Use texture + color ramp to set density vertex group for particle system I created a landscape of diffuse shaders, e.g. for meadow and rock and combine them with a Noise Texture + ColorRamp as factor of a Mix Shader:



Now I want to apply objects, e.g. trees to the area where the meadow is. Therefore, I would create a hair particle system with a density vertex group. This vertex group can be created by weight painting, but the information about the density is already there, in the output of ColorRamp in the material setup.

How can I use this directly to control the particle system without weight painting manually? Or in other words: How to convert the ColorRamp to a Vertex Group?
Vertex groups are one method of controlling where particles generate, but there are also particle textures.
Create a texture from your color ramp (one way is to connect the color ramp node direct to material output, then bake). This should be an image that is full black where you don't want particles and full white where you do.
Then in the Textures panel of your particle system menu, press the + to add a texture. In the settings for the texture, under the Influence tab, check the box for Density.