Wikipedia:Reference desk/Archives/Miscellaneous/2011 March 15

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March 15

Question about missile tracking

I have sort of a "where's the article that..." question; if this isn't the appropriate place to put it, please let me know.

I'm designing a computer game where one of the enemies tries to ram your player. Initially I moved the enemy directly toward your current position at each frame, but that doesn't work so well. The thing is, I remember seeing an article on Wikipedia about the history of anti-aircraft missiles, and they talked about this exact problem. The article said something to the effect of "the earliest air-to-air missiles tried to keep the enemy directly in front of the nose cone, but these performed poorly. Later designs took into account the enemy's motion..." and there was an explanation having something to do with the plane's velocity and an arc around the missile, but I'm afraid I don't remember much of it.

I've been trying to find this article again so that I can adapt that method for my own use, but my Google-fu has turned up nothing. I've been in Missile, Air-to-air missile, Guidance system, History of rockets, and some other usual suspects, but have been unable to find the article I was looking for. Does anyone know where this segment might be? Has it been deleted?

Again, I realize this is different from the usual format of these questions, so please tell me if this is not the appropriate place. Sloverlord (talk) 03:40, 15 March 2011 (UTC)

Have you tried following any threads from Trajectory optimization which shows some promise, though does not answer your question directly... --Jayron32 04:35, 15 March 2011 (UTC)

Or, perhaps Mathematical discussion of rangekeeping which is actually a kinda crappy article (by Wikipedia standards for naming and organization), but it does discuss the concept of "rangekeeping" which appears to be the idea of aiming a projectile where an object is going to be, rather than where it is; this seems to what you are looking for. Rangekeeper also discusses this as well. --Jayron32 04:38, 15 March 2011 (UTC)

Current location of missile--------A-----------B-----------C---->

A = position after 1 hour

B = position after 2 hours

C = position after 3 hours

You form a predicted trajectory of the missile, and then tell the intercepting missile to strike at the nearest point on that trajectory that it can make it to on time.

This is a really basic explanation. You may want to tell your enemies to head towards a few spaces in front of your player.AerobicFox (talk) 05:00, 15 March 2011 (UTC)

The simplest tracking method is to assume that the target will continue at the current speed and direction, ignoring any acceleration. Here's a rough method:

1) Find the difference in the target's X, Y, and (if 3D) Z coords in the minimum time interval over which movement can be detected. Let's say that's a tenth of a second, and we will call the target's movements in that time (ΔX,ΔY,ΔZ), and it's current position (X,Y,Z).

2) Calculate how long it would take to reach the target, if it remained stationary, where it is now, and your "missile" went straight toward it. This is simply the distance to the target divided by the speed of the "missile". Let's say that's 10 seconds.

3) Now calculate where the target will be in 10 seconds. Since that's 100 times the interval used in step 1, that would be (X+100ΔX,Y+100ΔY,Z+100ΔZ).

4) Repeat the calculate in step 2, but now use the location where the target will be in 10 seconds, instead of it's current location. This will give you a new time-to-target.

5) Now calculate where the target will be in that time-to-target. Continue to repeat these 2 steps until the intersection location rounds to the same coords in 2 consecutive steps.

6) Now you have your target intersection location. Head straight towards that.

7) Redo the calcs at each step of movement of the target, so your "missile" will adjust it's trajectory as the target changes speed and direction.

A more sophisticated model would take acceleration into account directly, rather than correcting for it, as we do here, but hopefully this is sufficient for your needs. To decrease the skill level, have the "missile" recalculate the trajectory less often. StuRat (talk) 05:43, 15 March 2011 (UTC)

I recall reading that a very simple geometric algorithmn was used during the air battles of WW2 (see Battle_of_Britain#The_Dowding_System - scroll down) to "vector" the fighter pilots to the enenmy. Cannot remember details, sorry. 92.15.5.217 (talk) 14:27, 15 March 2011 (UTC)

The mathematical name for this type of problem is pursuit-evasion problems. Our article mentions applications to missile guidance. Also see a specific example for discrete space, the Homicidal_chauffeur_problem. Here is a nice paper on the classic continuous dog-and-rabbit problem. [1]. SemanticMantis (talk) 17:41, 15 March 2011 (UTC)

I would ask this question on the Mathematics board - they should be able to solve it. You have two vectors and have to solve the triangle to find out where they cross, with the complication of different speeds being involved. The angle and speed of the target are known. The speed of the missile is known, but the angle is unknown. Its too late at night for me to have a stab at it. This http://www.gamedev.net/topic/451184-enemy-path-prediction-and-sieging/ may be of interest. Was the article you were looking for Pursuit guidance or Proportional navigation? 92.29.118.158 (talk) 00:35, 16 March 2011 (UTC)

A lot of good suggestions all around. Proportional navigation was in fact the one I had been looking for, although many of the other things mentioned seem like they would be helpful too. SemanticMantis' papers might be a little overkill for a chintzy computer game, but they look fascinating nonetheless and I'm going to read them anyway when I have a free moment. Thanks for your help, everyone! Sloverlord (talk) 01:35, 17 March 2011 (UTC)

