Video Safe Node
The Video Safe node modifies the color information in an image if needed to conform to broadcast standards for NTSC and PAL video transmission.
Because of the limited bandwidth of the broadcast video signal, certain high-intensity, highly saturated colors (pure red, for example) must be modified to reduce their amplitude to an acceptable level.
A given color is considered "unsafe" for transmission if the amplitude of its chrominance vector exceeds a predefined limit, or if the amplitude of the composite video signal exceeds a (different) predefined limit.
| NTSC Video | PAL Video |
Analog Color Space Referenced | YIQ | YUV |
Gamma Correction Value Applied | 2.2 | 2.8 |
Max Chrominance Amplitude | 50 IRE * | 50 IRE |
Max Composite Signal Amplitude | 110 IRE * | 110 IRE |
* For NTSC video, the reference black level is offset by a pedestal of 7.5 IRE; therefore, the maximum composite signal amplitude is actually 102.5 IRE (110 - 7.5) and the maximum chrominance amplitude is actually 42.5 IRE (50 - 7.5).
The Video Safe node evaluates each pixel in the input image using standard color space conversion factors for RGB-to- YIQ (if NTSC video is specified) or RGB-to- YUV (if PAL video is specified).
If the color of a pixel exceeds the boundaries of the color space used by the selected standard, then that color is corrected by reducing either the intensity or saturation, as specified by the user.
Video Parameter Tab
Video Type
Use the popup menu in the Video Type parameter to select the broadcast standard to which the output image should conform:
- NTSC Video : The television signal standard used for broadcast in North America and Japan. NTSC video uses the YIQ color space.
- PAL Video : The television signal standard used for broadcast in most of Europe and in many other countries. PAL video uses the YUV color space.
Fix Method
The Fix Method parameter features a popup menu that enables you to specify the method to use to modify any unsafe pixels in an image:
- Reduce Intensity : Select this method to reduce the intensity of the pixel; that is, to reduce both the luminance and chrominance values.
- Reduce Saturation : Select this method to reduce only the chrominance values of the pixel; the luminance value is unchanged.
The best method to use will depend on the nature of the image being processed. As a general guideline, it is useful to remember that the luminance and chrominance values are encoded separately in broadcast video signals.
The chroma signal is derived by subtracting the luminance value of the color from the blue and red components: (B - Y) and (R - Y). The green component value is reconstructed from both luma and chroma signal information when the composite video signal is decoded by the receiving device.
This implies that the Reduce Saturation method, which reduces only the chrominance values--(R - Y) and (B - Y)--could increase the relative contribution of the green component to the output image and thereby alter the hue of the affected pixels.
The Reduce Intensity method, on the other hand, is more likely to darken the affected pixels.
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