Here’s how to use the knife project tool in Blender:
This process has change a little bit in the recent versions of Blender. Now you just take the object you want to cut into edit mode, then control select the cutter from the outliner. Then cut like usual.
In order to demonstrate the knife project tool, let’s create some quick skyscrapers. We’ll take the default cube into edit mode and delete all the faces except one. We’ll scale it up a bit and then subdivide it into several smaller pieces. Then we’ll do a random deselection and extrude upwards.
So let’s say that we decide to add some more details into these long sides here.
We can’t loop cut through all of them at once and cutting them one by one would be quite tedious. Situations like this are when the knife project tool comes in really handy.
So let’s go to the object mode and create the object we’ll use for the cut. Let’s add a simple plane and bring it to edit mode. We’ll scale it to be a bit larger and move it up. Then we’ll duplicate the plane several times and move the duplicates below the original plane. We can quickly repeat the previous command by hitting shift-R.
Now we’ll go to the object mode and add the object to be cut to the selection by shift right-clicking on it. Then we’ll go to edit mode and search for the knife project command. The first result will not be what we are looking for, because we were at a diagonal viewing angle, and the knife project tool will consider that angle in it’s operation. So let’s undo and make sure that we are cutting in a straight view like the front view. We can also use knife project from this button here. Now the result looks better but we can see that only a small portion of the buildings was cut. So let’s undo once more and make sure that we check the “cut through” checkbox from the settings here. Now the results are exactly what we wanted: all the buildings have been cut through perfectly.
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