Daily Archives: June 29, 2015

Deja Vu Sim Crossing

deja vu sim crossing

Instant Replay

Here’s an interesting issue. When a boat sails into a new sim, it will often suddenly jump back twenty meters or so and then resail that distance.

To illustrate this point, here’s a small outtake from a video of the S4L 12M Qualifying Races.

deja vu2

click to enlarge

I’ve also posted a few frame captures below. In the first image (A), TEAM WYC2 approaches the Finish on a Run, with EUREKA slightly astern. Image B was captured slightly later, when WYC2‘s bow touched the line. You might think the race was over at that moment, but you’d be wrong. As shown in Image C, WYC2‘s boat suddenly jumped backwards a considerable distance; you can even see the boat’s wake in front of the bow. Image D shows the boat retracing it’s path, cutting the line a second time.

If you play the above video again, you’ll notice that Eureka also shows the same, strange behavior. As it nears the line, the boat sudddenly jumps back roughly 20m.

This is a significant effect that could easily disrupt a close race. Did WYC2 actually finish in Image B, or did it finish several seconds later in Image D? So… what’s going on here?

Funny Numbers

I’m pretty sure the problem above has nothing to do with the race line in Breadnut; I think it happens because the boats cross a sim boundary just before they finish.

Sailing across a sim boundary optimally involves a seamless hand-off of data from one simulator to the next, and that process requires efficient communication between server and client. This sounds straightforward but in practice it’s often imperfect, particularly when several vehicles are trying to cross the same border together. I sailed my first SL boat race back in 2006, and on that day every single boat crashed on the sim edge. 🙂 Although many things have improved in SL since then, sim crossings sadly remain a near death-defying challenge for many virtual skippers.

I’ve been sailing the Trudeau 12 Meter quite a bit recently, and it turns out to have very funny behavior at sim borders. Let me show you.

In the figure below, I’m sailing a boat on a beam reach in 15 kn wind. I sailed West in Linkous while taking a snapshot of the display every second. As the boat moves west within the sim, the X- position coordinate on the interface correspondingly decreases.

Linkous numbers

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In the first image above, the boat is at Linkous (+19).  A few seconds later it should enter Van Daemo sim, but instead the interface reads Linkous (-2) and next reading is Linkous (-8). I assume these negative numbers mean the boat is already moving into the next sim, but the asset “handover” is not yet complete. Proof of this comes one second later, when the interface reads Van Daemo (+241), implying the boat is already 15 m into the sim.

This all makes a certain amount of sense, and is hardly worth talking about; however, what happens next totally confuses me. (Okay okay, I admit that’s not hard to do)

Anyway, as my boat sails further west In Van Daemo, the horizontal position coordinates should continue to decline. The first two frames shown below demonstrate exactly that, with X-positions of  (+235) then (+230). However, the third frame below is one second later, and it registers (+251)! In other words, the boat suddenly jumped back a full 21 m !! The boat continues from that point on it’s original heading and with most of the previous momentum. At average rates of speed, I’d guess that will cost a racer roughly 4 to 8 seconds overall every time it happens.

This is the same problem shown in the video at the top of this page.

Linkous numbers 2

click to enlarge

This sudden “jump back” effect is consistent across many grid locations, and I don’t think it is unique to any specific group of sim server candidates. So far I’ve only looked for it in 12 m boats, but I think the problem is likely far more widespread and server-related.

Having said that, let me also add a few more observations:

  1. The effect is present in Trudeau 12 m traveling under engine only.
  2. I don’t see the effect when walking across a sim border.
  3. I don’t see the effect when driving a small outboard across a sim boundary.
  4. I don’t see the effect when sailing Wildwind OP60.
  5. I do think I’ve seen the effect in some Qwest boats, but I haven’t looked carefully yet.

Chaos Mandelbrot (LCC Admiral and SL-Pundit-In-Residence) tells me this issue of ‘bounce back’ after sim crossing is already widely known and it’s been prevalent across the grid for several months. Nonetheless, I can’t find any good discussion of the problem, and I’d love to hear from someone who can explain what’s going on here. 🙂

DilSpi 2