Adjusting Gamma

Final Cut Pro provides gamma control adjustments for RGB video and still-image formats.

About Gamma

Gamma is an implicit or explicit transfer function that maps input intensity to output intensity, usually in a nonlinear way. The most common example is a CRT monitor, where the brightness onscreen is less than expected based on the input voltage. If the input voltage ranges between 0 (black) and 1 (white), one would expect a voltage of 0.5 to generate a brightness value of about 50 percent gray on the screen, but in fact only 18 percent gray is generated. This is simply the nature of a CRT’s electron gun response to input voltage.

The CRT applies an implicit power function in which the input signal is raised to the power of 2.5 (V2.5). Gamma is usually defined by the value of the exponent in the power function—in this case, 2.5.

Nonlinear Coding

Human perception of brightness is also a power function that’s nearly opposite of CRT gamma. This means that exponential increases in physical light intensity are required to create perceptible shifts in brightness. To take advantage of this fact when coding luma values in a limited bit depth (such as 8 bits, or 256 codes), incoming video signals are modified with a gamma curve so that there are more codes concentrated in the dark part of the signal and larger code jumps as luma increases. This corresponds to the perception of brightness and therefore uses the available bits in the most perceptually efficient way. If luma were coded equally from black to white, you would likely see “banding”—perceptible jumps in brightness—in the black areas, and many subtle changes in white levels would be imperceptible.

About Gamma Correction

Gamma correction compensates for the fact that video display devices (such as televisions) inherently convert image signals to light intensity in a nonlinear fashion, usually with an assumed gamma value of 2.5. The ITU-R BT.709 specification recommends an in-camera gamma correction of 1/2.2 (or 0.45). When the camera gamma correction is multiplied by the CRT gamma (2.5/2.2), the result is an overall display gamma of 1.14, which has slightly more contrast than a display gamma of 1.0. A display gamma of 1.1–1.2 is desired for television viewing in dim viewing conditions.

In summary, a gamma of 2.5 is inherent in CRT displays. Gamma correction is automatically applied in a video camera so that the video is displayed on a CRT with a bit of contrast, and this value is usually 1/2.2.

Choosing Real-Time Playback Gamma Correction Options

Whenever you watch your video on a computer display (such as video displayed in the Canvas, the Viewer, or via Digital Cinema Desktop Preview), Final Cut Pro applies gamma correction to the video to more closely approximate the way it would appear on a video monitor.

Note: Gamma correction is not applied to external video outputs such as DV or third-party interfaces because this correction is inherently applied by external video monitors.

If your computer’s graphics card supports gamma correction, Final Cut Pro automatically uses the card for accurate gamma correction. If your computer has an older graphics card that does not support gamma correction, you have two gamma correction options in the Playback Control tab of the System Settings window.

  • Accurate: High-quality gamma correction. This option is the most accurate but also requires the most processing power.
  • Approximate: Lower-quality gamma correction. This option leaves more processing power available for real-time effects but results in less accurate color and brightness rendition.

    Tip: If your real-time performance is limited, try changing the gamma correction setting to Approximate (if available).

To choose a gamma correction option in the current sequence
  1. Open a sequence in the Timeline.

  2. Choose Final Cut Pro > System Settings, then click the Playback Control tab.

  3. Choose Accurate or Approximate from the Gamma Correction pop-up menu.

Changing Gamma Settings for Video and Imported Still Images

When you import certain video or still-image formats, Final Cut Pro attempts to determine what gamma correction has already been applied to the media and stores that value in the clip’s Gamma Level property. During playback, Final Cut Pro uses the Gamma Level property to determine whether a clip requires gamma correction.

In general, Final Cut Pro assumes the following:

  • Most RGB-encoded media (such as TIFF or the Apple Animation codec) has an implicit gamma of 1.8. This assumption is correct for images created on a Mac computer with display profile gamma set to 1.8. If the image file was created on a different platform, or on a Mac computer with display profile gamma set to 2.2, you may need to adjust the clip’s Gamma Level property to 2.2.

    Because the Apple ProRes 4444 codec has been designed to work with Final Cut Pro and has a native gamma of 2.2, it automatically eliminates potential gamma shifts.

