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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?

https://www.youtube.com/watch?v=TS6RwSqT0r8

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  • $\begingroup$ There must be some vertices the knife tool was snapping to? $\endgroup$ Jun 21, 2022 at 8:09
  • $\begingroup$ Hello and welcome. While files, images, and external videos or links may be helpful additions they should not be the only way to obtain information about your issue. Don't make understanding your question rely on downloading a file, watching a video or visiting an external site. Use the builtin tools to upload images or gifs, along with thoroughly explaining the problem in written form so it can be indexed and searched for thus helping future visitors with similar issues. $\endgroup$ Jun 21, 2022 at 8:16
  • $\begingroup$ To me it looks like snapping, too. Maybe you could show the wireframe of the plane you are trying to cut as well? If it's not a snapping problem it might be better to upload the file to inspect it further. $\endgroup$ Jun 21, 2022 at 8:17
  • $\begingroup$ Snapping is not enabled in the project, and has never been enabled. $\endgroup$ Jun 21, 2022 at 22:16

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Pretty sure this has to do with floating point imprecision. Your object looks like it's at ~1million meters from the origin.

enter image description here

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:

1.2000000

While a vertex at location 1 million might look like:

1000000.0

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:

Plane knife tool test, at  world origin

Relatively smooth.

The same plane located at 10,000m, 0,0:

Plane knife tool test, at  10000m x location

Not super noticeable but zoomed in you can definitely see the traces of floating point inaccuracy creeping in.

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    $\begingroup$ Oh yeah you're right, this object is way off... didn't take notice of that when looking at the video... $\endgroup$ Jun 21, 2022 at 12:57
  • $\begingroup$ Ah floating point. I've dealt with that issue in Unity quite a bit, but I didn't think of it with Blender. However it doesn't seem to be the issue. I'm using Blender OSM, so the real world scale is quite large, but I scaled down the whole project x100 and moved the center closer to the origin, and the problem still persists. I even upgraded to Blender 3.2 and it's still there. $\endgroup$ Jun 21, 2022 at 22:11
  • $\begingroup$ If you scaled it down by a factor of 100 that means it would still be 10000m wide which is probably still big enough to introduce some amount of floating point errors, and I misspoke, it's not simply the object origin that needs to be at 0,0,0, but the location of the cuts themselves. $\endgroup$
    – Jakemoyo
    Jun 22, 2022 at 8:37
  • $\begingroup$ I tested this and added my results to the answer, I'm fairly positive the FPI is the cause. $\endgroup$
    – Jakemoyo
    Jun 22, 2022 at 8:48
  • $\begingroup$ It just feels weird because it only noticeable in a few areas, and the areas where it's the worst are actually the closest to the origin point. There are areas much farther away were the issue doesn't pop up at all. I'm cutting out the shape of the eastern USA, and the orgin point is just off the coast of North Carolina, but North Carolina is the area where the problem appears the most. But working up by Maine is perfectly fine. $\endgroup$ Jun 22, 2022 at 14:50

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