A recent article on the Digital-Tutors' blog got us thinking about some of the tools that are used a lot. Because of the nature of the work, it's often needed to jump across a lot of different programs within a short time. Although there's a ton of overlap in the functionality of those programs, especially in modeling, some tools will eventually stick out as being particularly useful or time-saving. One tool that comes to mind in this instance is the knife tool in CINEMA 4D.
If you spend any amount of time modeling with polygons, you'll inevitably find yourself doing the same types of things again and again. Much of this work involves adding or manipulating edges in different ways. Depending on what application you're using, you might use three or four different tools to modify your edges in different ways. For instance, if you want to quicky add an edge loop in Maya, that's the Insert Edge Loop Tool. If you want to manually place edges without regard for the existing topology, that's another tool (or several depending on which version of Maya). In MODO, you might use Loop Slice and Edge Slice. These tools all work great, but sometimes it's refreshing to have one handy tool that can do just about anything.
The power of the Knife comes from its five different modes, and it will behave very differently depending on which mode is active.
Line mode will allow you to draw out individual edges in any configuration across the surface of your model. If you need to locally change the flow of edges in a certain area, this is the way to go. Line mode is good for edges that don't need to go too far or for edges that need to cross a lot of existing edges. It's analogous to the Multi-Cut tool or Split Polygon Tool in Maya. There's also a setting to add single edges if you need to quickly add single edges to different parts of the model. One really nice feature is that if you draw a new edge across several existing edges without adding points along the way, those points will automatically be added by the tool.
This mode will allow you to use the Knife to cut out a custom-shaped hole in any polygon face. Just draw out the shape on the polygon face (no need to touch any edges) and place your last point on top of the starting point. Keep in mind that the resulting face will be an ngon so you could go in and draw new edges out from the perimeter of the hole - with the Knife of course! From there you could extrude the border and bevel the edges to create a thickened opening.
Plane mode is a really powerful way of cutting up your model based on existing planes. These planes are independent of the edgeflow of the model and can be set to local planes or world planes. Or, to make things really interesting, you can cut based on a custom plane based on your camera angle. This can be a great way to get really straight cuts that go across the topological grain of the model. Adding loops wouldn't work and adding each edge individually would be really tedious. Plus you'd likely never get a straight result. As a bonus, you can also add multiple edges at once and you can decide how far apart those edges should be.
This mode will be really useful when your working with quad geometry- characters or models that you'll subdivide are good examples. Using this mode will allow you click on an edge and create a loop crossing all the way around (if possible) with a single click. You can also create partial loops, so pay attention to which edge you click on, especially on open geometry. In that case, you want to click on the border edge to extend it all the way to the other border. One tip is to hit the Shift key as you hover over the edge. This will create a temporary edge that you can then adjust numerically before committing it to the model.
This mode is sort of a combination of the Loop and Line modes. Just select a series of adjacent polygon faces on your surface. With Path mode selected, the Knife will automatically create a new edge that you can interactively position along that polygon path. This can be a good way to quickly create long edges across the existing topology while also being able to twist and turn if needed.
Keep in mind that changing modes does require a click, and there are other settings and attributes that you'll want to change as you go. So the Knife Tool in CINEMA 4D isn't going to save you a ton of click time, but it's really useful to be able to do so much with a single tool!