Chinese Mahjong Rules

I was playing Chinese Mahjong with friends, and we encountered an odd occurrence. I had a concealed kong (4 identical) and I revealed it to interrupt an opponent's turn. I drew make up tile and had to discard, but I was placed in an unusual situation where I seemed unable to complete my hand. I had the kong on the table, plus two other revealed sets. That makes 10 tiles, plus I had 4 in my hand, for a total of 14. The issue was that I had two pairs/eyes in my hand and I had to discard a tile to have 13 again. However, I would have had two matching tiles in my hand plus one that didn't, and I can't have a winning hand with two pairs. What rules did we miss so that someone can have three tiles in hand and successfully complete the hand without having two pairs?130.127.108.23 (talk) 06:11, 15 March 2011 (UTC)

By the the "rules" I know, you put the kong up as soon as you get it (with one tile turned down) so this would be at the start of the game or in your own turn. You then pick up an extra tile. Hence you keep the right number of tiles in your hand. However, if your rules allow you to interrupt an opponent you would have to pick up the extra tile and, since it is now your turn, pick up yet another tile as the normal part of your turn. However, I do not see why you should be able to interrupt the flow of play simply to declare a concealed kong. I can see though that you may be allowed to keep a concealed kong undisclosed though then you'd only be able to disclose it in your turn. But I am not a lawyer! Thincat (talk) 17:30, 15 March 2011 (UTC)

I now see Mahjong says "At any point during a players turn, if they have all four matching tiles in their hand, they may declare the Kong". Thincat (talk) 17:34, 15 March 2011 (UTC)

With any kong you lay out, get the extra tile from the other end of the stack. Graeme Bartlett (talk) 23:16, 18 March 2011 (UTC)

The subject of questions is mass communication

What is umbrella lead? What is dictation speed bulletin? —Preceding unsigned comment added by 117.211.76.2 (talk) 07:19, 15 March 2011 (UTC)

An 'umbrella lead' is a style of opening an article in a newspaper or magazine report. It may be characterised by inclusion of a number of different factors relating to a single story. See here, page 10. 12, section 5, the relevant section starts on page ten. Thanks 92. I can only guess at the second part of your question so I will leave it. Richard Avery (talk) 08:34, 15 March 2011 (UTC)

I've just downloaded that paper for future study and reference - thought I'd mention it to underline that answers here can be of interest to others besides the OP. Thanks, Richard! {The poster formerly known as 87.81.230.195} 90.201.110.135 (talk) 16:31, 15 March 2011 (UTC)

I make it page 12 or page 19. 92.15.5.217 (talk) 14:31, 15 March 2011 (UTC)

When I trained as a secretary, the BBC World Service would broadcast certain of its news bulletins at "dictation speed" of about 120 words per minute, about half the speed of normal news bulletins. This was so the bulletins could be taken down verbatim in shorthand by journalists (and budding shorthand-typists such as myself!). In the days before recorders, of course. I can only presume that this is what the OP is referring to. --TammyMoet (talk) 10:00, 15 March 2011 (UTC)

Identify this anime

I've been searching high and low in an effort to identify which anime (it looks like both have been taken from the same series) these pictures are from. Tineye didn't help one bit, and googling random phrases yielded nothing either. Can anyone help me please? Here is the url: http://copytaste.com/y293 Thanks in advance!! 223.177.244.236 (talk) 09:44, 15 March 2011 (UTC)

Might be La Clef: Meikyuu No Kagi. Googling this it appears to be cover art from a song from about 2007 (the cover art of the CD is spot on) by Kotoko. It might be a manga that was never produced? See: http://www.pireze.org/blog/?p=366 (I'm not affiliated with link). --Quartermaster (talk) 13:25, 15 March 2011 (UTC)

Added notes: Alternate transcriptions include La Clef: Meikyu No Kagi (only one "u"). The anime/manga itself is most likely just Meikyu No Kagi. Might also be Zettai Meikyuu Grimm. My google fu is strong. My Nihongo Fu is not. --Quartermaster (talk) 13:40, 15 March 2011 (UTC)

Final answer is: Ikoku Meiro no Kurowaaze --Quartermaster (talk) 15:16, 15 March 2011 (UTC)

This is the official site. The anime will be on air in this summer in Japan Oda Mari (talk) 19:10, 15 March 2011 (UTC)

I wonder why Wikipedia doesn't have an article on it. Isn't it noticeable enough? 223.177.247.82 (talk) 09:19, 16 March 2011 (UTC)

British Gas ad place names

There's an ad on UK radio for British Gas just now in which people read out a bunch of place names like Rochester, Yaddlethorpe, Abergavenny, Clenchwarton, and so on. However, try as I might, I can't seem to catch them all and it's driving me nuts. Can anyone tell me the full list? Cheers TheRetroGuy (talk) 17:58, 15 March 2011 (UTC)

