Luminosity masking in DaVinci Resolve: select by light, not by colour
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A luminosity mask selects pixels by how bright they are and nothing else. It is the oldest idea in retouching and still the most useful one in grading: most of the time, the thing you want to fix is not a colour, it is a range of light. Here is how to build one in DaVinci Resolve with the tools already in front of you, where it gets fiddly, and what a purpose-built version does differently.
What a luminosity mask is
Take the brightness of every pixel, throw the colour away, and use the result as a matte. Bright pixels are selected, dark pixels are not, and everything between is partially selected. That is the whole idea. Photoshop users know it as Lights and Darks — a stack of nested selections you make by intersecting the luminosity channel with itself until it is as steep as you want.
The reason it earns its place in a grade is that it degrades gracefully. A hue qualifier either has a pixel or it does not, and the edge of that decision is visible. A luminosity mask is a smooth function of something that already varies smoothly across the frame, so its edges tend to hide inside the gradients that are already there.
Building one in Resolve
You do not need anything extra for the basic version. Resolve's qualifier will key on luminance alone, and that is a luminosity mask.
The qualifier method
- 1Add a serial node and open the Qualifier.
- 2Turn OFF Hue and Saturation, leaving only Luminance enabled. This is the step everybody misses — with all three on you are keying a colour, not a range of light.
- 3Drag the Luminance low and high handles to bracket the range you want.
- 4Use the soft handles on either side to feather the edges, and turn on the highlight view to see what you have.
- 5Grade inside that node as normal.
The second method is Custom Curves with the Y channel, pulled into a shape that crushes everything outside the range you care about. It is less precise than the qualifier but it is instant, and for a quick highlight rolloff it is often enough.
Where the hand-built version gets fiddly
Three things, in order of how often they bite.
The edge is one control doing two jobs
A qualifier's soft handles set where the selection starts fading and where it reaches zero, but the plateau and the shoulder are tangled together — widening the soft region moves the fully-selected region too. What you usually want is to hold the middle exactly where it is and only change how gently it lets go at the edges.
Banding in anything smooth
Skies are where you will see it first. If the falloff is too abrupt, the boundary of the mask draws a visible contour across the gradient, and on 8-bit source material it will band even when the falloff looks generous. Widen the soft edge before you reach for anything else.
It moves when the timeline changes
This is the one that costs real time. A luminosity mask is drawn on the encoded signal, and log encodings put the same light in a very different place. On DaVinci Intermediate, mid grey sits at about 0.336 and diffuse white at about 0.514 — so a mask you placed on a Rec.709 timeline is pointing somewhere else entirely once the same footage is conformed to Wide Gamut. Nothing warns you; the numbers still look reasonable.
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Separating the plateau from its shoulder, and keeping a selection meaningful when the timeline is log rather than Rec.709, are the two things the hand-built version cannot do. Both are what Locus is for.
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| Where the light is | Rec.709 / sRGB | DaVinci Intermediate |
|---|---|---|
| Mid grey (18%) | 0.489 | 0.336 |
| Diffuse white (100%) | 1.000 | 0.514 |
The problem with faces
A luminosity mask cannot avoid a face by accident. Skin occupies the same exposures as everything else in the shot, so a band over the shadows will take the shadow side of every face in frame along with the shadows in the room. A hue qualifier sometimes misses faces by luck; a luminosity mask never does.
By hand, the fix is a second node with a skin key and a layer mixer, or a power window tracked to the face. Both work. Both are more nodes than the correction deserved, and the tracked window has to be checked shot by shot.
A starting point
If you want somewhere to begin rather than a rule, these are reasonable first positions on a Rec.709 timeline. Adjust by looking at the mask, not by trusting the numbers.
Three bands that do not fight each other
- Darks — centre
- About 0.18, roughly a third of the scale wide
- Midtones — centre
- About 0.50, roughly a third of the scale wide
- Lights — centre
- About 0.82, roughly a third of the scale wide
- Edge softness
- Generous. Start too soft and tighten, never the other way
- First move
- Exposure. Density and saturation read better once the level is right
Neighbouring bands overlapping is normal and wanted — that overlap is what stops the seams showing. What you do not want is two bands both fully claiming the same pixel and each applying a full-strength correction to it, which is how overlapping curve adjustments blow out.
Locus is this technique as a single node. Three bands, each with the plateau and the shoulder as separate controls so you can soften an edge without moving the middle; every mask one checkbox away from being visible; and the whole thing read on the encoded signal, so a band at 0.5 is centred where your waveform says 50 on a log timeline as well as a Rec.709 one. Its skin unit is built around the skin locus rather than a single reference tone, and its Protect mode holds every band off the faces it finds.