Weight painting vertex groups not entire object

Question

Blender version 2.9. Advanced noob.

I'm trying to weight paint a tree object for the particle generator which adds small sub-branches containing leaves. The tree object contains a large trunk with some main branches higher up, so it looks like, well, a tree.

Since I want zero sub-branches/leaves on the lower main trunk of the tree, I made a separate vertex group called "branch growth", which is only the upper portion of the tree containing the large branches where I want the small sub-branches/leaves to appear. So far so good. However, I cannot for the life of me figure out how to just weight paint this particular vertex group. I have the vertex group "branch growth" selected in the object properties pane, but when I switch to weight paint, the whole object, including the trunk, is painted blue, indicating to me that I am in fact NOT just painting the vertex group (more evidence to this theory is that I can in fact paint on the trunk, which is outside of the vertex group). "Fine" I thought, i'll just leave those areas weighted zero and make sure to only paint the areas I want the particle system emanating from. This approach works reasonably well, except that because of the large number of particles i'm generating (around 800), there is inevitably one or two particles coming off the trunk, which is annoying, even though it is solid blue. I'm guessing the law of probabilities is going to work it's way into this area given the large number of particles. Everything else such as branch size and density works great using the "branch growth" vertex selected in their respective fields. One solution I found researching this is to duplicate the "branch growth" vertex group as it's own mesh object, thus ensuring zero bleed over, but it seems that there must be a more proper way to do this, and I dislike using work-arounds when there is a "real" way to do things. I have tried this in Eevee and Cycles render engines, checked for duplicate vertices, hidden faces, etc.

Here is my workflow. Select entire "tree" object. Go into edit mode and select only the vertices I want to have branch growth on, and assign to new vertex group called "branch growth". Confirm this group by using the select/deselect buttons. While vertex group is selected under object properties, I go into weight paint mode, and this is where the problem begins (i.e. entire object is blue, not just the vertex group). I must be missing something at this point, unless i'm not and this is just how Blender behaves. Any help is greatly appreciated. I still get a liveable result at this point, but if one was trying to render hair on a face object I can see where a stray hair coming out of the nose would be unacceptable.

Answer 1

AFAIK, and maybe someone will correct me on this, weight painting and assigning verts to a group are the same thing, assignment just sets weight to 1 for all assigned verts. You may be able to assign this group to your particle system straight away.

A vertex group is basically just an extra variable attached to each vert that you can reference. So by creating a new vertex group you essentially create a new weight paint mask where all verts have weight 0 (Blue).

I have had issues with rogue particles, not sure if I ever had a surefire solution other than what you've suggested or looking through seeds to find the least problematic one.

Answer 2

As Allen Simpson said, weight painting and vertex groups are the same thing. Weight painting only gives your more flexibility and usability, but the same functionality is available from the vertex menu. (see 'weight' slider to assign values other than 1)

The solution would be to enable the Paint Mask M and select only what you want to be able to paint. This selection is shared with edit mode so it's easier to first select in edit mode.

Paint Mask will also enable you to hide verticies and faces for improved visibility.