Image Processing Reference
might find that your monitor or other equipment is set up to a special device-based color
space. The effect of this is visible if you mix moving video and still images in DVD menus.
If you have a still frame and then start rolling a video clip from an in point contain-
ing the same frame, the color appears to change as the video starts to play. This is an indi-
cation that you have processed the video in the wrong color space at some point in your
Color saturation affects the RGB values of a pixel, but when the video is stored in a
componentized form, the color saturation should have no effect on the luma. So if
you de-saturate the color, the only savings will be in the compression of the color
Analog video is designed so that synchronizing pulses are interleaved with the video sig-
nals. Certain signal levels need to be reserved so that the sync pulses are properly dis-
criminated. These reserved values cannot be used to represent a visible video intensity
value. Therefore, they must not be legal video levels. Figure 35-10 shows how this video
waveform fits into that legal range.
Sometimes when video is scaled or cropped, certain values result from the pixel
averaging that fall outside of the legal range of colors. This sometimes happens around the
edge of the frame. For example, someone wearing a bright red jacket walking off the
framed video area causes some very bright red values to be generated at the screen
boundary. These values push the intensity past the peak value and into the overshoot area,
which is illegal. Analog systems are forgiving of this and it will get clamped, clipped, or
crushed further downstream. But this kind of thing absolutely kills digital services. Some
digital-video processing equipment is so devastated by the arrival of illegal values that it
completely seizes up. If this is on the critical path to air, your service goes down.
Add more resilience to your system by placing color legalizers in the circuit. These
will substitute acceptable values for the illegal excursions. It is a little more expensive, but
far more preferable than going off air.
Your video capture card may provide protection against illegal color value genera-
tion. The Blackmagic Design video cards scale 255 RGB values to 100% video white and
restore the correct values on output. Not all video cards do this, and you should check the
specification of all the items of equipment in the video workflow path to find the problem
if you see illegal color values in the output.
Eyeheight LegalEyesHD: http://www.eyeheight.com/
Blackmagic Design: http://www.blackmagic-design.com/