Final Cut Pro: About Luminance

Measuring Black and White Levels

How Brightness is Measured

The way brightness (also referred to as luminance) is measured in analog video differs from the way it is measured digitally by Final Cut Pro's Waveform monitor. (The Waveform monitor uses the ITU (International Telecommunications Union) standard for digital video.)

Measuring Digital Video During Capture

When you use Final Cut Pro's Waveform monitor, black is always represented on a digital scale as 0 percent, and white is represented by 100 percent, regardless of the source.

Many consumer camcorders record super-white video levels, or video levels that exceed a digital value of 100 percent. Some record at up to 109 percent. Final Cut Pro is capable of capturing and maintaining these super-white levels through final output as long as the video codec you use to capture is compatible with YUV processing. The Apple DV codec included with QuickTime is compatible with YUV processing. It allows the seamless capture and processing of DV clips with super-white video levels. If you have a third-party capture card, check the documentation that came with it to find out if it is compatible with YUV processing.

If your video capture codec does not work with YUV processing, you may see an effect called luma clamping when Final Cut Pro renders clips with super-white luminance levels. When this occurs, clips with super-white luminance levels that have filters or transitions applied to them will have all portions of the signal from 101 to 109 percent clamped to 100 percent after rendering. Luma clamping appears as a slight darkening of the super-white areas in the picture. This effect is most noticeable when using a transition, such as a dissolve, between two clips. The super-white areas of the clip that is making the transition will appear to suddenly darken for the duration of the rendered transition.

Because the Apple DV codec is compatible with YUV processing, it preserves all super-white levels in both rendered and unrendered clips, eliminating luma clamping altogether.

Measuring Analog Video During Output

After you have captured and edited your video in Final Cut Pro, you may need to output it to tape. If you are outputting an analog signal (such as S-video or YUV) and you use a hardware Waveform monitor to measure the signal's brightness, it's measured in IRE (Institute of Radio Engineers) units. In professional analog video, 100 IRE represents absolute white. Absolute black, or Setup, is represented by 0 IRE for PAL, 0 IRE for NTSC in Japan, and 7.5 IRE for NTSC in the United States.

If you are outputting back to analog tape using the built-in FireWire interface of your Apple computer with a consumer DV device, the black level of your video may vary depending on the device you use. While the digital video signal output from FireWire always has a digital black value of 0 percent, some DV devices output analog black around 0 IRE, and others around 7.5 IRE, depending on the model. Professional video facilities are fairly stringent about keeping video levels at precisely 0-Setup for PAL and NTSC in Japan, and 7.5-Setup for NTSC in the United States, so you may need to use a hardware Proc Amp to adjust your analog video signal accordingly.

What is Setup?

Setup refers to the black level of an analog video signal. If you use a hardware Proc Amp to adjust an incoming or outgoing analog video signal, the Setup control will allow you to adjust the analog video signal's voltage level for black.

If you are outputting back to analog tape using a third-party analog video interface, you should check its manual to determine how to configure it for the U.S. (7.5-Setup) or the Japanese (0-Setup) NTSC standard. Most vendors of analog video I/O cards include a control panel that allows selection of which black level to use. Most vendors label this as "7.5 Setup" or "0 Setup". In some cases it ma be labeled "NTSC" or "NTSC-J".

If you are outputting back to digital tape as a digital signal (for example, "SDI", "601", or HD) using a third-party digital video interface, you do not need to worry about Setup levels because the digital values for black and white specified by the ITU are standardized worldwide. They are also the same as the digital values Final Cut Pro uses.


About the Super-White Tab

The options in this tab are available for codecs that work with YCrCb color space. To find out which codecs work this way, go to the Final Cut Pro Web site (http://www.apple.com/finalcutpro/).

Always Render in RGB
When this is checked, codecs that normally process color using the YUV (or YCrCb) color space are forced to process in RGB color space. Selecting this option may cause subtle changes in color in your rendered material. If the video capture codec you are using does not work with YUV rendering, this option is unavailable.

Use this option if you want to accomplish a specific compositing task, and you think color clamping in the RGB color space might give you a different desired result than using the straight YUV color space. For example, if you are using After Effects filters, which only process in RGB color space, with Final Cut Pro filters, which process in YUV color space, choosing this option ensures a consistent look.

Process maximum white as
Many DV camcorders record video with whites that are brighter than 100 IRE, the level allowed by the CCIR 601 engineering standard for video. White levels brighter than 100 IRE are called super-white.

When you use clips created in RGB color space (for example, imported graphics files, Final Cut Pro generator clips such as the Text Generator, or QuickTime movies that use codecs in RGB color space), Final Cut Pro sets the maximum whites values in these imported clips to the white level specified by this pop-up menu. In this way, you can make sure that the white levels of your graphics or generator clips will match those of your captured video.

This pop-up menu allows you to match RGB brightness to YUV (or YCrCb) clips using super-white. It is only available if 'Always Render in RGB' is not selected. It has two options:

• White
  If you choose this, the maximum brightness of imported graphics or generators using the RGB color space will be set to a digital level of 100 percent. If the exposure of your video was carefully controlled during your shoot and you are sure that the brightness of your imported video clips is no more than 100 IRE, you should use this setting. As a result, the brightest whites in your graphics or generators will be at 100 percent when viewed on Final Cut Pro's Waveform monitor. Using the White setting will result in the luminance of the graphics in your project being broadcast-legal, and is the recommended mode for video destined for on-air broadcast.
• Super-White
  If you choose this, the maximum brightness of imported graphics or generators using the RGB color space will be set to a digital level of 109 percent. This will allow you to match the super-white levels of clips from some DV camcorders with the maximum white levels of your graphics or generator clip. As a result, the brightest white in your graphics or generators will be mapped to 109 percent when viewed on Final Cut Pro's Waveform monitor. Using the super-white option can be important if you are trying to achieve a consistent look between RGB clips in your project and video clips with super-white levels.
  
  Important: While using the Super-White setting will match the super-white levels recorded by some consumer camcorders, the output will not be "broadcast legal" because the luminance levels will be too high according to the CCIR 601 broadcast definition for video. In this case, if you have to output a tape for broadcast, you will need to correct the white levels in your edited sequence either with filters, or by using external hardware such as a Proc Amp to adjust the levels of your video on output.