step 10. Now complete the description of the highlight range by entering a lower limit. These two values will describe the range that this particular LQV will display. The number you choose for this lower limit is one of personal taste. You may want to set a number now and then revise it later when you see how it works. The real range of millivolts is from 0-700, so to describe highlights as the brightest 10% of the image, set the lower level to approximately 630mv. To describe highlights as the brightest 25% of the image set the lower level to 575. To do your own math, turn your percentage to a decimal value (25%=.25, 33%=.33), multiply that decimal value times 700 and subtract that number from 700. So 10% = .10 x 700 = 70. 700-70=630. Don’t worry about exact values. The control is very granular and the difference between 1 or 2 millivolts won’t be noticed (see Figure 2-22).
- Press the Vector button to save your changes.
- Use the thumbnail button to move to the other trace display or to any other quadrant.
- Follow steps 2 through 9
- In steps 1-14 you created an LQV display for highlights (Figure 2-23). To create a similar LQV to display shadows (Figure 2-24), follow the directions in step 10, but change the LUMA HIGH value so that only the lowest 10% (or whatever your chosen level) is displayed. Using the math example in step 13, 70 millivolts is 10%. To set the upper range of your shadow range, add 70 to 0mv. If you prefer 20, set it at 140mv.
- Use the left arrow to move back to the LUMA HIGH menu choice.
- Use the down arrow to move down to the LUMA LOW menu choice.
- Use the right arrow to select the numeric range choice.
- Use the GENERAL dial, turning counter clockwise, to adjust the level as low as it will go. This level is already probably set to the default of -51.14 mv. If so, there is no need to adjust it (see Figure 2-23).
- Press the VECTOR button to save your changes.
Notice that the two quadrants that you just created look identical except that each has a small text overlay that says “LQV Enabled” and below that it shows the range of luminance values that will be displayed. These should be the numbers you just set.
Saving a Preset
That last setup required a lot of work, so you may want to save this combination of displays as a preset that can be recalled, saved and shared with others. To save this, execute the following steps:
- Press the PRESET button. The bottom buttons will light up and eight “soft” buttons will appear on the display screen along the bottom. Each of these soft buttons may have a name already assigned, or they could all read “
” if your unit has come straight from the factory.
- To save your preset (the two LQVs plus whatever the other two quadrants are set to), hold down one of the lit buttons at the bottom, corresponding to one of the “soft buttons” on the screen. (WARNING: If there is already a preset on the soft button you choose, it will erase that preset!)
- A dialog will appear on screen: “Save Preset #x” Continue will overwrite existing preset. Select Continue to proceed or Cancel to abort. To save your preset, use the right arrow key to highlight the Continue button in yellow and press the SEL button to execute the choice. A dialog will appear confirming that you have saved the preset.
- The default name of that preset is probably “A1.” To choose a more descriptive name for your preset, follow the following steps.
- Hold down the preset button for 2 seconds, or until a dialog box appears at the bottom screen including the menu choice “Save Preset.”
- Use the down arrow key to select the menu option, “Rename Preset.”
- Press the right arrow twice to navigate over to the menu choice “Rename Group A.”
- Use the down arrow key to select the menu option “A1” or whatever your newly created preset was called.
- Press the right arrow. A new dialog will appear called “Rename Preset.” Below, are eight boxes. The first two boxes may have the letters A1 in them.
- To change the name of the preset, press the right arrow key three times to navigate past Cancel, Clear, and Accept, moving the highlight to the first box.
- Use the General dial to scroll through the capital letters, numbers, symbols, space, lowercase letters, numbers, symbols and space choices. To call this preset simply LQV, use the General dial to scroll to the capital letter L.
- Use the right arrow key to advance to the next box in the Rename dialog.
- Use the General dial to scroll to the capital letter Q.
- Use the right arrow key to advance to the next box in the Rename dialog.
- Use the General dial to scroll to the capital letter V.
- To save your newly named Preset, press the right arrow key eight times until the highlight appears in the box just to the left of the word Accept in the Rename Preset dialog.
- Press the enter key to save the Preset.
- Check your saved preset by pressing the Preset button twice. Once to exit the Preset dialog, and once more to see the Preset soft keys along the bottom. You should see your newly named preset in the soft key above the WFM button.
