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Surging UFO laser

Verified against the spelunky.ai port v0.1.13 on .

The surging UFO laser is a quirk of Spelunky Classic in which a UFO's laser falls in surges, stopping dead every seven steps.

A UFO fires its laser straight down at the player, and the laser gathers speed as it falls. It should stop gathering at 4 pixels a step, but instead it stops moving altogether and starts over, so every laser falls in surges: a run of quickening steps, a pause, another run.

What happens

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The player throws a stunned yeti under a UFO. The UFO fires, and its laser falls in surges, speeding up and stopping dead every seven steps, until it hits the yeti, which flies up into the UFO and blows it up [1].

Replays it in the game, where you can take over.Replays it in the game, to pause and scrub.
A UFO's laser falls in surges. The player throws a stunned yeti under a UFO. The UFO fires its laser, which falls in surges, speeding up and stopping dead every seven steps, and hits the yeti, which flies up into the UFO and blows it up.

Why

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A laser starts with no speed and a gravity of 0.6 [2]. Each step, it moves by its speed, adds the gravity, and checks the speed against 4:

y += yVel;
yVel += yAcc;
if (yVel > 4) yVel = 0;
objects/oLaser/step.gml:1-3, from spelunky.gmk, Spelunky Classic 1.1's source, © 2008, 2009 Derek Yu and Mossmouth, LLC (Spelunky User License v1.1b).

The check is a cap that sets the speed to 0 instead of 4 [3]. So the laser moves 0, 0.6, 1.2, 1.8, 2.4, 3.0 and 3.6 pixels on seven steps; then its speed would reach 4.2, goes back to 0, and the run starts over: about 12.6 pixels every 7 steps.

Reproducing it

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Every laser a UFO fires falls this way. In the replay of the clip, a laser fired on frame 1865 has a speed of 0 again on 1871 and 1878, and never above 4, before it hits the yeti on 1881 [4].

In the port

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The spelunky.ai port reproduces it exactly: it's in docs/quirks.md's list of the 1.1 source's bugs [3], and the oracle's fingerprint of the replay pins the laser's fall. No option turns it off.

FAQ

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Why does the UFO's laser move in jerks in Spelunky Classic?

Its speed grows by 0.6 pixels a step, and once it would pass 4, the code sets it to 0 instead of capping it. So the laser speeds up for six steps, stands still for one, and starts again.

How fast does a UFO's laser fall?

About 12.6 pixels every 7 steps, 1.8 pixels a step on average, and never more than 3.6 in a step. Held at 4, as was meant, it would cover more than twice that on a long fall.

Can you outrun a UFO's laser?

It only falls straight down, so stepping out from under the UFO is enough, and the surges give the player a little more time to do it.

See also

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Related

References

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  1. Clip quirks/ufo-laser-surges
  2. objects/oLaser/create.gml:3-4
  3. Quirk 1 (docs/quirks.md)
  4. Replay fidelity/replays/enemy-ufo-stunned-yeti-seed-14.txt

Source code

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