YUV Stats Graphs

YUV refers to a particular a way of encoding color information in analog video where Y channels carry luma, or brightness information (wikipedia.org/wiki/Luma), and U and V channels carry information about color, or chrominance (wikipedia.org/wiki/Chrominance). QCTools can analyze the YUV Values of a particular encoded video file in order to provide information about the appearance of the video. These filters examine every pixel in a given channel and records the Maximum, Minimum, and Average values.

(For in-depth descriptions of how luma and/or chroma noise -- and other types of artifacts-- may present, the A/V Artifact Atlas is an excellent reference.) ##Y Values# ###MIN, AVG, MAX# Filter Domain: Y Channel

Filter Name(s): YMIN, YAVG, YMAX

Values: 0-255

Y Channels carry data about the brightness of a picture. Problematic variance in Y Channel values will manifest as a picture that is either too light or too dark, also known as containing luma noise. 8-bit video will have values falling in the range of 0-255 code values per pixel. A picture with well-balanced light levels will have an average, or mid-range Y Channel value of around 128 (YAVG). Graph readings outside of that range will indicate a picture that is either too bright or too dark. A Y Value of 0 would indicate total blackness and a value of 255 would present as entirely white. In the range of values, reference black is at value 16, while reference white is at value 235. Except during particular moments like scene changes where one might expect abrupt spikes , the average values of Y channels should remain relatively stable or constant with little variation. Portions of video showing extreme changes in average values (and not corresponding to a scene change or otherwise dramatic edit) likely indicate a picture error. Where you may expect luma spikes could be camera fade-ins/outs, or a sudden brightness in the picture, like a camera flash, for example.

###LOW, HIGH#

Filter Domain: Y Channel

Filter Name(s): YLOW, YHIGH

Values: 16-235

This filter works in a similar fashion as the YUV *MIN and *MAX filters, but instead of looking at the absolute minimum and maximum value for these channels, it looks at the 10th percentile (LOW, or 16 pixels) and 90th percentile (HIGH, or 235 pixels) which present the outside limits or 'headroom' of the legal broadcast range. An extreme minimum or maximum value could dramatically skew the graph but because they may be outside the viewable broadcast image (or the range of human perception), they may not necessarily be meaningful indicators of a problematic visual image that can be human-detectable. This is why Low/High measurements are so useful-- they ignore the extreme outliers (Min/ Max) in favor of those abnormalities which fall in the range of human perception.

Y values = Combined graph of YMAX, YHIGH, YAVG, YLOW, and YMIN

YMIN = Y channel minimum
YLOW = Y channel 10th percentile
YAVG = Y channel average
YHIGH = Y channel 90th percentile
YMAX = Y channel maximum

##U, V Values#

MIN, AVG, MAX#

Filter Domain: U Channel, V Channel

Filter Name(s): UMIN, UAVG, UMAX/ VMIN, VAVG, VMAX

Values: 0-255

The U and V Channels represent the chrominance, or color differences of a picture. U and V filters act to detect color abnormalities in video. It can be difficult to derive meaning from U or V values on their own, but they provide supplementary information and can be good indicators of artifacts, especially when occurring in tandem with similar Y Value readings. Black and white video contains no chrominance information so should present flat-lines (or no data) for UV channels. Activity in UV Channels for black and white video content, however, would certainly be an indication of chrominance noise. Alternatively, a color video which shows flat-lines for these channels would be an indicator of a color drop-out scenario.

###LOW, HIGH#

Filter Domain: U Channel, V Channel

Filter Name(s): ULOW, UHIGH/ VLOW, VHIGH

Values: 16-235

UMIN, VMIN = U channel minimum, V channel minimum
ULOW, VLOW = U channel 10th percentile, V channel 10th percentile
UAVG, VAVG = U channel average, V channel average
UHIGH, VHIGH = U channel 90th percentile, V channel 90th percentile
UMAX, VMAX = U channel maximum, V channel maximum