Game Development Reference
In-Depth Information
Upon entering the for loop in the preceding code sample, End1 of the current spring is
checked to see if it's connected to a fixed point in space. If so, the temporary variable
pt1 stores the global coordinates of the endpoint, and the variable v1 stores the velocity
of the endpoint, which is 0. If the endpoint reference is a valid Object , then we compute
the position of the endpoint, stored in pt1 , just like we did in the Initialize function,
using a coordinate transform as follows:
pt1 = Objects[j].vPosition + VRotate2D(Objects[j].fOrientation,
We compute the velocity of that point as shown in Chapter 9 by first computing the
velocity of the point due to rotation in body-fixed coordinates, converting that to global
coordinates, and then adding the result to the Object 's linear velocity. This is accom‐
plished in the following code:
v1 = Objects[j].vVelocity + VRotate2D(Objects[j].fOrientation,
We then repeat these calculations for the second endpoint of the spring.
Once we've obtained the positions and velocities of the spring endpoints, we compute
the spring-damper force in the same manner as in the rope example. The resulting spring
forces are aggregated in the vSprings property of each object. Note that if the spring
endpoint reference is a fixed point in space, we do not aggregate the force on that fixed
Since the Object s are rigid bodies here, we now have to compute the moment due to
the spring force acting on each object. You must do this so that when we integrate the
equations of motion, the objects rotate properly.
For the Object connected to End1 of the current spring, the following lines compute
the moment:
Fo = VRotate2D(-Objects[j].fOrientation, F);
r = Springs[i];
M = r^Fo;
Objects[j].vMSprings += M;
Fo is a vector representing the spring force computed earlier on the current Object in
the current Object 's local, body-fixed coordinate system. The line:
Fo = VRotate2D(-Objects[j].fOrientation, F);
transforms F from global to local coordinates of the current Object , Object[j] .
r is set to the local, body-fixed coordinates of the spring attachment point for the current
Object , and we compute the resulting moment by taking the vector cross product of r
with Fo . The result is stored in the vector variable M , which gets aggregated in the Object
property vMSprings . We then perform these same sorts of calculations for the Object
connected to the other end of the spring.
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