2770: Tapetum Lucidum

Explain xkcd: It's 'cause you're dumb.
Revision as of 17:56, 26 September 2023 by (talk) (Explanation: the reflective layer is behind the retina, and reflects the light that passed through the retina the first time)
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Tapetum Lucidum
Using a reflective wall in a game to give one shot two chances to hit is called a double-tapetum lucidum.
Title text: Using a reflective wall in a game to give one shot two chances to hit is called a double-tapetum lucidum.


Bill Nye, perhaps best known for his children's educational series Bill Nye the Science Guy, wearing the same lab coat as in 200: Bill Nye, beats an unseen player (presumably Randall) in an online multiplayer game resembling XPilot, in which players pilot spaceships using simulated rocket physics and attempt to shoot and kill each other. During a laser battle, Bill Nye provides a scientific explanation for the tapetum lucidum, the layer behind the retina of a cat's eye. He explains that the layer reflects back some of the light that passes through the retina, giving it a second chance to hit the retina again. This allows a cat's eye to capture more light than it otherwise would, and thus improves their night vision. It's also why cat's eyes appear to glow in the dark.

At the same time, Bill Nye's battle tactic in the online game perfectly analogizes the point he is making. His spaceship is firing energy pulses into the path of an approaching ship in an attempt to destroy it. Due to the difficulty of hitting a small, fast-moving target, it's likely that most or all of these shots will miss. However, because Bill Nye is firing at a reflective wall, each shot that misses bounces back into the path of the opponent's ship, giving it a second chance to hit the target and effectively doubling the density of the firepower. With double the number of shots to avoid, the opponent's ship is hit and explodes. This explanation is similar to how Bill Nye would explain scientific concepts by using analogous demonstrations of other things.

In the analogy, the weapon shots fired by Bill Nye's ship are the light photons entering the cat's eye, the reflective wall is the tapetum lucidum, and the opponent's ship is a retinal cell. Destroying the opponent's ship with a shot is analogous to a light photon being absorbed by the cat's retina (and therefore seen). If the reflective wall hadn't been there, the ship might have survived, which means the retina would never have seen that photon.

Randall presumably considers this "extra infuriating" because Bill Nye is showing both his scientific knowledge in some other field and his gaming prowess simultaneously, while he lacks the skill even to win the game normally.

The title text is a pun that refers to "tapetum lucidum" and uses "double tap" in the way that online games, memes, and films refer to shooting something twice in rapid succession to ensure its demise. This phrase is used in the film Zombieland, and is the subtitle of the 2019 "Zombieland: Double Tap" sequel.


[Two triangles with two long sides of equal length (isosceles triangles) but with the short side curving a bit in, are moving inside a black structure, their movement indicated with three and two curved lines below the left and above the right triangle. Both triangles shoots green lasers out of their sharp tip, indicating they represents space ships. They are flying inside a black structure, maybe a maze. There is nothing above the left ship. but below the ships are a black segment making a triangle in the lower left corner. The right ship is inside an opening created by this triangle below it, and another black triangle above it, that centers on the middle of the right side of the panel. The left ship has shot three green laser beams, one of them hits inside the opening on the right triangle and the beam bounces off this wall. But it is not close to the right ship. The right ship has only fires one green laser, which bounces off the left wall also far from the left ship. A voice emanates from the left ship, via a star burst at the top corner. And there are sounds because of the shots it has fired.]
Left ship: Cats have a shiny layer behind their retina called the tapetum lucidum.
Laser shots: Pew pew pew
[Same setting but the scene has panned a bit to the left and up so less of the black walls can be seen. The left ship has moved closer to the right wall and has turned so it's sharp tip points almost straight down, still with three curved lines to indicate movement, probably turning movement. It again fires green lasers at the other ship, four this time, with sounds coming from the shooting. The right spaceship is accelerating forward as indicated with three wavy lines behind it's short side moving in to the line of fire. It seems as though all four lasers might miss it, but one of those that already has passed it, is being reflected up against it from below the left black wall. Again a voice emanates from the left ship via a starburst in the left corner:]
Left ship: After light passes through the retina, this layer reflects it back through a second time.
Laser shots: Pew pew
[Same setting but the scene has again panned a bit to show a different segments of the black walls. The ship originally to the left is now above and a bit to the right of where the right ship originally was, and it is even closer to the right wall. It is not firing any more shots, because the reflected shot from the previous panel has hit the right ship which explodes in large green cloud with the black pieces of the ship inside it, and a huge sound. The tail of the laser shot that hits it can be seen entering the explosion. Three other laser shots from before are still moving down, but might all have missed the ship. There are no movement lines now but again the voice emanates from the left ship via a starburst in the left corner:]
Left ship: This extra bounce gives photons another chance to interact with the retinal cells...
Explosion: Boom!
[A man with short black hair wearing a labcoat is sitting in an office chair typing on his computer while speaking. Below him is a frame with a caption, from which it becomes clear that the man, and the owner of the voice from the left ship, is Bill Nye.]
Bill Nye: ...Improving their night vision! Isn't science cool?
Caption: There's something extra infuriating about losing online games to Bill Nye.

