Remember that in the last episode, Freddie had replaced the standard
Interpolators with a set of Behaviors that maintained a separate simulation
clock using a wakeupOnElapsedTime(time_step) criterion, rather than the
wakeupOnElapsedFrame(0) used by the standard ones. As we left him, Freddie
was puzzling over the odd results: 1.) the introduction of Behaviors that
use the wakeupOnElapsedTime(time_step) criterion evidently stopped the
rendering thread's characteristic free-wheeling behavior as evidenced by the
recovery of a significant portion of the CPU capacity, and 2.) there was an
odd relationship between the specified time_step and the actual elapsed time
(as measured by the System clock) that the Behavior's processStimulus() got
executed.
As we look in on him now, he's laboriously stepping through values of the
scheduled time_step and logging the observed difference in System clock
readings between processStimulus() calls. [Zoom in on the scrap of paper in
his lap. Toward the bottom is the graph:]
(sorry about the graph, if you don't have exactly the font I used to
construct it)
46 + *******
|
Delta Time |
on the 31 + ********
System |
Clock |
(milliseconds) 15 +*******
|
|
+-------+--------+--------+--------+
15 31 46
time_step
"Hah!" says Freddie as he slaps his forehead. "It's as though the Behaviors
post their wake up times to a queue, and the scheduler only checks the queue
every 15 or 16 milliseconds." He turns to Margo, his incredibly beautiful
ash blonde Chief Technology Officer, and says, "This means that if we don't
pick the right time_step, we won't be able to synch our simulation clock to
the System clock. If we pick 31 milliseconds, we'll be almost exactly in
synch, and at 32 milliseconds, we'll slip by 33%"
Margo sniffed and said, "Well, that's an interesting little cosmetic tidbit
you've stumbled on there, but the real crux is the effect on the adaptive
time stepping routines in our IK iterators and the numerical integrators.
If we don't tiptoe around that 15 milllisecond strobe, our solutions and the
display will look like garbage, even if they're correct."
"I know you would have figured that out by yourself eventually, but we have
to get a strategy for dealing with this NOW. Get on JAVA3D-INTEREST and see
if anyone knows whether this is documented anywhere. Can we count on this
across platforms? Across versions? Will we have to test each system on the
fly before we can begin our simulation?"
Margo began to leave but turned and leaned over the cubicle wall. "And
another thing. You just got the averages over 100 frames. Redo it and get
the distributions. We have to know how regular this is. Don't worry about
the delays for garbage collection and system services. They'll probably
take much longer than 15 milliseconds. They'll be outliers."
"Yes'm."
The closing credits roll as Margo turns and walks back to the daycare center
adjoining her window office, and Freddie hunches over his keyboard to
compose...
Freddie Klingener
Brock Engineering
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