- In the future, to recall this preset, simply press the Preset button then the WFM button. If you saved your preset to a position other than the first one, press the corresponding button to the soft key with your desired preset.
Preset Options for Using Luma Qualified Vectorscopes
If you have a Tektronix 7000 or 8000 series, you could create a preset with another LQV display to show the mid-tones. To do this, set the LUMA HIGH value to be the same as the LUMA LOW value of your Highlight LQV and set the LUMA LOW value to be the same as the LUMA HIGH value of your Shadow LQV. You could also set a fourth quadrant to show a standard vectorscope or any other display.
Another variant on this preset for a 5200 would be to have a Shadow LQV beside a regular vectorscope in one preset and another preset with a Highlight LQV beside a regular vectorscope. Then when you are adjusting shadows you can see the effect on just the shadows by calling up your Shadow LQV preset which will show you the effect of your adjustments just in the shadows in your Shadow LQV while at the same time seeing the effect on the entire signal in the standard vectorscope. Then switch presets to the Highlight LQV preset and adjust your highlights while viewing the effect on just the Highlight LQV display while at the same time seeing the effect on the entire signal in the standard vectorscope.
Another variant would be swapping the standard vectorscopes in the presets with Diamond displays so you could monitor whether your adjustments were causing gamut errors.
Note: A Preset saves the configuration of the instrument and also remembers the configuration of each measurement for each tile. Therefore you could configure a variety of different display for each of the tiles, so that when the measurement button is selected for that tile a different display is configured.
Color Grading Using Luma Qualified Vectorscopes
Numerous presets using Luma Qualified Vectorscopes are possible. For the following section on using Luma Qualified Vectorscopes, we’ll use the original example of two side by side Luma Qualified Vectorscopes: One set to display the brightest 10% of the signal and one set to display the darkest 10% of the signal. This is the preset we created in the previous section Figure LQVSETup,)
When using vectorscope displays, whether standard vectorscopes or LQVs, the most intuitive color grading control to use is the color wheel. With a hardware control surface (like the DaVinci Resolve, or Tangent Devices WAVE or ELEMENT, or Avid MC Color), this means using the three trackballs related to shadows, mid-tones and highlights. Avid software uses the term Hue Offsets to describe these color wheels.
The shadow trackball or shadow color wheel should have a very good correlation to the Luma Qualified Vectorscope that you’ve created with a range from about -51.14mv to 70mv. To balance the color of your shadows, the trace in the Shadow LQV should be centered on the middle target.
TIP: To get a better view of this center region on your LQV or regular vectorscope, you can press the GAIN button to zoom in. The default zoom is 2x. If you hold down the GAIN button for 2 seconds, a dialog appears allowing you to change the 2x option to 5x or 10x. Experiment with how much gain (zoom) helps you get the most accurate results quickest. Press the GAIN button again to exit the menu options.
Using the shadow trackball or shadow color wheel, try to “aim” the trace into the center of the LQV set to display shadows. The direction you move the trackball or cursor in the color wheel should correspond directly with the movement of the trace in the vectorscope or LQV.
Then, using the highlight trackball or highlight color wheel, try to “aim” the trace into the center of the LQV set to display highlights.
This method of color balancing has been used by colorists for decades using a standard vectorscope, but the ability to concentrate on a specific luma range makes balancing the two separate luma ranges much easier and more intuitive.
With your shadows and highlights color balanced, you can use the mid-tone (gamma) trackball or color wheel while monitoring a standard vectorscope or a LQV setup for mid-tones to balance your mid-tones. With your shadows and highlights dialed in properly, mid-tones can also be adjusted to taste using your picture monitor.
With a complex image with a lot of chroma and mid-tone values, you’ll quickly see the value of Tektronix’s exclusive Luma Qualified Vectorscopes.
Color Grading with the Diamond Display
Most seasoned professional colorists prefer the RGB Parade waveform monitor for balancing colors and setting levels. For many gradings, it’s an excellent choice, but for a single waveform display that gives the most information about the image, Tektronix’s Diamond display provides a wider range of information. Using it to make intuitive color grading decisions takes a little effort, but is worth the time to learn.
