Gamut Error
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what are they? by seamus on Dec 5, 2007 at 10:41:53 am Ok, heres the thing, I have supplied probably over 50 tvcs in the last couple of years for a particular client for broadcast. They are graphical tvcs
Composite Gamut Error
all made in After Effects. Very quick to do and I deliver to a how to fix gamut error company via FTP who farm out the tvc to the stations that the agency have booked with. I have used the same process from the beginning and never been rejected once. My trick for legal colours is an adjustment layer with levels filter and pulling in the outputs by a few 5 or so points. All good. Works a charm, everyone is happy. I have seen this on scopes in the past and that trick works. Now out of the blue I get a rejection on two tvcs saying they have gamut errors on the last frame. The thing is in these tvcs are part of a batch of ten all with the same end frame. But two have errors. So what gives?? What is a gamut error exactly? This is not a chroma luminance issue is it? How do I fix it? Particularly as it has not shown up on previous tvcs with the same frame. Any ideas are much appreciated. thankyou.....Return to posts indexReport Post •Re: Gamut errors, what are they?by Bob Zelin on Dec 9, 2007 at 9:03:01 pmhttp://broadcastengineering.com/mag/broadcasting_quality_control_digital/ A gamut error is defined as any video signal excursion outside the appropriate color space and is grounds for rejection of the material by the broadcaster. http://www.tek.com/Measurement/App_Notes/25_15609/25W_15609_0.pdf REPLY - simply google GAMUT error, and you will find lots of info. SO, how do you avoid Gamut errors, or illegal level error, so a waveform monitor does not "go off" and hit the Gamut Error Alarm on the scope - easy - YOU BUY A SCOPE AND USE IT TO ON LINE YOUR MATERIAL. "But I dont' own a scope, and they are expensive - how do I correct for Gamut errors without owning or renting a scope" - YOU DONT. Having broadcast monitoring equipment is part of the game of being in the broadcast business. Modern scopes from Tektronix, Hamlet, Leader, and Videotek have Gamut indications on them, but old analog "ebay" scopes from these very manufacturers probably will not. Depends on what model scope you buy. Bob Zelin Return to posts indexReport Post•Re: Gamut errors,
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also display the information on their display in RGB form. However, our studios and transmission systems use Y, Pb, Pr in order to process our work. There are several reasons that we http://www.gonos.com/index_files/Page1072.htm convert our RGB information to Y, Pb, Pr for processing. Y, Pb, Pr http://forums.autodesk.com/t5/smoke-forum/gamut-errors/td-p/4859944 reduces unwanted color detail and effectively dedicates more bandwidth to the luminance channel where detail is best appreciated by the audience. Also, it is easier and less demanding to build systems processing Y, Pb, Pr. It is this conversion issue from RGB to Y,Pb,Pr (and visa-versa) that makes it imperative to monitor not gamut error only Y,Pb,Pr but also the RGB channels. As it works out, there are legal value combinations in Y,Pb,Pr that result in illegal RGB values (over or under range). This results in a particular color (any combination of RGB) to be “clipped” either at the lows or highlights and creates serious unwanted artifacts. Take a look at the shot above. The bottom-left quadrant shows the real picture. A composite gamut error CINEZONE view (top-left) shows levels in false colors. The 5-Bar display (bottom-right) shows luminance (first bar) is a bit overlevel (clipping at the highlights). It also depicts RED (4th bar) to be way to hot (clipped). The top-right quadrant shows where the luminance (Y) and RED channel are being violated on the picture. Small gamut errors in specular areas and at the edges of highlights may be acceptable for artistic purposes but should be avoided as they cannot be corrected/adjusted in post production. The screens above indicate that the RED channel is way to hot in this shot and it is effecting the majority of the image. Want to know how we actually manage to take the shot and make it legal? Read below….. HOW WE TOOK THE SHOT By using the camera’s built in signal processor we were able to take this shot while maximizing the performance of our camera (JVC-250) for the given circumstances. HERE’s how we fixed it : We cut the exposure a fair amount until the gamut error disappeared but this made the image way too dark. We also applied a knee adjustment at 80%; this helped limit large luminance excursions (refle
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