By default most computers use a full RGB range, which uses 0 for black and 255 for white. Video instead uses 16 for black and 235 for white. In order for video to fit with the full RGB range typically computers will expand video so that 16 goes to 0, 235 goes to 255, the video information between 16-235 is expanded accordingly, and any possible video information from 1-15 or 236-254 is removed. In this scenario of playing video on a computer that outputs a full RGB range, you would only expect to see the 17-234 bars flash on the clipping patterns.
Video expansion on a computer is a practical way to get video to fit with the full RGB range commonly used for computers, and using video expansion is easily the most common way to play video on a computer. From a video purist standpoint expansion is not necessarily ideal, since the ideal is to pass the original video information to the display without such alteration. One item introduced from expansion is grayscale banding. Because there are 256 values for black through white in the full RGB range and only 220 values for black through white in the video range, some grayscale values are assigned a step of two digits, instead of the original intent that all grayscale steps would be a single digit. Fortunately most video includes color information, and the amount of strictly grayscale information in most video is limited, so usually the grayscale banding from expansion will not be noticeable. Another argument against expansion is how it removes the video information outside the 16-235 range. Certainly there is information that decodes outside the 16-235 range from a lot of video, but usually the majority of the information outside 16-235 appears to be due to video compression in my opinion. There are people that consider these sorts of topics to be their profession that claim video often contains "specular highlights" above 235, but personally I was never able to happen to run across any examples to support such claims, which makes sense considering most people are unlikely to ever be able to notice how expansion limits the range to 16-235 when watching video on a computer. Basically expansion is an easy way to play video on a computer, and it's uncommon for the effects to be noticed, but from a video purist standpoint the general recommendation would be to pass video information from the video player to the display with very little alteration.
Often video driver or video playback software will have controls to affect expansion. There can be ways to set a Windows computer to decode at least some video so that black remains at 16 and white remains at 235, and the entire video grayscale from 1-254 is retained within the computer output. The issue with this scenario is that the video values may no longer match the computer desktop, so you might have a black level at the desktop that does not match the black level from video, and you would need some way to switch between the two outputs. If your video card has two outputs you might be able to setup one display input for video as 16-235 and one display input as 0-255 for computer desktop use, but most people don't care to go to this sort of trouble in dealing with two non-matching levels from their computer depending on what task they are doing. So video drivers and video software may have controls to affect how expansion is handled in some situations, but for convenience usually video is expanded to match the full RGB range when using a computer.
Last edited by alluringreality; 10-26-2014 at 09:29 AM.