Graphical Mechanism Creation

When creating a model for a mechanism, such as a vehicle or a prop, it's important to have complete and accurate data to start with. While the mechanical engineer has complete controls over the physical dimensions of their dynamic parts, if the 3D model is correctly sized to start with, it can be used as a starting point to quickly assemble the simulated object.

A blueprint or technical drawing containing the major views (side, front, top) and dimensions would be a good example of reference data. All of the models at CM Labs are created in meters.

Building the Model

Place the side view so that the front of the model points in the positive X-axis. Reference images can be created in a photo editing software such as Adobe Photoshop or GIMP. These images can then be imported into the 3D modelling software and scaled to correct real size dimensions in their respective viewports. These will then become the template used to create the 3D model.

Using primitive polygon objects to block out the rough shape, refine the model by extruding faces and pulling vertices.

Avoid long, thin polygon strips, and weld vertices to close any gaps.

Make sure to remove unneeded inner faces.

The vehicle geometry's center line must be aligned to the X-axis, with the vehicle wheels precisely located opposite each other. For example, Left-Front-Wheel at 1.5 m in Y-Axis, and the Right-Front-Wheel at -1.5 m in the Y-axis.

Centering the model in the Y-axis helps to have symmetry between the wheels. The base of the wheels should be at height 0 in the Z-axis.

Note Rotate the viewpoint often and use the other viewports to confirm that the model maintains the correct shape at all times. Additional photographic references will help with this.

CAD Method

Another method of modelling is starting from and simplifying an existing high-resolution 3D model, such as a computer-aided-design (CAD) file. These are usually highly detailed, and as such unusable for real time applications (refer to Art Best Practices for recommended triangle and vertex counts).

Import the model and create a lower resolution version of each component by simplifying the geometry either in your favorite DCC (such as 3ds Max).

The advantage of this method is that the geometry is already at its proper dimensions, and the result of the simplification will provide a precise model. If high-resolution textures exist, these can also be projected onto the lower resolution, real-time version to provide the illusion of more surface details than actually exist in the mesh.

Degrees of Freedom (DOFs)

In any given model with moving parts, DOFs are needed. A simple example is a car. The geometry of the car body is parent to the car's tires, as displayed in the image above. The car has a main DOF that is parent to the car body geometry and to the DOFs of the tires. The main DOF could be at 0,0 in the world whereas the DOF for each tire is positioned at the center of rotation of each tire. By default, vehicles point in the X-positive direction. The tires rotate around the Y-axis. In most cases DOF placement is fairly simple.

Baking Nodes

In 3ds Max, an artist may accidentally scale a DOF, which can cause irregular behavior. It is important to verify that all DOFs be at 100% and not scaled.

In the event that DOFs are found to have scaling in the model hierarchy, the node structure should be noted, and all nodes unlinked. Once unlinked, select Utilities > Reset XForm. This will reset all geometry and DOFs back to scale 100%, Translation 0,0,0 and no Rotation.

After import into Vortex® Studio, it's also possible to return the model (local transform) to its identity matrix without affecting the 3D model. The local transform is set to identity for each selected node and its child nodes, and the associated geometry is updated to respect the aspect. See Model Simplification Tools for more. Fixing this problem is called "baking" a node.

Optimization

When creating a model it is important to plan ahead for polygonal budget. Knowing where to add more detail and where to let the texture show the detail is important. In the car image above, the door handles are not cut into the car body, but are part of the texture.

The car model has many curved surfaces and the edges are evenly spaced to ensure a smooth profile. The car has total triangle count of 14,000, which is a good number for this type of vehicle. Complex models can have up to 150,000 triangles. Depending on the use and position of the user, high polygon counts can be used in areas close to the camera (such as the operator cabin and the base of a crane's boom) and diminish as distance increases from the viewpoint (the pulley at the very tip of the crane doesn't need a lot of polygons).

In order for the model to be efficient in the simulation, major components need to be merged together. The body of the car, for example, can be one graphic node, though its wheels will be four separate nodes as they need to move. This approach of merging static parts helps reduce draw calls, whose number should be keep as low as possible.