Your Camera Adjustments & Video Measurements Corner

Leader Instruments Corporation

What Is The 5-Bar Display?

It is necessary to monitor not only Y, PB, Pr but also RGB levels during production.  The reasons are explained under “What Are Gamut Errors?” below.

Gamut error detection is essential otherwise production material will most likely not be useable.  In addition to the 5-Bar display, a on-picture gamut error indication facilitates quick determination of gamut levels and helps in eliminating the problem (see “What Are Gamut Errors” below for an example on how the on-picture gamut display can be used to adjust your camera for the optimum take).

On-Picture Gamut Display (Bottom-Right) :  Shows where gamut violation is occurring.  Reducing the camera’s Matrix Gain adjustment (set at MAX) will cause the gamut indication to disappear.  A great way of setting your camera to produce the maximum amount of possible color without violating gamut conditions.

What Are Gamut Errors?

Understanding Gamut Errors :  Our cameras’ sensors pick up information in RGB form.  Our monitors and even our home TV systems 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 convert our RGB information to Y, Pb, Pr for processing.  Y, Pb, Pr 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 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 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 (reflections).  We then applied a “Black Stretch” essentially modifying the “toe” adjustment of the camera and raising the low tones to midrange (that gave us a bit more brightness otherwise the shot was too dark from stopping down too far).  At the same time, we reduced the  black pedestal level (which gave us richer shadows in this very bright environment).  Finally, we reduced the color matrix (gain only) a bit until we saw the on-picture gamut display (top right) indicate the problem only at the edge of highlights.