Analyzing Images Using the Video Scopes

The following sections describe the use of each scope that Color provides:

The Waveform Monitor

The Waveform Monitor is actually a whole family of scopes that shows different analyses of luma and chroma using waveforms.

What Is a Waveform?

To create a waveform, Color analyzes lines of an image from left to right, with the resulting values plotted vertically on the waveform graticule relative to the scale that’s used—for example, –20 to 110 IRE (or –140 to 770 mV) on the Luma graph. In the following image, a single line of the image is analyzed and plotted in this way.

Figure. Illustration of a single line waveform analysis.

To produce the overall analysis of the image, the individual graphs for each line of the image are superimposed over one another.

Figure. Complete waveform graph.

Because the waveform’s values are plotted in the same horizontal position as the portion of the image that’s analyzed, the waveform mirrors the image to a certain extent. This can be seen if a subject moves from left to right in an image while the waveform is playing in real time.

Figure. Sample image and accompanying waveform.

With all the waveform-style scopes, high luma or chroma levels show up as spikes on the waveform, while low levels show up as dips. This makes it easy to read the measured levels of highlights or shadows in the image.

Changing the Graticule Values

The Waveform Monitor is the only scope in which you can change the numeric values used to measure the signal. By default, the Waveform Monitor is set to measure in IRE, but you can also switch the scope to measure using millivolts (mV) instead by clicking one of the buttons to the right of the waveform selection buttons.

Figure. IRE and mV buttons.

Waveform Analysis Modes

The Waveform Monitor has eight different modes. For more information, see:

The Parade Scope

The Parade scope displays separate waveforms for the red, green, and blue components of the image side by side. If Monochrome Scopes is turned off, the waveforms are tinted red, green, and blue so you can easily identify which is which.

Figure. Sample image with accompanying RGB parade graph.

Note: To better illustrate the Parade scope’s analysis, the examples in this section are shown with Broadcast Safe disabled so that image values above 100 percent and below 0 percent won’t be clipped.

The Parade scope makes it easy to spot color casts in the highlights and shadows of an image, by comparing the contours of the top and the bottom of each waveform. Since whites, grays, and blacks are characterized by exactly equal amounts of red, green, and blue, neutral areas of the picture should display three waveforms of roughly equal height in the Parade scope. If not, the correction is easy to make by making adjustments to level the three waveforms.

Figure. RGB Parade graph of image with color cast.
Figure. RGB Parade graph after correction.

The Parade scope is also useful for comparing the relative levels of reds, greens, and blues between two shots. If one shot has more red than another, the difference shows up as an elevated red waveform in the one and a depressed red waveform in the other, relative to the other channels. In the first shot, the overall image contains quite a bit of red. By comparison, the second shot has substantially less red and far higher levels of green, which can be seen immediately in the Parade scope. If you needed to match the color of these shots together, you could use these measurements as the basis for your correction.

Figure. Image with an elevated red channel in RGB parade graph.
Figure. Image with elevated green channel in RGB parade graph.

The Parade scope also lets you spot color channels that are exceeding the chroma limit for broadcast legality, if the Broadcast Safe settings are turned off. This can be seen in waveforms of individual channels that either rise too high or dip too low.

Figure. RGB parade graphs exceeding 100 and 0 percent limits.

The Overlay Scope

The Overlay scope presents information that’s identical to that in the Parade scope, except that the waveforms representing the red, green, and blue channels are superimposed directly over one another.

Figure. Sample unbalanced image and overlay graph.

This can make it easier to spot the relative differences or similarities in overlapping areas of the three color channels that are supposed to be identical, such as neutral whites, grays, or blacks.

Another feature of this display is that when the video scopes are set to display color (by turning off the Monochrome Scopes parameter), areas of the graticule where the red, green, and blue waveforms precisely overlap appear white. This makes it easy to see when you’ve eliminated color casts in the shadows and highlights by balancing all three channels.

Figure. Sample balanced image and Overlay graph.

The Red/Green/Blue Channels Scopes

These scopes show isolated waveforms for each of the color channels. They’re useful when you want a closer look at a single channel’s values.

Figure. Sample image and red channel graph.

The Luma Scope

The Luma scope shows you the relative levels of brightness within the image. Spikes or drops in the displayed waveform make it easy to see hot spots or dark areas in your picture.

