Layers are procedural effects that you can apply to a vertex group.
They can be chained together to create advanced distributions, for example by creating a slope layer on a landscape to only have trees grow in flat areas, and then using a height layer to make sure that they only grow above or below a certain point.
Layers act much the same as modifiers on objects, or layers in photoshop.
You can add a new layer by clicking the "Add layer" dropdown, and selecting one from the list.
Each layer has a few properties/operators at the top:
Show/hide details: (This is shown as the icon of the layer at the top left) Whether to show the properties of the layer, or just the top bar.
Layer name: The display name of this layer
Show/hide layer: Whether to show the effects of this modifier (in both the viewport and rendered view)
Move layer up/down: Move this layer up or down in the stack
Delete layer: Remove this layer from the stack
Blending mode: How this layer is blended over the ones that have come before. Add is the default, but some other useful ones are Multiply (Acts as a mask) and Mix (A linear interpolation between this layer and the ones before it)
Blend factor: Acts as the strength of the layer. At one, it is fully blended, and at zero it isn't blended at all.
Then below that the layer specific properties are shown, in this case, Rotation.
Creating a forest distribution map
1. For this, we want trees:
only on flat areas
Not at high altitudes
In a random pattern
Not outside the camera view
Let's start with a blank terrain
2. To start, let's add a slope layer, so that the trees won't scatter on the steep areas of the terrain. Let's also add a colour ramp adjustment, so that we can better control the contrast of the layer:
Now the trees will only appear in the flat areas
3. Then, to make the trees only appear at low altitudes, let's add a height layer, and invert it by adding another colour ramp adjustment. By default, this is just added on top of the previous layer, but we can make it act as a mask by setting the blend mode to "multiply":
Don't forget to set the blend mode to multiply!
4. Now, to add a random pattern, we can add a procedural texture layer, and again, set it to multiply so that it masks the layers above it. Then we can set the texture mode to "noise", and adjust the scale and other parameters to our liking (once again with a colour ramp adjustment):
This helps to break up any patterns in the trees
5. Finally, we can add a camera cull layer that will set all vertices of the mesh that aren't visible to the selected camera to zero. You can also click the "Use scene camera" button to automatically pick the scene camera:
This optimizes away trees that would not be visible from the camera
Here's the final layer stack:
The final result, where the weight map is used to control the distribution and scale of trees in a forest