I'm using the knife tool to cut out landmass shapes on a simple plane object. Most of the time it works fine, but on certain areas, I get this weird ghosting effect with the cursor rapidly shifting positions. When I click, the new cut vertex gets put wherever the cursor is, so it's really hard to be accurate. It seems random when and where this error occurs, but when it does show up it happens consistently in the same area. The plane I'm using currently has 7,000 vertexes after the cuts I have already made. I'm on blender version Here is a video demonstrating the error. Any advice?
Pretty sure this has to do with floating point imprecision. Your object looks like it's at ~1million meters from the origin.
Blender only has a limited number of bits that it can store vertex locations at.
Say for example, Blender has 8 bits of data to store each vertex location in one axis. That means a vertex location at 1.2m on the X axis in memory "looks" something like this:
While a vertex at location 1 million might look like:
You only have one place after the decimal to clarify the vertex position, vs. 7 in the smaller example.
This is obviously not exactly accurate on a computational level but only serves to illustrate the purpose.
The farther away from the origin you get, the fewer digits you have to store exact vertex locations. Blender has to "round down" in order to stay within the bounds of the bit depth vertices are stored at.
If the location of your object (or more specifically the section you are cutting) is moved closer to 0,0,0 in world space this bug will likely disappear.
Testing to confirm
Simple 2m plane in a blank scene, located at 0,0,0:
The same plane located at 10,000m, 0,0:
Not super noticeable but zoomed in you can definitely see the traces of floating point inaccuracy creeping in.