Luma and Chroma Clamping

Symptom #1: Footage Dims

"My footage dims as I go into a transition and returns to normal after I leave the transition."

Background

Although FCP is a digital video editor, meaning that it uses RGB values to calculate luminance and chroma values, it adopts a YUV workspace so that you are always working with true broadcast values. (Professional editors work with YUV source material and need to deliver YUV products.) You can run into problems if some element in your project does not use YUV to process video colors.

Luma and chroma clamping appear as a slight darkening in bright areas of a video frame after rendering. When present, you can normally see it in transitions and areas where part of a clip is rendered and the other is not. Also, a sequence might look completely normal when in playback, but when playback is stopped, luminance levels appear to jump a few IRE levels.

Under most circumstances, the appearance in the Viewer or Canvas will not appear to change, because the phenomenon has to do with the translation of luminance values between RGB and YUV color spaces, which occurs only in true video output rather than in the computer monitor preview. Computer monitor playback always shows RGB levels directly from the FCP digital video engine and does not display in YUV. This is why definitive color correction should always be done on a calibrated production monitor rather than the Canvas or Viewer.

Although FCP edits in a YUV space, it doesn't always have to render new video frames within that YUV space. When you render video, you have a choice between rendering using YUV 8-bit (the norm for DV), YUV-10 bit (a much larger color space still based on YUV-legal limits only available when editing an uncompressed 10-bit sequence), and RGB, which you can choose in the Video Processing tab in Sequence Settings. If you use either of the YUV rendering modes, your YUV material is rendered using calculations that do not jeopardize the YUV luminance and chroma limits. It's "YUV in equals YUV out."

But if you choose the RGB option, FCP scales the smaller YUV luma and chroma range up to the larger RGB space to perform its luma and chroma calculations. This translates as stretching the YUV range of 16 to 235 up to a 0 to 255 range. This can be necessary, for instance, if you are using After Effectstype plug-ins in FCP, which only process color in the RGB color space. In order to make sure that your sequence's rendered clips all use the same consistent color space, when you use RGB-native plug-ins, you must always use RGB-only rendering as applied from the Sequence Settings Video Processing tab.

Luma clamping is caused by the potential clipping of broadcast-illegal values when scaling from YUV to RGB. RGB values, which extend from 0 to 255, include luminance values that are broadcast-illegal. (To give you a frame of reference for digital-to-broadcast values, the international CCIR 601 digital video standard set the YUV legal luminance values in the RGB realm at 16 to 235. So 16 in the RGB scale equals 0 IRE on the YUV scale, and 235 equals 100.)

FCP normally maps any digital values it receives (whether RGB or YUV) to its own independent scale of 0 to 100 percent. For YUV values, this gives an easy translation of one to one, 0 being absolute black and 100 being absolute white, both for YUV and for the FCP independent luma percentage value.

But if you want to render YUV material using RGB calculations, FCP has to map YUV's 16 to 235 set of values to the full RGB scale, which exceeds in definition YUV's possible luminance values.

It becomes necessary to scale 16 to 235 YUV up to the full range of RGB's 0 to 255 scale in as near as possible to a one-to-one relationship. Although that's technically impossible, FCP does a pretty great approximation. It's likely that if you render YUV material using RGB rendering for such purposes as using After Effects filtration, you will not notice the difference.

Where you will see a difference is in the situation that causes luma clamping. YUV, particularly from DV source material, is actually capable of "greater than white" values. We call these values Superwhites. Although such values are broadcast-illegal, DV still records visual detail up to 109 IRE (110 IRE corresponding to complete white).

Tip

The ability to record detail above the legal broadcast level is a valuable asset to "run and gun" videographers, because it assures that even if you overexpose your footage a couple of stops, you have a buffer from clipping that would normally occur at 100 IRE. Color correction can be used in post-production to pull the illegal values back down and rescue the overexposed, though not clipped detail. DV is not alone in this regard; most professional formats will also record detail far above the legal levels.

Having illegal levels in FCP and then using RGB rendering can lead you into the trap that causes luma clamping. When FCP scales YUV's values up to RGB's, it maps based on the legal values of 16 to 235. (16 is scaled down to 0 and 235 is scaled up to 255.) The YUV illegal Superwhite values from 101 to 109 cannot be scaled above 255 (RGB's limit) and thus are clipped. Afterwards, when the values are scaled back down to YUV for playback, the range of luminance values above 100 IRE have been lost through the clipping, generating lower values than were present prior to the clipped scaling.

Thus, when a clip with Superwhite values is played back in the sequence and part of it has been rendered in RGB, you see its luminance jump a little where the Superwhite values were clipped. It's subtle, but when the clipping is within the same clip, such as in a transition, it's quite noticeable.

Solution

The only way to avoid luma clamping is to make sure that no footage used in a sequence where RGB rendering is required has luminance values that exceed 100 IRE. As long as none of your clips exceed 100 IRE, FCP will always scale luminance without clipping. Although the RGB color space may introduce very slight, nearly undetectable, differences in the extremes of color values, the luminance will be accurate both going to and coming from RGB.

Symptom #2: Graphics Are Brighter Than Highlights

"When I import still graphics they seem brighter than the highlights in my video footage."

Background

Now that you're familiar with the White or Superwhite values in YUV DV (see previous section), let's look at one other circumstance in which you might have luminance level issues. It's possible to run into problems if you are using Photoshop stills, QuickTime movies using RGB-based codecs (such as Animation), or the Text Generators in the FCP Effects tab, which all process color in the RGB color space. Using them can create problems if the luminance of the graphics and text does not match the smaller range of the YUV video you're editing.

Solution

To solve this problem, you must choose the correct settings so that FCP will make non-YUV footage match the range of your edited footage. To do that, use the White/Super-White pop-up menu in the sequence settings window. Use Superwhite if any of the video footage in the frame (other than your titles) exceeds 100 IRE in the Waveform Monitor. Use White if all your video levels are safely below 100 IRE and legal.

Changing this setting will not affect your YUV DV footage; it will only adjust the luminance level of the imported footage when it is rendered. When you view the footage in the Video Scopes on the Toolbench and change the pop-up menu back and forth, you will see the luminance levels scale up and back as the graphic's luminance is adjusted to match your target YUV range.