Masks are entities that control where instances can and cannot be scattered. They can either be Paint masks, Area masks, Path masks, Image masks, or Composition masks.
A mask can be inclusive, which means that instances can only be scattered in its area, or exclusive, which means that instances cannot be scattered in its area. To switch between those two modes, click the
Inclusive / Exclusive
If a composition contains both types of masks, inclusive masks are applied first and then exclusive masks are applied over the remaining instances.
Masks are automatically reordered when toggling their inclusive/exclusive mode in order to reflect this rule.

Paint masks

A Paint mask allows you to manually draw the shape of the mask. You draw the shape with a brush, just like you would in Photoshop or similar image editors.
To draw the mask, click the
icon, then click & drag over your hosts. You can later click the
icon at any point to continue drawing the mask.
To erase the mask, click the
icon, then click & drag over the existing shape.
Erasing an inclusive Paint mask is not the same thing as drawing an exclusive Paint mask!
Change the drawing mode when pressed (toggle paint/erase)
Draw a straight line
Maintain pressed and move the cursor laterally to change the brush's radius

Area masks

An Area mask is a group or a component instance that defines a space under which instances will not be scattered.
To add a new Area mask, click the
icon and pick a group or component in the model.
Part of the geometry used for masking.
In Faces mode, instances directly below the entity faces will be masked. This is useful to preserve holes in the mask.
In Contours mode, the whole silhouette of the picked entity is used. Only edges are considered, faces do not have any effect.
Projection axis
The direction in which instances are masked.
If unchanged, the default behavior is to use the same projection axis as the composition.
Left: Faces mode. Right: Contours mode.

Path masks

A Path mask uses a curve with a configurable thickness under which instances will not be scattered.
Path masks are particularly convenient to simulate dirt paths and tire tracks.
To add a new Path mask, click the
icon and pick a group/component containing curves.
Lateral distance under which instances are masked
Projection axis
The direction in which instances are masked, like for area masks

Image masks

An Image mask uses an external image file to control where instances can be scattered. This is useful to reproduce complex patterns.
To add a new Image mask, click the
icon and select an image file on your disk.
The black and white Skatter logo is used as an image mask to scatter vegetation.
Image masks rely on the surface hosts having valid texture coordinates. Please make sure that your surface hosts are properly set up.
Image file to use as a mask.
Moves the image along the X and Y axis.
Texture coordinates are traditionally defined in the [0, 1] range, so an offset of 0.5 usually moves the image by half its size (this convention might not be followed depending on the origin of the texture coordinates).
Size of the projected image. Tweak this value to create zoom or tiling effects.
Orientation of the image, in degrees.
Display the texture
Shows the image in the viewport (only for SketchUp 2020+).
SketchUp only allows extensions to draw textures in the viewport since SketchUp 2020 so the preview will not be available for older versions.

Darkness values

The whiter a pixel of the image is, the more likely it is for an instance to be scattered there.
A 100% white pixel means that instances can be scattered there. A 100% black pixel means that instances cannot be scattered there. Values in between are interpolated linearly.
Image masks only consider grayscale values, as shown in the SketchUp preview. Skatter accepts colored images and converts them to grayscale.

Composition masks

A Composition mask uses the instances from another composition to filter where instances can and cannot be scattered.
For instance, a composition Trees might be used as the composition mask for a composition Fallen leaves, so that they only appear around the trees.
Fallen leaves are part of a composition that uses another composition's trees as a mask so that leaves only appear around trees.
The other composition to use as a mask
Distance to mask around each instance of the other composition
It is possible to chain multiple compositions as masks of each other ("composition A masks composition B which itself masks composition C...").
However, Skatter does not allow to create cycles where all compositions depend on each other ("A → B → C → A") as it would be impossible to compute.