The AI grabbed too much. One subtract brush stroke and it's gone.
Subtract mode removes areas from an existing mask — the most precise way to clean up AI selections that bled into unwanted areas, or to punch holes in a gradient.
✂️You don't start over when an AI mask bleeds. You subtract. Every masking workflow ends here.
Core Concept
Final Mask = Original − Subtracted
Before subtract
→
After subtract
💡Wherever the subtract region overlaps your mask, those pixels are cut out and stop receiving your adjustment. The rest of the mask is untouched.
How-To
How to Access Subtract
Select a mask in the masking panel, then click Subtract — a tool picker dropdown appears.
1
Open Masking panel — Shift+W or the mask icon in the toolbar
You need at least one mask already created before you can subtract from it.
2
Click the mask in the list to select it
A highlight border appears on the selected mask. Its sliders are visible below.
3
Click "Subtract" → choose a tool from the dropdown
All masking tools are available: Brush, Linear Gradient, Radial Gradient, Select Sky, Select Subject, Objects, Range.
4
Use the tool — the mask updates in real time
Wherever the tool acts on the existing mask, those pixels are removed. The overlay reflects the change instantly.
Most Common Tool
Subtract with Brush
Before — Bleed
Subject masked + background bleed at 60–75%.
After — Clean
Subtract brush removed the bleed. Subject only.
🖌️
Same Controls
Size, Feather, Flow, Density all work identically in Subtract mode. Use [ and ] to size the brush while painting.
👁️
Toggle Overlay (O)
Press O to see the red mask overlay while painting. Seeing the mask edge makes precise subtraction much easier.
✅Subtract Brush is the go-to for cleanup. Zoom to 100%, toggle the overlay, paint slowly near edges.
Gradient Tool
Subtract with Linear Gradient
Remove the mask from an entire band across the image in one drag — ideal when a full zone (sky, foreground) shouldn't be masked.
Linear Gradient Subtract — Sky Removed
🎯Drag a subtract gradient from the top to remove the sky from a mask. The feathered edge creates a smooth transition at the horizon — no hard cut.
AI Tool
Subtract with Select Sky
Remove the sky from a Select Subject mask that bled into the background — AI removes AI bleed.
1
Create a Select Subject mask — sky bleed is included
Common with portraits against open sky. The AI sees sky as part of the subject silhouette near edges.
2
Click Subtract → choose Select Sky
Lightroom's sky detection AI runs and builds a sky region. That region becomes the subtract area.
3
The sky is cut from the subject mask — instantly clean
Wherever the sky and subject mask overlapped, those pixels are removed. Subject edges preserved. Background excluded.
🤖AI + AI = precision. Select Subject to add, Select Sky to subtract. Neither tool alone achieves this — combined, they're unbeatable.
Key Distinction
Subtract vs. Brush Erase Mode
✂️
Subtract Mode
Creates a permanent subtract component in the mask list. Has its own entry with a minus icon. Can be toggled, adjusted, or deleted independently. Works on any mask — including AI masks you didn't paint.
🖊️
Alt / Option Erase
Temporarily switches the brush to erase mode while held. Erases strokes from the current brush component only. Useful for correcting your own painting mistakes — not for removing AI mask areas.
💡Use Alt/Option erase to fix your own brush strokes. Use Subtract mode to remove areas from any completed mask — especially AI masks.
Component View
Subtract Entries in the Component List
Each subtract operation appears as a discrete, labeled entry — identifiable by its minus badge and tool name.
Mask 1 — Component List
🤖
Select Subject
+ ADD
➖
Brush 1
− SUBTRACT
➖
Linear Gradient 1
− SUBTRACT
↑ One add + two subtracts. Each component is independently togglable and editable.
👁️Click the eye icon next to any subtract component to toggle it off — see what it removed. Click again to restore it.
Toggle Technique
Toggling a Subtract Component
Turn a subtract component off to see what it removed. Turn it back on to restore the cleanup. Your mask debugger.
1
Expand the mask's component list — click the disclosure triangle
All components appear with eye icons on the left side for toggling visibility.
2
Click the eye on a subtract component — it turns off
The mask expands to include the previously-removed area. You see the original bleed return — confirming what the subtract was doing.
3
Click the eye again — subtract snaps back on
No data is lost. Toggling is purely a visibility operation. Toggle repeatedly to diagnose a mask that looks wrong.
🔬Toggle subtract components to audit your mask. If something looks wrong, toggle each component off one at a time to find the culprit.
Workflow #1
Select Subject → Subtract Brush
🤖
Select Subject
AI masks person + bleed
→
✂️
Subtract Brush
Paint away bleed
→
🎯
Clean Mask
Subject only, tight edges
Open Masking panel (Shift+W) → Click "Select Subject"
Press O to see the overlay — identify bleed areas
Click Subtract → Brush. Zoom to 100%.
Paint over bleed with a feathered brush. Use [ ] to resize.
Press \ to compare before and after
Workflow #2
Gradient Sky → Subtract Brush on Treeline
🌅
Linear Gradient
Covers sky + clips treeline
→
🌳
Subtract Brush
Paint out treeline tops
→
☀️
Sky Only
Trees untouched, sky masked
🔭
Zoom In
Work at 100–200% when painting near treeline or ridge silhouettes. Precision here prevents halos.
🖌️
Small + Low Feather
Use a small brush with Feather 20–30 near tree tops. Reduces feathering prevents color bleed along the edge.
🌲A gradient alone cuts straight across a curved treeline. A subtract brush fixes what the gradient can't — follow every ridge and branch tip.
Your Challenge
🏔️
Subtract Challenge
Find a landscape with sky + mountains or treeline. Complete this workflow.
🤖 Create a Select Sky mask — see what the AI selected
✂️ Subtract → Brush — paint out any mountain or ridge bleed
👁️ Toggle the subtract component off, then on — verify what it removed
🌅 Apply a sky adjustment and compare before/after with \
💬 Share your before/after in the club gallery or comments
Recap + What's Next
3 Things to Remember
1
Subtract = Punch Out
Final mask = original minus subtracted areas. Subtracted pixels stop receiving the adjustment. Everything else stays.
2
Brush Is Your Go-To
Subtract Brush handles most cleanup. Zoom in, press O for the overlay, use [ ] to size. It creates a permanent, editable component — not a one-time erase.
3
AI + AI = Power
Select Subject + Subtract Select Sky (or any combination) gives precision neither tool achieves alone. Toggle components to audit your work.
Lesson 26 — Lightroom Classic
Intersecting Masks
Intersection keeps only the overlap between two masks — the most precise selection operation in Lightroom. Affect only the sky within a radial gradient. Target only skin tones within a subject mask. Surgical precision that nothing else can match.