Can you get a hard edge with Weight Paint?

Can you get a hard edge with Weight Paint like you can with Vertex Paint? It seems that even if you mask your Weight Painted area of the mesh, any weighted vertex will graduate its colour to the next vertices even if those next vertices are not being weighted, which means you ALWAYS end up with a gradual decrease in weight over the surrounding faces.

I am trying to get some grass to stop abruptly, and at the moment I am having to use a duplicate of the weighted mesh combined with Mask modifiers.

No, it will not unless you have enough geo to make the transition sharp. I mean that if you use simple face select and masking, you will see a gradual ease to the value to the next vert - but if your mesh has a loop cut or inset/bevel that allows you to have that ease be given a smaller run to travel.

Thinking again, could you use a texture instead? Black and white areas can be very sharp in the texture, at least sharp enough to make for a good stencil

at the moment I am having to use a duplicate of the weighted mesh combined with Mask modifiers

That’s done like so on the scalps too as far as i know and seems to be efficient - set separate Emitter mesh to not rendered. What for do you use Mask modifiers?

Let’s say I have a cube, and i want only the top face to have grass. So you create a Vertex Group from the top 4 vertices. The trouble is, these top 4 vertices will graduate downwardly to the bottom 4 vertices - and not give you a sharp edge that ends outside the top facing face.

So i use a Mask modifier to make it so that only the top face is visible. This modifier then needs to be moved above the particles in the modifier stack so that it hides the graduated bits - making only a sharp cut off at the edge of the top face / grass.

However, that mask also hides the slice of mud (the rest of the cube) from which the grass sprouts. So duplicate the cube, on the new duplicate, you can remove all particles and all masks so that it is ‘mud only’. Parent them together and you have the cube of mud with the grass growing ONLY out of its top face with the sides and underside totally grass-free!

Said Cube: if you duplicate top face and make it separate Obj with Particle System you do not need anything else except parent it if that is an animation. Even the vertex paint/group is not needed with 4 vertices or one face (might come handy if the surface is bigger).