Join Rob Garrott for an in-depth discussion in this video Modeling with the Knife tool, part of Cinema 4D R12 Essential Training.
Cinema 4D has a very extensive modeling toolkit. It can be a bit overwhelming to look at the entire list, but there is hope. It was the goal of the programmers to give you tools for every conceivable situation, but you can model some very complex shapes using just four basic tools. We'll start off with the most important, the Knife tool. The Knife tool is used to create slices in your objects, and let's see what that means. I'm going to add a cube to the scene, and this cube is a parametric object. It's called a primitive, and it means we can change the parameters of it over here in the Attribute Manager. I'm going to convert this into a polygon object by using the Make Object Editable button. When I click that--I can also hit letter C on the keyboard--and my cube now changes into a polygon object. That means I can overhear the modes icons on the left-hand side of the interface and click through these modes, so I get Point mode, Edge mode and Polygon mode.
Now, I normally like to use the Knife tool in Point mode, so I'm going to click on Point mode and then access the Knife tool. Though Knife tool can be accessed a couple of ways. I'm going to switch layouts here to something called the Modeling layout. It's still the same Cinema 4D application, it's just going to be a rearrangement of the icons and palettes on-screen. So I'll go over here to the Layout button and click and hold, and go to the Modeling layout. And when I let go that, you can see a very quick flash of the screen, and it's reconfigured all the palettes. I still have the Object Manager over here on the right and the Attribute Manager, but the Coordinate Manager shifted, and now I've a whole host of tools across the bottom here.
Now some of them are grayed out, but the grayed-out icons will change depending on which mode you're in. As I switch from Point, Edge, and Polygon mode, you'll see the icons shift and change based on where those tools can be used. Now the Knife tool can be used in any of the three modeling modes: Point, Edge, or Polygon. I prefer to use it in Point mode, just because it allows me a little bit more control. So let's access the Knife tool. The Knife Tool icon looks like this, and you can see, as I hover over it, it says Knife. The shortcut is MK, or just the letter K by itself, so I'm going to select the Knife tool.
When I do, I see the attributes for the Knife tool. The Knife tool has several modes to it. The basic is Line mode, and let's take a look at what that means. The way the Knife tool works, it is the click and drag operation when you're using Line mode. If I click over here on the left-hand side and drag a line across my object, when I let go, it makes a slice through the object. That knife slice went right diagonally through my object based on the angle that I was holding the Knife tool at. If I make a slice this way through my object, it makes a slice at that angle. Polygons come in several flavors, and when you using the Knife tool, really what you're doing is you're creating polygons on your object.
Each time I use that Knife tool, I'm slicing through it, and the result is new polygons on my object. And you can see that I've made a polygon here, I've made polygon here, I've made polygon here, and the same on the other side. When I use the Knife tool in this mode, it's slicing my object, but it's not slicing all the way through. When I orbit around my cube, what I see is the result of these knife cuts on the back side of the cube. These green lines indicate the presence of N-gons. Now the reason I'm seeing these green lines in my interface is because I went to the Filter menu and did All, and that activates the N-gon indication.
Normally, if you had that N-gon indication off, you would not see green line here. But these green lines tell me that when I use the Knife tool, I created something called an N-gon. And N-gon is a polygon with more than four sides, and they're not usually a good thing. And the reason they're not a good thing is because they can be misinterpreted by the computer and give you really weird end results. And so I like to make sure that I don't create those N-gons when I'm using the Knife tool. And so we're going to switch modes with the Knife tool to make sure that we don't get those N- gons anywhere on the surface. Just to repeat the process, delete this cube, add a new cube in the scene, make it editable, and then make sure that I'm in Point mode.
I'm going to right-click in the interface to get the Knife tool. Same as going down to the icon below, I just like to right-click a lot. So I get the Knife tool here, and you see the mode is still set to be Line mode. Now I'm going to switch this mode to Loop, and when I switch to Loop mode, the Knife tool behaves in a different way. Now what it is doing is it's going to cut all the way around my cube, and this is a much better way to make a cut, because I know for a fact that it's going to not create an N-gon when I do this, because when I click the knife--boom--and make a cut, it has made a cut evenly all the way around my object.
Loop mode causes the Knife tool to look for something called an edge Loop. An edge loop is a selection of edges that travel all the way around your object. So in this case they're all the way around the Y axis and back to the other side again. If I use the Knife tool along this edge, it's going to go all the way around the Z axis and back again, so I can make a cut here. You'll notice that each time I make that cut, I'm introducing new polygons in your object, and that's really the whole point of the Knife tool is to introduce new polygons in your object, so that you can then manipulate them further. Now the Knife tool has several other modes, but I primarily use it either in Line mode or Loop mode. And so let's take a look at some of these check marks here. The Restrict to Selection button and Create N-gons buttons don't really have an effect when you're using the Knife tool in Point mode, using the Loop mode.
Now let's see how those do have an effect when you're in Polygon mode. I'm going to switch my mode over here on the left-hand side interface, and they go into Polygon mode, so I'm now cutting polygons. I'm going to switch to the Selection tool and select just these two polygons here. I'm going to grab my Knife tool again, by right-clicking and going to the Knife tool, and then I'm going to make a cut along this edge right here. When I make that cut, look what happened: instead of cutting all the way around my object, it only cut the selected polygons, and that's what this button right here does, Restrict to Selection.
Now the Create N-gons button was on as well, so when it created that cut it closed off that cut by creating an N- gons to one side, and that's what the Create N-gons button does. Now, I don't normally like to use the Knife tool in this mode just for this reason. But if you do need to cut the polygons for some reason, you want to make sure and turn off Restrict to Selection and turn off Creating N-gons, That way when you make a cut--I'll undo to get rid of that cut: Command+Z or Ctrl+Z to undo that-- when I make a cut now, even though I have the polygons selected, it still behaves the same way as it did when I was in Point mode.
So you've got to be really careful about how you select those buttons over there and which mode that you're in, to make sure that the Knife tool behaves in a predictable way. The Knife tool is a great way to add detail that can be used to create complex objects.
- Exploring the importance of object hierarchy
- Modeling with splines
- Modeling with the Knife and Extrude tools
- Applying materials and texturing
- Creating and manipulating light sources
- Animating in the timeline with keyframes
- Controlling camera movement
- Compositing in After Effects
- Texturing with BodyPaint
- Using XPresso and MoGraph
- Creating particle systems
- Rendering and adjusting final render settings