Plan Your Attack - Setup & Considerations
Now that we have an overall idea of what shape our car is going to take, you want to analyze the reference material and really consider what approach you are going to take. For myself, HyperNURBS modeling comes in two main flavors.
1) Using points to construct polygons in such a fashion that you are essentially 'surface modeling'.

This method relies on placing vertices, and using the bridge tool to create polygons between these points. By selecting resultant points and polygons, you can fine tune the positioning of them to achieve the result you want. The advantages of this method are that you can perfect different areas of your model as you go without having to worry a whole lot about parts you will get to later. It allows for more direct control, requires less planning. But it tends to be more tedious (placing points, bridging points, placing points again, and so on...).

2.) Using modifications to a simple primitive to create a detailed model from an original simple one.

This method takes a simple object (like a cube), and through various techniques, (like knife cuts, polygon extrusions, bevels, smooth shifts, inner extrusions, etc..) you can create a detailed result. The advantages of this method are that you don't have to do quite so much manual point placement and vertex bridging (the tedious stuff...) The disadvantages, or rather, the tricky parts, are that you have to have a LOT clearer idea of where you are going with the model from the outset. You need to plan the model in such a way that you can execute modifications to one aspect, without messing up another part.

There could well be more, but these are the ones I use! The success one has at either method is likely going to be determined by personal preference of modeling approaches. It is good to get familiar with both approaches though, because when one approach fails for a particular model, that usually means the other approach is ideal.
In this case, by analyzing the result we want to achieve, we can decide what approach will work best.




1. This car in question is a pretty generic cube shape... that's an indication so far that we might think of using the second method I mentioned.
2. Looking at where the details lie, they seem to occur at pretty even, geometric locations on the car's body. Notice the tops of the wheel wells are roughly at one third and two thirds of the body height?
3. Also, the car door is pretty straight up and down, so could be approximated by a couple knife cuts. In fact, this car is so geometric, I already have decided to use the box modeling method!


Getting it Started
Create a cube that roughly encompasses the dimensions of the car hull in our reference picture... Apply a basic color material with some transparency so that we can work in shaded mode, but still see points that would otherwise be obscured by a solid surface.






The cube needs only be 1 segment tall and one segment long at this point, but for our purposes, it needs to be two segments wide. Make it editable and switch to polygon mode.



The first step after creating the cube is to get rid of most of it! Cars are generally symmetrical down their centerline, right? In that case, we'll use a symmetry object and only do half the work! Delete all polygons from the bottom of the cube, and also all polygons to the one side. For now, we won't worry about adding the symmetry object until later.




Select all polygons. Now we are going to add a bunch of knifecuts based on an analysis of the model in areas we know we are going to want more detail in. You want these knifecuts to pass completely through the cube at this point. If you don't make cuts from one side to the other, you'll end up with triangulation, which is undesirable, messy, and just generally harder to get a pleasing result from. (There are other reasons to avoid it too...)


Side View- Knife theory
With all polygons selected, we can make knife cuts through the following areas:




Horizontal knife cuts
1. This is a "lip" or line in the car body that defines the edge of the headlight swell and also creates a recess where some chrome trim will be seated later. While the rest of the car body at this level is fairly smooth, we will need some control over the edge of the headlight swell, and this is the only way to do it! Coincidentally, this is about at the level where the hood ends on the very front. Ordinarily we'd want a double knife cut to define the hood edge, but not at this stage... We'd either have trouble getting a nice gradual curve to the side of our car body, or we'd end up with a sharp ridge running around our model if we double-knife here too soon.
2. This knifecut defines the top of the front wheel well, and the trim line for the rear half of the car. It also provides a necessary edge for working in some details on both the front and back of the body.
3. This knifecut defines the top of the rear wheel well. It adds a necessary edge to the front and back as well, to facilitate the modeling of details later.
4. This knifecut defines the bottom of the door.





