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• Chipmunk Physics or Box2D for C++ 2D GameEngine ?

  - by Mr.Gando

  Hello, I'm developing what it's turning into a "cross-platform" 2D Game Engine, my initial platform target is iPhone OS, but could move on to Android or even some console like the PSP, or Nintendo DS, I want to keep my options open. My engine is developed in C++, and have been reading a lot about Box2D and Chipmunk but still I can't decide which one to use as my Physics Middleware. Chipmunk appears to have been made to be embedded easily, and Box2D seems to be widely used. Chipmunk is C , and Box2D is C++, but I've heard the API's of Box2D are much worse than chipmunk's API's. For now I will be using the engine shape creation and collision detection features for irregular polygons (not concave). I value: 1) Good API's 2) Easy to integrate. 3) Portability. And of course if you notice anything else, I would love to hear it. Which one do you think that would fit my needs better ?

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• Is writing to a socket an arbitrary limitation of the sendfile() syscall?

  - by Sufian

  Prelude sendfile() is an extremely useful syscall for two reasons: First, it's less code than a read()/write() (or recv()/send() if you prefer that jive) loop. Second, it's faster (less syscalls, implementation may copy between devices without buffer, etc...) than the aforementioned methods. Less code. More efficient. Awesome. In UNIX, everything is (mostly) a file. This is the ugly territory from the collision of platonic theory and real-world practice. I understand that sockets are fundamentally different than files residing on some device. I haven't dug through the sources of Linux/*BSD/Darwin/whatever OS implements sendfile() to know why this specific syscall is restricted to writing to sockets (specifically, streaming sockets). I just want to know... Question What is limiting sendfile() from allowing the destination file descriptor to be something besides a socket (like a disk file, or a pipe)?

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• How come ftp protocol produces transmission errors sometimes if the data is using TCP, which is checksummed?

  - by Cray

  Every once in a while, downloading (especially large) files through ftp will produce errors. I am guessing that's also partly the reason why all major sites are publishing external checksums along with their downloads. How is this possible if ftp goes through TCP, which has checksum inbuilt and resends data if it is transmitted corruptly? One could argue that this is due to the short length of the CRC in the TCP protocol (which is 16bit I think, or something like that), and the collisions are simply happening too often. but 1) for this to be true, not only must there be a CRC collision, but also the random network error must modify both the CRC in the packet, and the packet itself so that the CRC will be valid for the new packet... Even with 16 bitCRC, is that so likely? 2) There are seemingly not many errors in, say, browsing the web which also goes through TCPIP.

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• Making my XNA sprite jump properly

  - by Matthew Morgan

  I have been having great trouble getting my sprite to jump. So far I have a section of code which with a single tap of "W" will send the sprite in a constant velocity upwards. I need to be able to make my sprite come back down to the ground a certain time or height after begining the jump. There is also a constant pull of velocity 2 on the sprite to simulate some kind of gravity. // Walking = true when there is collision detected between the sprite and ground if (Walking == true) if (keystate.IsKeyDown(Keys.W)) { Jumping = true; } if (Jumping == true) { spritePosition.Y -= 10; } Any ideas and help would be appreciated but I'd prefer a modified version of my code posted if at all possible.

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• How do I find hash value of a 3D vector ?

  - by brainydexter

  I am trying to perform broad-phase collision detection with a fixed-grid size approach. Thus, for each entity's position: (x,y,z) (each of type float), I need to find which cell does the entity lie in. I then intend to store all the cells in a hash-table and then iterate through to report (if any) collisions. So, here is what I am doing: Grid-cell's position: (int type) (Gx, Gy, Gz) = (x / M, y / M, z / M) where M is the size of the grid. Once, I have a cell, I'd like to add it to a hash-table with its key being a unique hash based on (Gx, Gy, Gz) and the value being the cell itself. Now, I cannot think of a good hash function and I need some help with that. Can someone please suggest me a good hash function? Thanks

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• Cygwin bash syntax error - but script run perfectly well in Ubuntu

  - by Michael Mao

  #!/bin/bash if test "$#" == "4"; then echo "$*"; else echo "args-error" >&2; fi; This little code snippet troubles me a lot when I tried to run it on both Ubuntu and Cygwin. Ubuntu runs bash version 4.0+ whereas Cygwin runs 3.2.49; But I reckon version collision shall not be the cause of this, this code runs well under fedora 10 which is also using bash version 3.+ So basically I am wondering if there is a way to code my script once and for all so there are not to have this awful issue later on. Many thanks in advance.

