To paint on a 3D object, you must first make the object paintable. When you paint directly on a paintable object, Paint Effects creates a curve on the surface along the stroke path and attaches the stroke and new brush to it.
If you make an object paintable and then try to paint on one that is not paintable, no stroke occurs. If you don’t make any objects in your scene paintable, your strokes are placed on the ground plane.
When you paint directly on a paintable object, Paint Effects creates a curve on the surface along the stroke path and attaches the stroke and new brush to it. If you move the plane, the paint stroke moves with it.
The kelp brush orients the tubes to point along the normal of the surface you paint on. In the ocean, kelp grows towards the surface of the water in a vertical manner. If you want your kelp to appear realistic, it should point vertically upwards. If you paint the kelp on an angled part of the surface, the kelp tube will not point in a vertical direction.
If you want to change the width of the brush stroke, to paint a narrower or wider tube path, for example, a wider swath of kelp plants, press the shift and b key and drag to the left or right.