Using the RGB Limit Filter

The RGB Limit filter has controls to prevent illegal RGB levels while working in Y′CBCR color space.

About Illegal RGB Levels

Most video formats are recorded, edited, and transmitted in Y′CBCR (component) color space. Although you may never intentionally convert your footage to RGB color space, it is fairly certain that it will become RGB at some point—most likely during display on a television or monitor. Video is almost always converted to RGB color space within display devices.

When you work in Y′CBCR color space, levels are either described in their native color space or they are referred to as RGB-equivalent values, meaning the values of your Y′CBCR video signal when it is eventually converted to RGB.

Illegal RGB levels are generally caused when certain combinations of luma and chroma levels (in Y′CBCR color space) are converted to RGB color space. Even if both luma and chroma levels are legal in Y′CBCR color space, the combination of these two may cause illegal RGB levels.

Tip: The RGB Parade scope in the Video Scopes tab is useful for viewing RGB levels when working in Y′CBCR color space. For more information, see Learning to Read the RGB Parade Scope.

Legal RGB levels are usually defined as 0 percent to 100 percent, where 0 percent is the lowest legal value and 100 percent is the highest. When Y′CBCR footage is converted to RGB, RGB levels can be illegal in two ways: if they are below 0 percent or above 100 percent.

Note: If your video originated using a codec that is already in RGB color space (such as the Animation codec), it is unlikely that the RGB levels are illegal.

RGB Limit Filter Controls

The following section describes the controls in the RGB Limit filter.

Figure. Filters tab showing the RGB Limit Filter controls.
Minimum RGB Limiting Controls
  • Enable: Select or deselect the checkbox to turn on or turn off RGB clamping below the Clamp Levels Below parameter value.
  • Clamp Levels Below: Set a value to define the minimum allowable RGB levels. RGB levels below this parameter value are clamped to this value.
Maximum RGB Limiting Controls
  • Enable: Select or deselect the checkbox to turn on or turn off RGB clamping above the Clamp Levels Above parameter value.
  • Clamp Levels Above: Set a value to define the maximum allowable RGB levels. RGB levels above this parameter value are clamped to this value. For example, if your image has RGB-equivalent levels of 115 percent, 110 percent, and 105 percent, and you set this slider to 105 percent, the RGB levels are set to 105 percent, 105 percent, and 105 percent.
Maximum RGB Reduction Controls
  • Enable: Select or deselect the checkbox to turn on or turn off chroma desaturation, luma reduction, or both, if necessary.
  • Desaturate or Darken Levels Above: Applies chroma desaturation, luma reduction, or both, to bring RGB levels down to the value set here.

How the RGB Limit Filter Works

The RGB Limit filter processes your video in 32-bit floating-point RGB color space and then outputs the native color space of your footage (either RGB or Y′CBCR). The high precision of this color space minimizes quality loss in your video during processing.

The RGB Limit filter affects only RGB-equivalent values above the Clamp Levels Above parameter value and below the Clamp Levels Below parameter value. The filter applies up to three stages of level reduction at a time, as necessary:

  • Clamping: Levels above the Clamp Levels Above parameter value are clamped to the parameter value. Levels below the Clamp Levels Below parameter value are clamped to the parameter value. For example, if the Clamp Levels Above slider is set to 100 percent, all RGB levels above 100 percent are clamped to 100 percent.
  • Chroma desaturation: After clamping, any pixel with an RGB level above the Desaturate or Darken Levels Above parameter value is desaturated until the red, green, or blue channel reaches the parameter value. Desaturation is an effect easily understood in Y′CBCR color space: the color difference channels are merely reduced. However, in RGB color space, desaturation is achieved by reducing the level of the color channel with the highest level while simultaneously increasing the levels of the two channels with lower levels. This reduces saturation while maintaining the same luma level. A pixel with equal RGB channel levels has no saturation (in other words, the pixel is black-and-white only—like the luma channel in Y′CBCR color space) and therefore cannot be desaturated.
  • Luma reduction: Finally, for any RGB levels that still remain above the Desaturate or Darken Levels Above parameter value after desaturation, luma levels are reduced so they are at the level set by the Desaturate or Darken Levels Above slider. Reducing luma levels in RGB color space is achieved by simply reducing the levels of all three channels simultaneously.