S0195_luma.png

The difference between the highest peak and the lowest dip of the Luma scope’s graticule shows you the total contrast ratio of the shot, and the average thickness of the waveform shows its average exposure. Waveforms that are too low are indicative of images that are dark, while waveforms that are too high may indicate overexposure.

Figure. Underexposed, overexposed, and well-exposed luma waveforms.

If you’re doing a QC pass of a program with the Broadcast Safe settings turned off, you can also use the scale to easily spot video levels that are over and under the recommended limits.

The Chroma Scope

This scope shows the combined CB and CR color difference components of the image. It’s useful for checking whether or not the overall chroma is too high, and also whether it’s being limited too much, as it lets you see the result of the Chroma Limit setting being imposed when Broadcast Safe is turned on.

For example, the following graph shows extremely saturated chroma within the image:

Figure. Extremely saturated image in chroma graph.

When you turn Broadcast Safe on with the default Chroma Limit value of 50, you can see that the high chroma spikes have been limited to 50.

Figure. Clipped saturation seen in Chroma graph.

The Y′CBCR Scope

This scope shows the individual components of the Y′CBCR encoded signal in a parade view. The leftmost waveform is the luma (Y′) component, the middle waveform is the CB color difference component, and the rightmost waveform is the CR color difference component.

Figure. The YCbCr graph.

The Vectorscope

The Vectorscope shows you the overall distribution of color in your image against a circular scale. The video image is represented by a graph consisting of a series of connected points that all fall at about the center of this scale. For each point within the analyzed graph, its angle around the scale indicates its hue (which can be compared to the color targets provided), while its distance from the center of the scale represents the saturation of the color being displayed. The center of the Vectorscope represents zero saturation, and the farther from the center a point is, the higher its saturation.

Figure. Sample image and Vectorscope graph.

If the Monochrome Scopes option is turned off in the User Prefs tab of the Setup room, then the points of the graph plotted by the Vectorscope will be drawn with the color from that part of the source image. This can make it easier to see which areas of the graph correspond to which areas of the image.

Figure. Vectorscope graph with the monochrome scopes option turned off.

Comparing Saturation with the Vectorscope

The Vectorscope is useful for seeing, at a glance, the hue and intensity of the various colors in your image. Once you learn to identify the colors in your shots on the graph in the Vectorscope, you will be better able to match two images closely because you can see where they vary. For example, if one image is more saturated than another, its graph in the Vectorscope will be larger.

Figure. Lesser and greater saturation compared in Vectorscope.

Spotting Color Casts with the Vectorscope

You can also use the Vectorscope to spot whether there’s a color cast affecting portions of the picture that should be neutral (or desaturated). Crosshairs in the Vectorscope graticule indicate its center. Since desaturated areas of the picture should be perfectly centered, an off-center Vectorscope graph representing an image that has portions of white, gray, or black clearly indicates a color imbalance.

Figure. Color cast shown in Vectorscope.

The Color Targets

The color targets in the Vectorscope line up with the traces made by the standard color bar test pattern, and can be used to check the accuracy of a captured video signal that has recorded color bars at the head.

These targets also correspond to the angles of hue in the color wheels surrounding the Color Balance controls in the Primary In and Out and Secondaries rooms. If the hues of two shots you’re trying to match don’t match, the direction and distance of their offset on the Vectorscope scale give you an indication of which direction to move the balance control indicator to correct for this.

Figure. Color bars signal and Vectorscope targets.

At a zoom percentage of 75 percent, the color targets in the Vectorscope are calibrated to line up for 75 percent color bars. Zooming out to 100 percent calibrates the color targets to 100 percent color bars. All color is converted by Color to RGB using the Rec. 709 standard prior to analysis, so color bars from both NTSC and PAL source video will hit the same targets.

Note: If Broadcast Safe is turned on, color bars’ plots may not align perfectly with these targets.

The I Bar

The –I bar (negative I bar) shows the proper angle at which the hue of the dark blue box in the color bars test pattern should appear. This dark blue box, which is located to the left of the 100-percent white reference square, is referred to as the Inphase signal, or I for short.

Figure. Negative I bar in the Vectorscope.

