Introduction

The video industry commonly evaluates color using known test patterns. In this way, it is straightforward to learn how video was processed based on how the pattern deviates from that known standard. Classically, in an analog video environment, SMPTE (Society of Motion Picture and Television Engineers) color bars are used (these are generally referred to as Engineering Guideline EG 1-1990, see the SMPTE website for more information).

QuickTime Test Pattern movie files

QuickTime Test Pattern movie files are available for download on the QuickTime Developer page. These color bars have been designed to better help you to check the color reproduction of your QuickTime-based application or workflow. See Figure 1 for an example test pattern.

The test patterns contain color and grayscale patterns that let you test issues such as:

a) 16 black levels on black (black crush test/range expansion)

b) Continuous gradient (quantization/super-black,super-white handling)

c) 100% color bars (matrix, color match)

d) 75% color bars (matrix, color match)

e) Quantized gradient (gamma)

f) 16 white levels on white (white crush test/range expansion)

These features can be evaluated by opening the test pattern in the desired application and visually inspecting it, reading back the display buffer pixel values using the DigitalColor Meter utility (see Interpreting 75% Grey Levels), or via a professional waveform monitor and/or vectorscope (see Output to Vectorscope and/or Waveform Analyzer).

Interpreting 75% Grey Levels

As shown in Figure 1, the QuickTime Test Pattern movie files contain 75% color bars.

The easiest way to check the color management in your application is to use 75% grey bars.

On Mac OS X 10.6 Snow Leopard and later with modern media, 75% grey bars will be processed by a 1.96 to 2.2 gamma conversion, resulting in a 198 value. See Figure 3. Here's a detailed explanation:

AV Foundation and QTKit will use a value of approximately 1.96 as the gamma of video, which is the gamma value without the boost required for classic dim surround viewing environments. Here's how that number is derived: if you had measured the response of a CRT, most had a gamma of between 2.4 and 2.5 (it is dependent on how the brightness and contrast is set on the monitor). If you choose a value halfway in between, 2.45, and remove the contrast enhancement (the 1.25 gamma boost provided for viewing in dimly lit environments) you'll get a value very close to 1.96 (2.45/1.25 = 1.96).

On Mac OS X 10.6 Snow Leopard and later, the video gamma is then converted to the 2.2 display buffer gamma when the application performs the color match to the display.

To recap, if you take the 75% grey bar and perform the 1.96 to 2.2 gamma conversion, the calculations are as follows: 191/255 (75% grey bars in an 8-bit (0-255) RGB space have 191 energy levels, because 191 is 75% of 255), raised to the 1.961/2.2 exponent (gamma conversion), scaled back to 255, producing a value of approximately 198:

With rounding error, the results will be in this range.

That's the default behavior for a modern AV Foundation, QTKit and QuickTime Visual Context application during playback. If you get 191's or other values, it probably means there's no color management being applied. See How to ensure your app video is color managed.

Vectorscope

A vectorscope is a type of instrument that measures the color relationship of a video signal by plotting the two chroma components of a signal. Vectorscopes often include printed boxes indicating standard colors. Ideally, a correctly reproduced color bar signal should 'hit' the vectorscope boxes.

Typically, there is a set of six boxes (Yellow, Cyan, Green, Magenta, Red and Blue) representing 75% color bars, and another set for 100% color bars -- these correspond to sections c) and d) of the QuickTime Test Pattern movie files as shown in Figure 1. If the representation's color vector 'hits' inside the corresponding box during testing, then that color is correct.

Waveform Monitor

A waveform monitor displays the composite of the amplitude of all of the lines of a given frame of video over time. Among other things, this instrument can help ensure that all values are used with no range of blacks or whites being truncated (crushed). Here's elements of the QuickTime Test Pattern Movie file (see Figure 1) to examine on a waveform monitor:

a) Black boxes on a black background. On the waveform monitor, this portion of the test signal should be visible as 16 steps beginning with true black.

b) Gradient. This monotonically increasing gradient includes super-whites and super-blacks that are expected to be crushed by a rendering process that includes full-range RGB. Consequently, the result on the waveform monitor should range from true-black to full-white, but the beginning and end of the repeating pattern may be clamped to black or white for the extent of the range corresponding to super-white and super-black. The gradient should form a straight line on the waveform monitor's display (otherwise it indicates a gamma conversion was applied). The line should also be smooth, otherwise quantization may have occurred.

f) White boxes on a full-white background. On the waveform monitor this portion of the test signal should be visible as 16 steps beginning from full-white.

Additional pattern features

e) Quantized gradient. This is used to perform a numerical method on plotting the area under those targets and solving for the gamma of the signal.