Technical Development: Advanced Track Systems

I have been working for a few years to develop a simple and efficient track system that can be used to render and animate rail lines, roads, pipelines, power lines, and general object movement.  All these systems contain  vertices that describe a set of curves which object movement is interpolated upon.

Here is a video showing a basic implementation of the rail system, first where the line follows terrain height, then where the line is smoothed, and finally where the terrain height is modified to prevent terrain from overlapping the rail line.  The video also shows a sample for pipeline animation-

General Track Lines 

Generation

One of the goals of this system is to have an implementation where you can connect the current line to any other point-  This is done most easily by connecting segments of circles, which with an endpoint and angle, a smooth arc can connect to any other point.

One problem with this system is when connecting points, the end angle can not be defined.  So for connecting new sections with objects or other arc segments, a more complicated cubic function would have to be used.  Creating smooth circuits and connecting to bridge objects will necessary for full implementation, but for now linear segments work very well. 


These arc segments exist only to generate vertex arrays for the roads and rails, as well as a set of nodes that object movement is based upon.  The height of these vertices are set to the terrain height, so the lines do not cut underground. Naturally, these lines are very rough depending on there terrain, where in reality, train lines and roads are smooth with a maximum gradient.  The track vertices then have a set value they can change over a certain distance, effectively smoothing the line based on the system (roads can have more bumps and variation)  Lowering and raising vertices from the terrain creates the need to modify the terrain, which is done most simply by setting near by vertices to equal the height of the track during a traverse.  This works well, but the resulting flatness is pretty rough and obvious.  I will probably try to allow a certain level of slope differentiation, most likely operating on triangles rather than squares, but it is hard to prevent terrain lines from cutting into the upper parts of the track or eroding from beneath. 


Movement

After the lines are generated, objects move based on a separate set of vertices that contain the position, length, and rates of change for direction and angles.  Positions of objects are essentially interpolated between vertices, as the objects contain data for what index they are currently on, and the position in the segment [0-1].  Distance moved on the segment is determined by the length of the segment, and speed of the object so movement is consistent across the entire system.

These systems are fully scalable, and operate well in 3D.  Though there are a few issues of angles changes jumping between 0-360, these problems could definitely be smoothed out.  There are also a few features such as grading, where objects and lines would tilt into curves, but this is beyond the current scope of this project. 


Rail Lines

Train motion is currently based off of moving two separate wheel truck objects, and then determining the mean rotation and position of the train body based upon these two objects.  Each train car is moved in this way, and as long as each truck moves by the same speed, the train appears to move smoothly. Currently the the different train cars are initially offset from the starting position by relative distances dependent on the length of the curve.  Eventually I would also like to implement simple slack action between the train cars.

Pipelines
This system is more simple, as the only animation that really will take place is pipeline generation similar to that shown in the video.  For this visual traversal, I added an alpha value to the vertices which is set by the current position on the pipeline.   This system is quite simple but effective, I would also like to connect the lines to objects such as above river crossings, which would require seperate implementation for similar animation.

RoadsRoads are similar to the Rail Lines, where each object contains data for its current index on the road line as well as the current possition on the line segment.  I also implemented a system to fade the begining and end of the road segments.  Using an alpha value similar to the piplines, the fade is also present in the movement data, which can be used to multiply by the speed of the current object to slow it down as it approaches the end of the segment.


These systems are all quite simple, but difficult to explain without showing the math and implementation in detail-  I will try to write a comprehensive tutorial on these systems in the future.