What is a Layer Mask?

A layer mask is a greyscale image attached to a layer that controls that layer's transparency - Without permanently deleting any pixels. Where the mask is white, the layer is fully visible. Where the mask is black, the layer is hidden. Grey values create partial transparency. For a concise overview, see layer masks explained.

Mask Tones

White = Show

Grey = 50%

Black = Hide

Non-destructive: The original pixels are never deleted. Paint the mask back to white to recover any hidden area instantly.

Reversible: Unlike erasing, masking can be undone at any point in the future - Even after saving the XCF file and reopening it.

Paintable: Use any brush, gradient, or selection to control exactly which pixels are visible.

Key insight: "Paint with black to hide, paint with white to reveal" is the mantra of layer masks.

Mask vs Eraser: Why Masking Wins

Adding a Layer Mask

Method 1: Via the Layer Menu

Select the layer you want to mask in the Layers panel
Go to Layer → Mask → Add Layer Mask…
Choose an initialisation option (see table below)
Click Add

Method 2: Right-click in Layers Panel

Right-click the layer in the Layers panel
Choose Add Layer Mask… from the context menu
Select initialisation and click Add

Initialise Mask To	Result	Use When
White (full opacity)	Layer fully visible; paint black to hide areas	You want to start visible and selectively hide
Black (full transparency)	Layer fully hidden; paint white to reveal areas	You want to paint in the effect gradually
Greyscale copy of layer	Luminance of the layer becomes the mask	Luminosity masking, HDR effects
Selection	Selected area is white, rest is black	Convert an existing selection to a mask
Layer alpha channel	Existing transparency becomes the mask	When layer already has alpha information

Important: To paint on the mask, you must click the mask thumbnail in the Layers panel (not the layer thumbnail). The mask thumbnail will show a white border when active. Paint with black (foreground colour) to hide, white to reveal. Press D to reset colours to black/white, then X to swap them.

Painting & Refining Masks

Basic Painting Workflow

Click the mask thumbnail in Layers panel
Select Paintbrush (P)
Set foreground to Black (press D, then X if needed)
Paint over areas you want to hide
Switch to White to reveal areas you over-painted
Use a soft brush for natural edges, hard brush for precise cuts

Refining Edges

Feather selection first: Select → Feather (2–5px) before converting to mask for smooth edges
Soft brush tips: Use 30–50% hardness for natural blends
Reduce opacity: Paint at 40–60% opacity to build up mask gradually
View mask: Alt+click the mask thumbnail to see the mask in greyscale

From Selection to Mask

The fastest way to create a precise mask is to make a selection first, then convert it to a mask:

Create a selection using any selection tool
Layer → Mask → Add Layer Mask → Initialize to Selection
The selection becomes a white area on the mask (visible), the rest is black (hidden)
Refine by painting on the mask with soft brushes

Gradient Masks

A gradient mask creates a smooth transition from fully visible to fully hidden - Perfect for blending two images together or fading an effect. Pair this technique with blend modes for even more creative control.

Add a layer mask to the top image layer (initialise to White)
Click the mask thumbnail to make it active
Select the Gradient tool (G)
Set gradient type to FG to BG (Black → White)
Drag the gradient across the image - The direction controls the fade direction
The top image will fade smoothly into the image below

Tip: For horizon blends (sky replacement), drag the gradient vertically. For left-to-right blends (panorama seaming), drag horizontally. Hold Shift while dragging to constrain to 15° increments.

Left to right fade

Sky blend (vertical)

Radial vignette

How Blend Modes Work with Layer Masks

Blend modes control how a layer interacts with the layers beneath it. They are applied to the layer, not the mask. When combined with a layer mask, blend modes let you control both what is visible and how it blends with everything below it.

Blend Mode	Effect with a Mask	Masking Use Case
Normal	Opacity of visible mask area controls how much of the layer shows through	Standard compositing - paint the mask to reveal exactly what you want
Multiply	Darkens the composite wherever the layer is visible through the mask	Shadow layers, darkening vignettes, tint overlays
Screen	Lightens the composite wherever the layer is visible through the mask	Glow effects, light leaks, brightening via masked adjustment layer
Overlay	Adds contrast to the composite in masked areas; 50% grey disappears	Masked contrast boosting, texture blending
Difference	Shows differences between the masked layer and the layer below	Alignment checks, creative inversion effects

See the complete GIMP blending modes reference →

Photo Compositing with Layer Masks

Compositing means combining multiple images into a single believable scene. Layer masks are essential to this process.

Sky Replacement Walkthrough

Open background photo: Your base landscape image
Open sky image: File → Open as Layers - Places sky as a new layer above background
Reorder layers: Make sure the sky layer is above the landscape layer
Scale sky to fit: Layer → Scale Layer to match the canvas size
Select the original sky: Use Fuzzy Select on the old sky area of the background layer
Refine selection: Select → Grow 1px, then Select → Feather 2px for soft edges
Add mask from selection: On the sky layer: Layer → Mask → Add Layer Mask → Initialize to Selection
Paint mask edges: Paint with black/white to refine the horizon line
Colour match: Add a Colour Balance adjustment layer clipped to the sky layer to match lighting

Layer Groups

Layer groups let you organise related layers together and apply a single mask or blend mode to the entire group.

Creating Groups

Layer → New Layer Group
Or click the folder icon in the Layers panel
Drag layers into the group
Click the triangle to collapse/expand

Group Masks

Add a mask to the group layer itself
The mask affects all layers in the group simultaneously
Perfect for masking a multi-layer subject against a background
Each layer in the group can also have its own individual mask

Group Use Case	Layers Inside	Benefit
Subject with effects	Subject layer + colour grade + sharpening	Single mask cuts subject from background
Text with shadow	Text layer + Drop shadow layer	Move, transform, or mask both together
Sky replacement	Sky + colour balance + clouds	Adjust all sky elements with one mask
Reusable background	Background + texture + vignette	Toggle or mask the whole background group

Clipping Masks

A clipping mask constrains a layer's visibility to the shape (alpha channel) of the layer directly below it.

Create a base layer with the shape you want (e.g. a text layer, or a shape)
Place the texture/image layer above it
Right-click the texture layer → Create Clipping Mask
The texture now only shows through the shape of the layer below

Common use case: Put a marble texture inside text. Create a text layer (the shape), then place the marble texture above it and clip it. The marble only shows where the letters are.

Quick Worked Examples

Double Exposure Effect Creative

Open portrait photo; convert to greyscale (Image → Mode → Greyscale → Colour)
File → Open as Layers - Add a landscape photo
Set landscape layer blend mode to Screen
Add a layer mask to the landscape (White → fully visible)
With a black soft brush, paint the mask to hide the landscape outside the portrait silhouette
Adjust opacity to taste (60–75% often works well)

Product Photo - Remove Background Practical

Open product image; duplicate background layer
Use Fuzzy Select + Grow + Feather to select the white background
Add Layer Mask from Selection (inverted) to keep only the product
Refine edges with a small white/black brush at 50% opacity
Add a new solid-colour layer below for the desired background

Vignette with Gradient Mask Quick Effect

Add a new layer filled with black above all other layers
Add a layer mask initialised to White
Select the Gradient tool - Use Radial gradient, FG to Transparent (White to Transparent)
Draw a large radial gradient from the centre of the image outward
Set the black layer opacity to 50–70% for a subtle vignette

Non-Destructive Editing in GIMP: Masks, Blend Modes and Groups