Important: As with any content, the color bars themselves must be 'nclc' color tagged to be correctly interpreted. See Video Color Management in AV Foundation and QTKit to learn how to tag your video.

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).

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 ).

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

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.

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:

Figure 2 Gamma Conversion Calculation for 75% Grey Bars

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

Figure 3 75% Grey Levels in AV Foundation and QTKit apps on Mac OS X 10.6 Snow Leopard and later

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.

Note: By default, color matches are performed using perceptual rendering intent, but because the profiles are "matrix-based", the results of the color match are identical for all rendering intents.

Color match will keep grey levels 'pure' grey In a color managed application, given a correct display profile, a color match won't change a grey value from appearing grey. It may adjust the brightness based on the gamma correction, but it won't add color to it (grey's remain R=G=B). Therefore, the RGB components of a given grey value from one of the color bars will have identical values after the color match. If they don't, that's an issue you should investigate (see How to ensure your app video is color managed).