Section 1 of this primer describes the way the Diamond display represents the incoming signal. To recap, black is in the center of the display where the two diamonds join. White is displayed at the top of the top diamond and, less intuitively, also at the bottom of the bottom diamond. Monochromatic tones run straight up the middle of the two diamonds, vertically. Saturation is represented as the trace extends away from that middle axis. The green channel is presented on the left side of both the top and bottom diamonds. The red channel is presented on the right of the bottom diamond and the blue channel is represented on the right of the top diamond. There are letters representing these color channels on each side of the diamonds, so there’s no need to memorize these relationships (Figure 2-31).
Call up a Diamond display in one of the quadrants of your Tektronix waveform monitor. Bring the Diamond display to full raster. This is done differently on various Tektronix waveform monitors. On the 5200, with the Diamond display quadrant selected, hold down the Thumbnail button. This fills the screen with the Diamond display.
To create the display in Figure 2-32, an image was routed from a color corrector to the WFM5200 and all saturation was removed from the image. The Diamond Display shows a fine white line that travels perfectly vertically from the bottom of the bottom diamond up through the center, where the two diamonds meet, and continues all the way to the top of the top diamond.
Using the shadow trackball or color wheel, the shadows were pushed to be as saturated as possible and then we rolled through the various vectors (hues). We monitored these gradings on the Diamond display. With the blacks pushed very green, the trace on the Diamond extends outside of the diamonds to the left (Figure 2-33). This makes sense because the green channel is represented to the left.
Rolling to a perfect blue, the trace extends outside to the right of the top diamond and is straight up and down in the bottom diamond (Figure 2-34).
Rolling the trackball to magenta, the trace extends to the right on both diamonds, which makes sense because magenta is made up of red and blue. The red and blue channels are represented to the right side of the diamonds (Figure 2-35).
Rolling to a perfect red, the trace in the top diamond is centered and the trace extends outside and to the right of the bottom diamond. This makes sense because the red channel is indicated by the bottom right half of the diamond (Figure 2-36).
Rolling to a perfect yellow, the trace goes to the left of the top diamond and the center of the bottom diamond (Figure 2-37). This connection is slightly harder to grasp, because yellow is made up of red and green. Red is indicated to the right of the bottom diamond and green is indicated at the left of the diamonds. In the top diamond, there is no blue in yellow (yellow and blue are opposites) so the trace goes to the green side, but in the bottom diamond, since yellow is made up of equal parts of red (right) and green (left), yellow is indicated in the middle of the red/green diamond. The other color that works in this same way is Cyan.
Rolling to cyan, the trace in the top diamond centers because the top diamond is green/blue and cyan has equal amounts of cyan and blue (Figure 2-38). In the bottom diamond, the trace extends off to the left because cyan has no red in it (cyan and red are opposites), so the trace goes to the green side because cyan has green in it.
As an experiment, try this on your own system by lowering saturation on your color corrector so that there is no saturation, then use trackballs or color wheels in the shadows to push the color to some extreme, then, just using the trace on the Diamond display, try to guide the trace back into a perfect vertical alignment. With some practice, this will become second nature.
You can run through this same experimentation with the highlight trackball or color wheel. Without looking at the cursor on the color wheel, watch the Diamond display and try to “dial in” each of the primary and secondary colors perfectly. Then try to null out the colors, balancing the image in the center again. Then try the same experiment with the gamma or mid-tone trackball.
Another good experiment is to run this same process, but to use a preset where you can also see an RGB Parade monitor alongside the Diamond display or a vectorscope with the Diamond display. Seeing the Diamond alongside a display that you may be more used to will give you a good reference for what’s happening, almost like a side by side translation.
The advantage of using the Diamond display is that you can set your luma levels, saturation levels, and balance your colors all with a single display while intuitively monitoring gamut levels in every range.
Color Grading with the Spearhead and Vectorscope
As explained in the section titled “Color Grading with the Spearhead Display“, if you imagine looking down at color space from above as being the vectorscope, then the Spearhead is the same color space examined from the side (Figure 2-39).
The Spearhead shows black at the bottom left corner, white at the top left corner and saturation extends out from the left side to the right side. The value of the Spearhead is that in addition to showing luminance (up the left side vertically) and saturation, it also provides a quick method of checking for gamut issues and a good understanding of where those gamut issues are (in the blacks, whites or mid-tones). This makes level gradings easy to make with the Spearhead.
Balancing colors is not very intuitive with just the Spearhead, but combining it with the vectorscope really creates a very complementary view of color space.