The I bar (positive I bar) overlay in the Vectorscope is also identical to the skin tone line in Final Cut Pro. It’s helpful for identifying and correcting the skin tones of actors in a shot. When recorded to videotape and measured on a Vectorscope, the hues of human skin tones, regardless of complexion, fall along a fairly narrow range (although the saturation and brightness vary). When there’s an actor in a shot, you’ll know whether or not the skin tones are reproduced accurately by checking to see if there’s an area of color that falls loosely around the I bar.

Figure. I bar and skin tones in the Vectorscope.

If the skin tones of your actors are noticeably off, the offset between the most likely nearby area of color in the Vectorscope graph and the skin tone target will give you an idea of the type of correction you should make.

The Q Bar

The Q bar shows the proper angle at which the hue of the purple box in the color bars test pattern should appear. This purple box, which is located at the right of the 100-percent white reference square, is referred to as the +Quadrature signal, or Q for short.

Figure. Q bar in the Vectorscope.

When troubleshooting a video signal, the correspondence between the Inphase and +Quadrature components of the color bars signal and the position of the –I and Q bars shows you whether or not the components of the video signal are being demodulated correctly.

The Histogram

The Histogram provides a very different type of analysis than the waveform-based scopes. Whereas waveforms have a built-in correspondence between the horizontal position of the image being analyzed and that of the waveform graph, histograms provide a statistical analysis of the image.

Histograms work by calculating the total number of pixels of each color or luma level in the image and plotting a graph that shows the number of pixels there are at each percentage. It’s really a bar graph of sorts, where each increment of the scale from left to right represents a percentage of luma or color, while the height of each segment of the histogram graph shows the number of pixels that correspond to that percentage.

The RGB Histogram

The RGB histogram display shows separate histogram analyses for each color channel. This lets you compare the relative distribution of each color channel across the tonal range of the image.

For example, images with a red color cast have either a significantly stronger red histogram, or conversely, weaker green and blue histograms. In the following example, the red cast in the highlights can be seen clearly.

Figure.  Sample image and RGB Histogram.

The R, G, and B Histograms

The R, G, and B histograms are simply isolated versions of each channel’s histogram graph.

The Luma Histogram

The Luma histogram shows you the relative strength of all luminance values in the video frame, from black to super-white. The height of the graph at each step on the scale represents the number of pixels in the image at that percentage of luminance, relative to all the other values. For example, if you have an image with few highlights, you would expect to see a large cluster of values in the Histogram display around the midtones.

Figure. Sample image and Luma histogram.

The Luma histogram can be very useful for quickly comparing the luma of two shots so you can adjust their shadows, midtones, and highlights to match more closely. For example, if you were matching a cutaway shot to the one shown above, you can tell just by looking that the image below is underexposed, but the Histogram gives you a reference for spotting how far.

Figure. Underexposed image and histogram.

The shape of the Histogram is also good for determining the amount of contrast in an image. A low-contrast image, such as the one shown above, has a concentrated clump of values nearer to the center of the graph. By comparison, a high-contrast image has a wider distribution of values across the entire width of the Histogram.

Figure. High-contrast image and histogram.

The 3D Scope

This scope displays an analysis of the color in the image projected within a 3D area. You can select one of four different color spaces with which to represent the color data.

The RGB Color Space

The RGB color space distributes color in space within a cube that represents the total range of color that can be displayed:

  • Absolute black and white lie at two opposing diagonal corners of the cube, with the center of the diagonal being the desaturated grayscale range from black to white.

  • The three primary colors—red, green, and blue—lie at the three corners connected to black.

  • The three secondary colors—yellow, cyan, and magenta—lie at the three corners connected to white.

In this way, every color that can be represented in Color can be assigned a point in three dimensions using hue, saturation, and lightness to define each axis of space.

Figure. Illustration of RGB color cube.

The sides of the cube represent color of 100-percent saturation, while the center diagonal from the black to white corners represents 0-percent saturation. Darker colors fall closer to the black corner of the cube, while lighter colors fall closer to the diagonally opposing white corner of the cube.

Figure. 3D scope shown in RGB mode.

The HSL Color Space

The HSL (Hue, Saturation, and Luminance) color space distributes a graph of points within a two-pointed cone that represents the range of color that can be displayed:

  • Absolute black and white lie at two opposing points at the top and bottom of the shape.

