Lumetri Scopes: Clamp Signal – 8 Bit?!

[Filip Stillerska]

Well, can you at least give me any guidelines Dave? From my understanding, you should uncheck the “Clamp Signal”-checkbox when you want to see exactly how much of the information is being clipped. When working in 8 bpc, you can leave the drop-down menu set to “8 Bit”, since that’s what you’re working in. When working in a HDR colorspace, you want to set the drop-down menu to “HDR”, since you want to see the wider color space.
Can you confirm if this information is correct? ↑

Also, should I choose “8 Bit” or “float” when working in a 16 bpc or 32 bpc project in a Rec. 709 color space?

[Walter Soyka]

These settings control how the scopes show information to you. Don’t try to match them to specific project settings, but rather use the settings you need for the task at hand.

“Float” displays data based on Lumetri’s internal floating-point processing (0.0 – 1.0 normal scale, plus superblack below and superwhite above).

“8-bit” displays an 8-bit video interpretation of that float data. This both reduces the precision of the display and effectively limits the waveform range from -7.5 to about 109 IRE, but it does show roughly what the signal would look like after 8-bit digital processing (if that’s part of your pipeline).

“Clamp signal” restricts the input to the scope (but not the actual output of the footage!) to a normal range of 0-255(8-bit) / 0.0-1.0 (float) / 0-100 (IRE). The Lumetri waveform normally uses a variable scale that defaults from 0-100, but can expand when superblack or superwhite colors are present in the signal. During video playback, this can cause the scope to “bounce” as the scale is dynamically adjusted for the data in the current frame. Clamp restores sanity in these situations and keeps the scale constant, but also prevents you from seeing just how much data is beyond the normal display range.

HDR is a special monitoring mode that displays a fixed logarithmic scale that goes well beyond the normal range on the high end; it’s meant to be used when grading for HDR displays.

For most cases, I think Float is most appropriate. Turn Clamp off when you’re trying to evaluate the potential for highlight or shadow recovery, and turn Clamp on when you want to keep the scale steady to read the scopes for playback or compare different still frames.

Walter Soyka
Designer & Mad Scientist at Keen Live [link]
Motion Graphics, Widescreen Events, Presentation Design, and Consulting
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[Filip Stillerska]

Thank you, Walter!