  • Y′CBCR media has an implicit gamma of 2.2. You cannot adjust the gamma of Y′CBCR clips.

A Common Gamma Correction Scenario

One of the most common situations that requires you to change the Gamma Level property of a clip is when you are rendering an RGB clip within a Y′CBCR sequence. Final Cut Pro uses QuickTime to import RGB media and always assumes that RGB media (except Apple ProRes 4444) has a gamma of 1.8. When Final Cut Pro renders these clips in a Y′CBCR sequence, the gamma is adjusted by a factor of 1.22 to match the 2.2 gamma compensation required for Y′CBCR.

The problem with this default behavior is that RGB files are not always created with a gamma of 1.8, even on Mac computers. In this case, you can adjust the gamma level of your imported RGB clips accordingly.

Supported File Formats

Final Cut Pro supports gamma adjustment for the following file formats.

  • Still images: JPEG, PNG, TIFF, SGI, PlanarRGB, MacPaint, and layered or flattened Photoshop (PSD) files.
  • QuickTime movie files: Movie files using the None or Animation codec

Adjusting Gamma in Imported Still-Image and Video Clips

There are two places to adjust gamma.

  • Gamma Level pop-up menu in the Editing tab of the User Preferences window: This pop-up menu globally determines gamma at the time media is imported.
  • Gamma Level clip property in the Item Properties window or Browser column: You can adjust the gamma of individual clips in the Browser or in the Item Properties window. This allows you to override the global setting applied by the Gamma Level pop-up menu in the Editing tab of the User Preferences window.

Choosing a Gamma Setting

Final Cut Pro analyzes imported still images and attempts to determine their encoding format automatically. Sometimes this interpretation is incorrect. If imported still images appear too light, or do not have enough contrast, then selecting Gamma Level 2.2 could correct for this. Alternatively, if the images appear too dark or have too much contrast, then this could be corrected by selecting Gamma Level 1.8.

To choose the gamma value applied to an imported file
  1. Choose Final Cut Pro > User Preferences, then click the Editing tab.

  2. Choose one of the following options from the Gamma Level pop-up menu.

    • Source: This option uses QuickTime to interpret the gamma of imported media files.
    • 1.8: Choose this option when importing media files created by an application in which you specified a gamma value of 1.8, or created with Mac OS X v10.5.x or earlier.
    • 2.2: Choose this option when importing media files created on a system whose display gamma was set to 2.2, or created by an application in which you specified a gamma value of 2.2.
    • Custom: Choose this option to enter any gamma value you wish in the Custom number field.

Note: When you adjust the gamma of a clip within Final Cut Pro, the media file is not modified; only the clip in the project is affected.

If you need to modify gamma for clips after importing, you can adjust the Gamma Level property in the corresponding Browser column or in the Item Properties window. The Gamma Level property is not shared between master and affiliate clips, so changing the gamma for a master clip has no effect on the affiliate clips, and vice versa.

To change the Gamma Level property of a clip in the Item Properties window
  1. Select a clip in the Browser or in a sequence.

  2. Choose Edit > Item Properties > Format (or press Command-9).

  3. Do one of the following:

    • Control-click the value next to the Gamma Level property, then choose a predefined gamma value for the clip (Source, 1.8, or 2.2) from the shortcut menu.

    • Click in the Gamma Level field and enter a gamma value or type “Source,” then press Enter.

      Tip: To see how a clip’s gamma affects its brightness, try changing the Gamma Level property of a clip while viewing the clip in the Viewer or Canvas.

To view the Gamma Level column in the Browser
  • Control-click in any column in the Browser, then choose Show Gamma Level from the shortcut menu.

To change the Gamma Level property of a single clip in the Browser
  1. Select a clip in the Browser.

  2. Click in the Gamma Level column next to the selected clip, enter a value between 0.01 and 10.00, then press Enter.

To change the Gamma Level property of multiple clips in the Browser
  1. Select multiple clips in the Browser.

  2. Control-click in the Gamma Level column, then choose Source, 1.8, or 2.2 from the shortcut menu.