I've not heard the ad, but could it be a UK version of "I've Been Everywhere"? Rolf Harris did a version with UK placenames in 1963 (youtube has it). -- Finlay McWalter ☻ Talk 18:09, 15 March 2011 (UTC)

Interesting song. I don't think the ad is based on it as they're just random place names said (I think) in regional accents with no particular structure to them. However, having just listened to this song, I do recall a series of television ads for a building society during the early 1990s based around it. I thinmk that was for Abbey National or something like that. TheRetroGuy (talk) 18:42, 15 March 2011 (UTC)

Actually, I stand corrected, the building society ad was for Nationwide (see here) TheRetroGuy (talk) 18:47, 15 March 2011 (UTC)

Yes, that's fundamentally the same song, with what must be their own, rather twee, lyrics (not the Rolf Harris ones). All the versions of "I've Been Everywhere" are just random recitations of places in their respective countries that rhyme (but in an order with no reason). Note, by way of trivia, that there's only one Letsby Avenue in Britain - it's a road in the former Sheffield City Airport & Heliport (where the police station was) map. -- Finlay McWalter ☻ Talk 20:37, 15 March 2011 (UTC)

I can't believe there is actually a Letsby Avenue, and with a police station on it as well. :) It puts me in mind of a joke I heard years ago about the policeman who lives at 999 Letsby Avenue, though I can't remember the details of it now. TheRetroGuy (talk) 21:57, 15 March 2011 (UTC)

The street (which is really a private service road) was clearly named (in a fit of bureaucratic whimsy) after the joke. -- Finlay McWalter ☻ Talk 23:02, 15 March 2011 (UTC)

Howeever, the song "I've been Everywhere" is nothing to do with said advert. I wonder if contacting the company will bear fruition? I might be unemployed, but even I draw the line at sitting next to the radio all day trying to catch the town names to answer a Wikipedia question! --TammyMoet (talk) 10:57, 16 March 2011 (UTC)

Might do that. I wouldn't want anyone to spend hours listening to the radio just on the off chance the ad might come on, more if someone happened to hear it and could catch the names. :) TheRetroGuy (talk) 21:07, 16 March 2011 (UTC)

I've been looking into this just now, but their contact us page appears to want me to become a customer and limits enquiries to those concerning their services, paying bills, etc. As I've no desire to switch from my current provider then I think the above option could be out. The main problem with the ad is that each place name is spoken in the regional accent corresponding to the location, and I find many of them are difficult to understand. A little more information on the ad. The music played in the background is from The Universal by Blur. TheRetroGuy (talk) 22:49, 18 March 2011 (UTC)

Lykke Li

HEEEEEEEELP!
Lykke Li is 25 years old. How do I change the age? B-Machine (talk) 19:41, 15 March 2011 (UTC)

Usually use the Help Desk for editing related questions, or perhaps the Entertainment Reference Desk for this kind of thing... but our article indicates her birthday is on the 18th. It's not yet the 18th. Do you have a citation for the different birthdate? If so we can use it update, but you should provide that citation as there's a lot of shenanigans related to date changes of celebrities. Shadowjams (talk) 20:05, 15 March 2011 (UTC)

"...her birthday is on the 18th. It's not yet the 18th." Oops. B-Machine (talk) 00:54, 16 March 2011 (UTC)

The infobox uses a "birth date and age" template within it, so the age should update automatically on the 18th. DuncanHill (talk) 01:17, 16 March 2011 (UTC)

I added the section title for easier reference. Cuddlyable3 (talk) 09:59, 16 March 2011 (UTC)

An adversion to "affecting" others

Hi. Why do some people seem to be very cautious to prevent themselves from affecting other people in some way? This is not strictly an opinion-based question as many people state at least implicitly that they keep from influencing or affecting other individuals either in a negative or positive way. Is this same tendency linked to one shying away from helping others? Thanks. ~AH1^(TCU) 23:25, 15 March 2011 (UTC)

Noninterference directive ? :-) StuRat (talk) 01:15, 16 March 2011 (UTC)

Please could you give some specific examples of what you mean? 92.29.117.90 (talk) 12:56, 16 March 2011 (UTC)

I'm not certain that there are any specifics, as this is mostly a general description. For example when something is not done for the reason that it would "affect" somebody especially in the case when you know that other person. ~AH1^(TCU) 17:12, 16 March 2011 (UTC)

Sounds like good old-fashioned consideration for other people for me. Ah the days when we weren't quite as selfish and thought of other people before ourselves! --TammyMoet (talk) 18:39, 16 March 2011 (UTC)

I agree with TammyMoet, it sounds like not being a maniac; having some consideration for other's feelings; not being a self-centred ogre. 92.24.180.239 (talk) 14:51, 17 March 2011 (UTC)

That would be altruism, however, I think the OP means any sort of interference, good or bad. A sort of personal isolationism? Grandiose (me, talk, contribs) 18:55, 16 March 2011 (UTC)

A more specific example would help. You could change peoples names. 92.24.186.176 (talk) 00:04, 17 March 2011 (UTC)