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exercises.md
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@ -174,7 +174,8 @@ Bear in mind the main purpose of this quiz is for you to test your understanding
71. You've probably seen a game freeze and become unresponsive and then you likely heard audio get stuck too in a weird way: a short piece of sound is just played over and over like a broken vinyl record. Why does this happen? How and WHY is audio typically implemented here?
72. Mention at least one advantage and one disadvantage of using [matrices](matrix.md) to represent transformations in 3D engines.
73. A nudist is lying completely horizontally on a beach with his dick pointed up towards the sky when a hot 16 year old jailbait walks by and he gets an erection, the sun is shining under the angle 20 degrees (measured from horizon), his dick is now pointed up completely vertically and casts a shadow that reaches up to his feet, i.e. the shadow (completely horizontal) has a length of 60 cm. How long is his dick (with erection)?
74. Did you enjoy this quiz?
74. If your computer resides in a private network that's connected to the Internet through a router that performs network address translation ([NAT](nat.md), common with many ISPs), why you typically cannot host a server that would be publicly accessible from the outside [Internet](internet.md)? I.e. explain how NAT works and say what's preventing outside computers from reaching your server behind it. How can you make your server work even behind NAT?
75. Did you enjoy this quiz?
### Answers
@ -251,7 +252,8 @@ Bear in mind the main purpose of this quiz is for you to test your understanding
71. Continuous audio is normally implemented with a [circular buffer](circular_buffer.md) -- that is we have a buffer of audio samples of certain size *N* and that is being played over and over, with the play head going from start to finish and then back to start again; the program has to keep updating this buffer regularly to fill it with new samples to play and it has to keep ahead of the play head. Circular buffer is nice because we don't have to shift it as a whole (which would require moving a lot of values in memory), the only thing that is moving is the play head, that's why it's used as opposed to e.g. a queue. When a game freezes, it stops operating correctly and it stops updating the audio buffer, so whatever is in it will just be played over and over in a loop.
72. Main advantage is that a matrix can hold any combination of transformations and that applying all the transformations is then simply performed by performing a single matrix multiplication which additionally may be implemented with quite fast matrix multiplication algorithms. Not only can a matrix represent for example the whole translation+rotation+scale transformation of a single object, it can hold any number of such transformations performed in any order so that we can for example precompute a matrix that will perform world transformation, camera space transformation and view space transformations all at once! That's very fast. Disadvantages of matrices may be that they can only hold affine transformations (i.e. they can't represent ANY transformation whatsoever), it may also be a bit harder to extract back the parameters of the transformation from a matrix (though it can be done) etc. Also in case of some extreme memory limitations matrices may take up more space than would be strictly needed.
73. From the right triangle: *dick_legth / shadow_length = tan(20 degrees) => dick_legth = tan(20 degrees) * shadow_length ~= 21.83 cm*.
74. yes
74. Behind NAT you're in a private network, computers inside it have no public addresses (their IP addresses are in the private range and potentially same as addresses of computers in other private networks), only the router has a public IP address that's unique within the Internet, i.e. from Internet's point of view there is only one device connected -- your router. Computers behind this router are invisible, so no one can connect to the server that's behind it. The possibility of you having a two way communication from within this private network with the outside Internet is enabled by the router who communicates for you when you ask for it, i.e. when you (from inside the private network) initiate the connection -- the router then creates the connection for you and talks to the outside world for you (translating your private address to its public address, hence *network address translation*). But no one can initiate communication from the outside, the router wouldn't know to whom he wants to speak. This can be solved e.g. by port forwarding (setting some default computer to which communication from outside will be redirected) or tunneling (keeping a constant connection with some outside proxy server that's listening to requests and resending them).
75. yes
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