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Added possibility of XPilot as inspiration, as a different branch of "triangular ships in 2D worlds" from Asteroids, which is a single-player PvE with no static walls or rebounding shots. The shots from xpilot ships can, if the map is set to do so, bounce off walls. Or be distorted by gravity, so aren't lasers. (Or maybe they can be, but it's been decades since I played it and development moves on!) But I'm sure the ecosystem of clone-games arising from both these inspirations (and other predecessors to both/between their respective releases) is nigh on uncatalogued so who knows if Randall's depicting some actual specific release or just a memetic interloper that just has the right narrative features for this particular comic's "message". 12:56, 1 May 2023 (UTC)

Lasers are distorted by gravity. If we are talking about black holes, at least. -- Hkmaly (talk) 20:54, 1 May 2023 (UTC)
(Technically lasers aren't distorted. The space they pass through is. ;) )
Not as much as physical things. I too was an XPilot player/hoster, a few decades ago now. Ah, nostalgia...
The "bullets" could be fired into a cluster of gravity-blocks and whip around like mad until they timed-out or managed to escape from whatever 'random' side their chaotic paths led them to. Great fun to include such features when editing maps (adding detailing and fine-tuning with a text editor to the conversion of some interesting image to the map format, often obtained via netpbm piping and a bit of other automatic/manual editing), especially with shields a disabled feature so as to make it a risky proposition to enter combat in close proximity (and a rather desperate escape-route, if piled up between walls).
Or set your map with huge time-to-die (bullets, drifting mines, etc), a high number of wall bounces allowed and skew the pickup populations with many multiple spread-shooters (front and rear) and let the players create a 'bullet soup' situation oh so easily! 22:40, 1 May 2023 (UTC)
Not sure about xpilot, but these must be bullets, or burst of say plasma/energised particles, as laser beams would go from ship to wall of target (almost) instantaneously.RIIW - Ponder it (talk) 16:19, 2 May 2023 (UTC)

Come on, Randall- Tapetum Ludum was right there! SaturnVI (talk) 11:49, 2 May 2023 (UTC)

https://www.ign.com/articles/2019/10/18/zombieland-rules-full-list-columbus-official-and-deleted-scenes This is one of many possible references for the "double tap" Zombieland quote and annotation noting its use as title in the sequel. (talk)

"Double-tap", by that name but in various subtly different forms (that yet still count upon two shots being made from a single aiming), goes back at least to SOE training manuals in WW2. Zombieland clearly just quotes common parlance, just like Charlie's Angels: Full Throttle wouldn't be considered the source of the phrase "Full Throttle". 19:42, 2 May 2023 (UTC)
I agree, I've known the term "Double Tap" since long before Zombieland came out, and I seriously question the idea that it's where it's best known, except among people young enough not to have heard it before. I don't think it was even used that much, it was just one of the rules he lists at the beginning. It only gained significance by being put in the title of the sequel. (And first thing I think of for "Full Throttle" is the 90s LucasArts adventure game) :) Out of interest I browsed the quotes on IMDb to find the quote to use as a citation, but nobody found it interesting enough to submit (although EXPLAINING the rule is in there). NiceGuy1 (talk) 03:43, 6 May 2023 (UTC)

is making the transcript this verbose productive? is it supposed to be a guide to fully recreate the image if it's not loading on your end? -- 10:57, 2 May 2023 (UTC)

Yes, it should be this verbose. It is meant for the visually impaired. 14:16, 2 May 2023 (UTC)
I disagree. If I was blind, this much text would annoy me, especially if I found out how simple the comic actually is. For example, it's ridiculous to specify that the ships are isosceles triangles, "triangles" is enough. "A video game field with two triangular ships, one firing green lasers bouncing off walls at the other" should be roughly sufficient to start with, come on! There's no reason to get so overly specific, those of us who can see aren't looking at all those details, why should the visually impaired be forced to? NiceGuy1 (talk) 03:43, 6 May 2023 (UTC)
You must be fun at parties 00:29, 12 May 2023 (UTC)
Yup! My love of keeping it simple and straightforward comes in handy! :) NiceGuy1 (talk) 04:35, 27 May 2023 (UTC)
Agree completely. Will simplify. Jkshapiro (talk) 16:23, 14 April 2024 (UTC)

Too much spam was found on the explanation page. This makes it very difficult to tell which text is spam. ClassicalGames (talk) 08:27, 3 May 2023 (UTC)