  • The primary and secondary colors are distributed around the familiar color wheel, with 100-percent saturation represented by the outer edge of the shape, and 0-percent saturation represented at the center.

In this way, darker colors lie at the bottom of the interior, while lighter colors lie at the top. More saturated colors lie closer to the outer sides of the shape, while less saturated colors fall closer to the center of the interior.

Figure. 3D scope in HSL mode.

The Y′CBCR Color Space

The Y′CBCR color space is similar to the HSL color space, except that the outer boundary of saturation is represented with a specifically shaped six-sided construct that shows the general boundaries of color in broadcast video.

The outer boundary does not identify the broadcast-legal limits of video, but it does illustrate the general range of color that’s available. For example, the following image has illegal saturation and brightness.

Figure. 3D scope in YCbCr mode.

If you turn on the Broadcast Safe settings, the distribution of color throughout the Y′CBCR color space becomes constricted.

Figure. 3D scope in YCbCr mode with Broadcast Safe turned on.

The IPT Color Space

The IPT color space is a perceptually weighted color space, the purpose of which is to more accurately represent the hues in an image distributed on a scale that appears uniformly linear to your eye.

While the RGB, HSL, and Y′CBCR color spaces present three-dimensional analyses of the image that are mathematically accurate, and allow you to see how the colors of an image are transformed from one gamut to another, they don’t necessarily show the distribution of colors as your eyes perceive them. A good example of this is a conventionally calculated hue wheel. Notice how the green portion of the hue wheel presented below seems so much larger than the yellow or red portion.

Figure. Illustation of color wheel.

The cones of the human eye that are sensitive to color have differing sensitivities to each of the primaries (red, green, and blue). As a result, a mathematically linear distribution of analyzed color is not necessarily the most accurate way to represent what we actually see. The IPT color space rectifies this by redistributing the location of hues in the color space according to tests where people chose and arranged an even distribution of hues from one color to another, to define a spectrum that “looked right” to them.

In the IPT color space, I corresponds to the vertical axis of lightness (desaturated black to white) running through the center of the color space. The horizontal plane is defined by the P axis, which is the distribution of red to green, and the T axis, which is the distribution of yellow to blue.

Here’s an analysis of the test image within this color space.

Figure. 3D scope in IPT mode.

Sampling Color for Analysis

The 3D video scope also provides controls for sampling and analyzing the color of up to three pixels within the currently displayed image. Three swatches at the bottom of the video scope let you sample colors for analysis by dragging one of three correspondingly numbered crosshairs within the image preview area. A numerical analysis of each sampled color appears next to the swatch control at the bottom of the 3D video scope.

Figure. Color sampling swatches in 3D scope.

The color channel values that are used to analyze the selected pixel change depending on which color space the 3D scope is set to. For example, if the 3D scope is set to RGB, then the R, G, and B values of each selected pixel will be displayed. If the 3D scope is instead set to Y′CBCR, then the Y′, CB, and CR values of the pixel will be displayed.

You can choose different samples for each shot in the Timeline, and the position of each shot’s sampling crosshairs is saved as you move the playhead from clip to clip. This makes it easy to compare analogous colors in several different shots to see if they match.

This analysis can be valuable in situations where a specific feature within the image needs to be a specific value. For example, you can drag swatches across the frame if you’re trying to adjust a black, white, or colored background to be a uniform value, or if you have a product that’s required to be a highly specific color in every shot in which it appears.

Note: These controls are visible only when the 3D scope is occupying an area of the Scopes window.

To sample and analyze a color
  1. Click one of the three color swatch buttons at the bottom of the 3D scope.

    Figure. Clicking a color swatch.
  2. Click or drag within the image preview area to move the color target to the area you want to analyze.

    Figure. Dragging a color target in the preview area.

    As you drag the color target over the image preview, four things happen:

    • The color swatch updates with that color.

    • The H, S, and L values of the currently analyzed pixel are displayed to the right of the currently selected swatch.

    • Crosshairs identify that value’s location within the three-dimensional representation of color in the 3D scope itself.

      Figure. Crosshairs showing analyzed value in the 3D scope.

      Each color target is numbered to identify its corresponding color swatch.

    • A vertical line appears within the Hue, Sat, and Lum curves of the Secondaries room, showing the position of the sample pixels relative to each curve.

      Figure. Sampled pixel analysis in Secondaries curves.