Vertical knife cuts
1. These four knife cuts define more or less the vertical portions of our curved wheel wells. We don't need to provide very much definition in this view because once the three straight edges of each wheel well are placed under HyperNURBS, the lack of tight polygon edges in this dimension will lead to a fairly loose, rounded wheel well that closely approximates this shape.
2. Front door edge. This is a triple cut because it will define the separation point between body panels, and will require a very sharp edge to look good. The middle knife cut is the separation point, while the two outer cuts are to "pin" the edge of the panels to a certain level of sharpness. Don't worry yet that the reference pic shows a curved door edge in front. We will shape this edge more closely once we have done some basic manipulations first.
3. Rear door edge. Same as above, but due to the straight vertical nature of the rear door edge in our reference pic, once we cut this, we won't have a lot of editing to do in this area.... Sweet!
4. This double knife cut is here to facilitate the stark change in curvature of the body styling at this location. You can see by the reference picture that there is quite a dip going on right behind the door.



Top View- Knife theory
Still with all polygons selected, we'll have a go at cutting in some of the features of the top view. (Try to mentally block out the pre-existing cuts from the side view. The numbers below refer only to new cuts made from the top view!)




Vertical knife cuts
1. This knife cut serves little other purpose other than to add a bit more control to the curvature of the headlight swell we will model. There is a slight 'S' curve to the body as the hood slopes away, and the point provided by this cut will enable us to better approximate it.
2. This double cut is the hood edge, even though right now it appears to be far away from the edge of the hood! it starts at the furthest point back, and will require us to move the points into position to depict the round hood edge. Incidentally, the triple cut from the door edge on our side view (visible just to the right) will also become the cab edge in the top view.... How's THAT for convenient?
3. This is the rear cab edge. We don't need to define this one with such a tight formation of polygons as you can see the curvature of the cab corners in the back will likely be approximated by the natural "loose skinning" of larger polygon formations.
4. This is the trunk edge, an requires a double cut to "pin" the corners in place for when the trunk's separation from the car body is done.




Horizontal knife cuts
1. These two lines at roughly equal intervals are for adding some curvature to both the hood and to a lesser degree the trunk. They will also allow us to approximate the curve of the front cab edge a lot closer.
2. This cut is going to help define the hood edge, and also set up the "pinning" of the HyperNURBS surface to a lot tighter point when the transition to the headlight swell is made. Also helps with placement of the cab edge points in front and back and will start to define the trunk in the back of the car. When defining the trunk edge, we'll eventually want a double cut, but to avoid overcutting the front of the car, we'll wait until we've separated our body panels to add the second cut.
3. This double cut allows for the sharp transition we'll need from the hood and trunk slopes to the headlight swells and tail fins respectively. It also defines the side cab edge and will keep the edges of the door pinned tightly in place when we separate the door panel from the body.
4. This cut will give us a way to add a nice round curvature to the tail fins and headlight swell when the points along it are selected and moved in a positive Y direction.



Front View- Knife theory
You MUST be getting sick of this by now! :-) With all polygons selected... knife cuts through the entire cube as shown...




Uhhh, no new knife cuts!
Okay, so I lied! On closer inspection, it looks like the ones we made on the other views are going to suit us nicely in this view as well. There are already lines that roughly define where our grill opening will be, where the bottom edge of our hood will be (may have to add a second cut to that later though...)
Sure there will be a little bit of point manipulation and maybe some sundry adjustments to be made, but this is all we need right now to get a good start.
(Note... the back view is also fine for now, so I won't bother showing it here.. I'm sure we'd all like to get on with modeling the damn car already!!!) :-)



Review- Planning
When doing the box modeling HyperNURBS method, this planning is probably the most important step... Unless you have sufficient skill to model yourself out of a corner, you'll find if not enough time was spent planning how to accomplish the model, you'll be hitting Undo, deleting, reworking, and just generally plowing your way through the model in the least efficient ways possible... (Well, that's true for ME anyway...)
You can see knife cuts happening at places such as door edges, panel edges, because we'll want these polygon edges to define actual edges in our model. But you'll also notice there are knife cuts where we will want to affect greater changes in the model's shape. As of the writing of this tutorial, Cinema 4D XL does not have vertex weighting in HyperNURBS control cages. Consequently, the only way to create varying curves and edges in the model is by manipulating the polygon density of corresponding areas of our control cage. Thin narrow polygons can be used to create sharp edges, while large polygons cause gradual curves in our model.
Blah blah blah, enough already! :-)