ZModeler does not have units setup feature and uses it's native nameless generic units. In most of cases these units match with games "meters" units, but it's not a rule. When you move mouse cursor in a view, the rightmost pane in status bar shows coordinates of cursor. They are in ZModeler native units (or simply "units"). Before modelling, you should consider for yourselft what this unit will be. If you are about to model a huge island with roads and such, you should consider unit to be 10 or 100 meters and work with this units in your mind. While modelling a vehicle, it could be 1 meter (for simplicity) and if you model a small object (a gun or an ammo belt for example), you can consider unit to be 1 centimeter. You can rescale your models before exporting, but working in non-comfortable units should not be your choice. Choosing proper units of scale will avoid some drawing issues in ZModeler (e.g, when model is way too small, there will be very lengthy normals and zooming in and out will be way too fast).
While modelling a 3D mesh you should always be aimed to create a model that is as close to it's original object as possible - car should not "just look like original", it should match it to look perfectly; human should have proper proportions of torso, legs head and arms, or it will look ugly. In any case, you will need to use a reference material in your hands to make things with proper sizes and details.
In 3D Modelling the most important reference material is a set of "tracing images". These are an images from certain angles (frontal view, side view, top view etc.) If you have no tracing photos or blueprints, then it's highly unlikely that you'll succeed at representing original object in your 3D model. You can't go into ZModeler and expect to just start clicking here and there and end up with an accurate representation of what you were going to create. Ryan Travisol has written a pretty good guide on what are tracing images and tracing photos. There will be not written that much here on this issue. Assume You have found blueprint BMW on the right. There is no view from top on this blueprint, so we shall work without it.
When preparing a background images (blueprints) for ZModeler, you have to follow basic strategy: Edge-sizes have to be "power of 2" (128,256, 512, 1024 and so on). When loading any images into zmodeler, it is adopted to your videocard texture compatibilities. Most of cards require textures to be "power of 2" in width and height. Moreover, some of cards uses only square textures. We shall respect these restrictions or otherwise loaded image might appear stretched or rescaled. To make a proper tracing images from blueprint, you have to cut front, read and side views and save them as separate images. First of all, When cutting images, you have to be as precise as only possible - cut exactly on car's boundaries. Second, when placing cutted image on a canvas, make sure it's centered. So the sequense of operations in your photo/images editor should be as follows:
On the right is an example of front view placed on 512x512 canvas.
This lesson will envolve creation of BMW 3 series and I've prepared blueprints. They are in Files folder with several .z3d files.
Having a dozen of photos of the vehicle you create will help you a lot. Certain details are omitted on blueprints and in some cases you will need to check on photos how exactly certain area is shaped. Also, before modelling, it's recommended to examine photos first - to check where the shape is more curvy, and where it's nearly-flat; where it has gaps and other details (this allows to lay polygons properly and prevent re-modelling). Images on the sides are an examples of analyzing the photos. I've drawn some curved lines over them - these lines form the shape of the vehicle. Notice that in the "corners" where shape of the side smoothly turns and forms the rear part, there are more lines, then on the ares where shape is almost flat (sides, trunk, hood, etc). In general, the more curvy the shape, the more shaping-lines required. This guideline directly relates to 3D modelling - You need to place more vertices/edges where shape is curved, and much less vertices/edges required on flat areas.
It usually appears that such an analyzing curves (or even actually-modelled edges) comes into a certain point and result in a small, but very detailed area. (Corners near rear-light and head-light are an example). In such a case, this extra-detalization can be ommited and modelled with less polygons. Also, keep in mind that laying polygons exactly over shaping curves does not always neccessary.
So, we came very close to modelling of vehicle. Before creating anything in scene, you should set ZModeler layout and load required backgournd images. I stronly recommend to use default views layout which we shall set up right now. You need four panes (two rows, two columns). If you don't, pick in main menu View\Layout\Split top-level\Four panes cross. Then make sure the views are laid the following way:
If any of views is wrong, click on a viewport label-button and pick proper viewport in pop-up menu.
The second step is to disable grid in viewports. Since we have almost all required blueprints, we don't need grid in the views. At least not such a detailed. In each viewport uncheck in viewport menu Disply\Grid\Draw Minor. You can optionally uncheck Disply\Grid\Draw Major too (at least I disable it in perspective view).
Finally, we shall load all three blueprints, starting from Front view.
Pick in front view's menu Display\Image\Select Image.... A "Textures
Browser" will pop-up. You should press "Add..." button in the bottom,
and locate blueprints. I've placed them in Files folder of this tutorial.
They are "front.png", "side.png" and "rear.png". You can select them all (holding
down; or load them one by one - it doesn't matter. Image on the left shows
textures browser with all images loaded.
Once you have loaded images, you should pick front.png item in the list
on the left and click OK button. A "Background image affect" options box
will come up, where you should select a Affect views of same type item.
Since you will use background images on your own later, I'll exaplain all three options here:
Click OK button and "front.png" blueprint should be in "Front" viewport background now:
Repeat the same by loading "side.png" into "Left" view and you are ready for modelling the car.