Sat / Lum
Saturation over luma scope
The Sat/Lum scope plots each pixel in a 2D space where the X axis is luma (Y') — the weighted-sum of the non-linear R'G'B' channels used in Y'CbCr — and the Y axis is saturation, computed as the length of the CbCr (or UV) chroma vector from the neutral axis. A fully desaturated pixel (grey, white, black) sits on the bottom edge regardless of its luma; a fully saturated primary sits at the top. This makes the scope a compact at-a-glance view of how saturated the picture is across the tonal range.
Reading the plot
Shadows on the left, highlights on the right
Low saturation at the bottom, high saturation at the top
A tight, narrow trace at low saturation → the image is nearly neutral
A trace that bulges toward the top-right → saturated highlights (common cause of clipping perceived as "burnt" colour)
A trace that bulges toward the top-left → saturated shadows (often indicates chroma noise or crushed blacks with a colour cast)
Because naturally exposed footage has most pixels in the midtone range, the trace usually hugs the centre. Outliers are what you're looking for.
Colour Space Selection
The CbCr coefficients used to compute the chroma vector depend on the active colour space — Ccir 601 for SD, Rec. 709 for HD, Rec. 2020 for UHD. This affects both where pixels land horizontally (luma coefficients differ) and how far they extend vertically (chroma scaling differs).
Ccir 601
Legacy SD broadcast (Kb = 0.114, Kr = 0.299)
Rec. 709
HD broadcast / most SDR (Kb = 0.0722, Kr = 0.2126)
Rec. 2020
UHD / HDR container (Kb = 0.0593, Kr = 0.2627)
Options
Colorize — tint each trace sample with the pixel's own colour
Colour space selection — Ccir 601 / Rec. 709 / Rec. 2020
Related
Vectorscope — raw CbCr plane without the luma axis
Histogram — saturation mode
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