See image. I'm trying to cut out a small piece of the terrain to use for particle system (scatter trees, grass etc). I used the knife tool to draw the area (orange line). How can I then delete the rest of the terrain and just keep the portion within the orange line? (In Max I just use the shapemerge modifier, not sure what to do here in blender).
Create a desired shape from 2D curves, below is an example, and close it enabling cyclic option. Model the plane to shape, convert to mesh and knife project it onto the terrain. Then invert the selection and delete it. Or, having knife projected, separate the selection and delete or keep the portion that's left. NB: to knife project correctly you need to be in ortho view.
For selecting mesh inside the loop:
- Using Knife tool create you loop as you want. You also can slect one vertex an pressing Ctrl select other, and blender will select shortest way between them. After selecting one way, you can continue selecting with Crtl + Shift.
- In Edit mode, at the top click Select > Select Loops > Select Lopp Inner-Region
- For separate selected, click on the top Mesh > Separate (P) > Selection. This will separate your selected mesh in new object.
As already written by "moonboots" user, if you want use this for particle system, you just can use Weight Painting, and selecting not required. Also you can paint white-black texture and use this as mask for transparent material, but you need particle system, so on transparent regions still will be particles. But, using this with Weight Paint, you can paint weights for particles and mask for transparent material, so it will be non-destructive.