How to Use the Knife Tool in Cinema 4D

show more Using the Knife tool provides you with in-depth training on 3D + Animation. Taught by Rob Garrott as part of the CINEMA 4D Essentials 2: Polygon and Spline Modeling show less
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Using the Knife tool

Points, Edges, and Polygons are the fundamental building blocks of all 3D objects, and those building blocks can be manipulated using some really important tools, the most important of which I think is the Knife tool. The Knife tool can be used to create slices in your objects. Let's take a look at that. I'm going to add a cube to the scene, and I'm going to make that cube editable by clicking on the Make Editable button. I can also hit the letter C on the keyboard. Now, I'm going to switch into Point mode. I almost always use the Knife tool in Point mode, and there are some important reasons for that in the behavior of the Knife tool.

There is three ways you can grab the Knife tool. I can go to the Mesh menu>Create tools and select Knife, I can also hit the letter K on the keyboard, or I can right-click. Now let's go ahead and select the Knife tool now and see how it works. I'm in the Perspective window, and I'm going to click and drag. The way the Knife tool works in its default state is a click and drag method. So, if I click anywhere in the grey area, I can drag across the cube, and when I let go, that creates a slice through my cube that lines up exactly from the camera's point of view with that cube, and the slice that travel through it.

Now, what's actually happened? CINEMA 4D has taken that slice and added points to my cube. You can see I now have new points there, there, and there. If I orbit around my cube though, you notice on the back side that the knife has not sliced all the way through and that's created something that is kind of evil. Sometimes it can't be avoided, but most of the time it can. That evil thing is called an N-gon. Now, N-gons aren't actually evil, I just call them that to be dramatic. But, what makes them evil is that you lose control over how your objects break when they transform and move.

Polygons come in three flavors; triangles, quadrangles, and N-gons. Quadrangles can be subdivided evenly. They can even be converted into triangles. But, N-gons are polygons that have more than four sides, and they do weird things when you're trying to form them. What I mean by that is you'll notice this polygon on the back side, there is an edge here, there's an edge here, here, here, and here, and that's five edges. That's one-to-many for a square. Now, what happens is when I go to move this point right here, let's get back the Selection tool and grab that point right there, when I take that point and drag it on this axis, what I end up with is that same weird fall-off issue on this polygon.

CINEMA 4D does not know how to interpret this deformation of this polygon and so it's going to do weird things as I render. You can see from this angle, it looks like a straight edge through there. From this angle, it looks smooth. And so, if you were to animate around this section, it would do very strange things in the Render Engine. So, the way you get around that is by slicing all the way through your object. Now, let's delete this cube and start over again. In fact, let's close this whole document up; Command or Ctrl+W, and no, we don't want to save it, and we're back in a new fresh document. Let's click on a cube to add it, make it editable, and then go into Point mode.

Now, let's grab the Knife tool the right-click method. I'm going to right click. I right clicked on the cube but I could right-click any place in the editor window. And this is a Contextual menu, and this Contextual menu changes based on what mode you're in. Different tools are available in different modes and so this Contextual menu would change. Now, because we're in Point mode, these are all the tools that can be used with points. So I'm going to select the Knife tool, and when I let go of that, I now have the Knife tool just as if I grabbed it from the menu; much faster than going up and grabbing it from the menu though.

Over in the right-hand side of the interface in the Attribute Manager, you can see that my Knife tool has some options, most important of which are these guys; Restrict to Selection, and Visible Only, and Create N-gons. I'm going to uncheck all three of those. Now, when I click and drag through my object, let's orbit around and see what happened, you can see that it actually made a slice all the way through. And that's really great because now I've got nothing but quadrangles in my model and can maintain that quadrangle efficiency all the way through my modeling process this way.

Now, there's a much better way to use the Knife tool, and that's called Loop mode. Let's see how that works. I'm going to undo; Command or Ctrl+Z. And now, over on the Knife options, I'm going to select Loop mode. When I do that, I get a line that draws around my object. Let's zoom in here a bit. Now, as I move the Knife tool up and down these edges, you'll be seeing a white line. Now, that white line is not drawing on my screen here because of the screen capture software that I'm using. CINEMA 4D and the screen capture software are not getting along very well and so it's inhibiting the drawing of that line.

But, you'll be seeing a white line drawing around your object, and that's indicating the loop. Now, on my screen, you'll see four white dots, and those white dots are telling me that the Knife tool is going to cut all the way around those edges. Now, when I click, it just made a cut right at that location. And you can see that as I move around, I can make multiple cuts around this object by clicking on different lines. Now, the reason I like to use the Knife tool in this mode is because no matter what options I have unchecked. Even if I were to turn on Restrict to Selection and Create N-gons, I can still use the Knife tool in this mode.

Now, if I switch over to Polygon mode, in Polygon mode, you'll see that I can make cuts with this Knife tool in my cube because I have nothing selected. I'll use the Spacebar to get back to the Selection tool and I'm going to make some selections. Now that I have these selections made, if I switch back to the Knife tool by hitting the Spacebar, when I try to make a cut, notice what happens; it only cut the selected polygons. Now, the problem with that, if I make a cut in there, now I've got my Snap settings on. I accidently turned my snapping on right here.

If this happens to you, you can turn it off by just clicking that. That's going to make your lining up a lot easier. Now, if I make another cut there, you'll see that what's happening is it's only cutting this area right around those selected polygons. And you see the cut no longer travels all the way around the object. And that's really important to notice because I've created N-gons. If I hit the Spacebar and grab that polygon right there, that's an N-gon and that's what I want to try to avoid creating. So, that's why I don't use the Knife tool in Polygon mode very often.

I'd much prefer to use it in Point mode.

Using the Knife tool
Video duration: 6m 17s 1h 40m Beginner


Using the Knife tool provides you with in-depth training on 3D + Animation. Taught by Rob Garrott as part of the CINEMA 4D Essentials 2: Polygon and Spline Modeling

3D + Animation
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