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• When are you truly forced to use UUID as part of the design?

  - by Pyrolistical

  I don't really see the point of UUID. I know the probability of a collision is effectively nil, but effectively nil is not even close to impossible. Can somebody give an example where you have no choice but to use UUID? From all the uses I've seen, I can see an alternative design without UUID. Sure the design might be slightly more complicated, but at least it doesn't have a non-zero probability of failure. UUID smells like global variables to me. There are many ways global variables make for simpler design, but its just lazy design.

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• PHP and Enums

  - by Henrik Paul

  I know that PHP doesn't have native Enumerations. But I have become accustomed to them from the Java world. I would love to use enums as a way to give predefined values which IDEs' auto completion features could understand. Constants do the trick, but there's the namespace collision problem and (or actually because) they're global. Arrays don't have the namespace problem, but they're too vague, they can be overwritten at runtime and IDEs rarely (never?) know how to autofill their keys. Are there any solutions/workarounds you commonly use? Does anyone recall whether the PHP guys have had any thoughts or decisions around enums?

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• Amazon S3 collisions with heroku and paperclip

  - by poseid

  I have an app on my localhost for development and an app for testing on heroku. Image upload with localhost and paperclip always works. However, doing the same experiment with image upload on my heroku app, the app hangs... and the upload seems to be going on forever. I suspect that there is a collision going on. What is needed to get but uploads working? Or do I need to use different buckets for each environment?

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• Scale an image every time it collide with another

  - by jean mayot

  here is my code : -(void)collision{ if(CGRectIntersectsRect(imageView.frame,centre.frame)){ imageView.alpha=0; [UIView beginAnimations:nil context:nil]; [UIView setAnimationDuration:1.0f]; centre.transform=CGAffineTransformMakeScale(1.3, 1.3); [UIView commitAnimations]; } } When imageView collide with centre centre become bigger. my problem is that when "imageView" collide with "centre" a second time the animation doesn't work. I want to make centre bigger and bigger and bigger every time imageView collide with center, but it become bigger just one time . Sorry for my english I'm french :/ How can I solve this please ?

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• Is it okay to truncate a SHA256 hash to 128 bits?

  - by Sunny Hirai

  MD5 and SHA-1 hashes have weaknesses against collision attacks. SHA256 does not but it outputs 256 bits. Can I safely take the first or last 128 bits and use that as the hash? I know it will be weaker (because it has less bits) but otherwise will it work? Basically I want to use this to uniquely identify files in a file system that might one day contain a trillion files. I'm aware of the birthday problem and a 128 bit hash should yield about a 1 in a trillion chance on a trillion files that there would be two different files with the same hash. I can live with those odds. What I can't live with is if somebody could easily, deliberately, insert a new file with the same hash and the same beginning characters of the file. I believe in MD5 and SHA1 this is possible.

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• Dynamic dispatch and inheritance in python

  - by Bill Zimmerman

  Hi, I'm trying to modify Guido's multimethod (dynamic dispatch code): http://www.artima.com/weblogs/viewpost.jsp?thread=101605 to handle inheritance and possibly out of order arguments. e.g. (inheritance problem) class A(object): pass class B(A): pass @multimethod(A,A) def foo(arg1,arg2): print 'works' foo(A(),A()) #works foo(A(),B()) #fails Is there a better way than iteratively checking for the super() of each item until one is found? e.g. (argument ordering problem) I was thinking of this from a collision detection standpoint. e.g. foo(Car(),Truck()) and foo(Truck(), Car()) and should both trigger foo(Car,Truck) # Note: @multimethod(Truck,Car) will throw an exception if @multimethod(Car,Truck) was registered first? I'm looking specifically for an 'elegant' solution. I know that I could just brute force my way through all the possibilities, but I'm trying to avoid that. I just wanted to get some input/ideas before sitting down and pounding out a solution. Thanks

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• How secure are GUIDs in terms of predictability?

  - by ssg

  We're using .NET's Guid.NewGuid() to generate activation codes and API keys currently. I wonder if that poses a security problem since their algorithm is open. .NET Guid uses Win32 CoCreateGuid and I don't know it's internals (possibly MAC address + timestamp?). Can someone derive a second GUID out of the first one, or can he hit it with some smart guesses or is the randomness good enough so search space becomes too big? Generating random keys have the problem of collision, they need a double check before adding to a database. That's why we stuck with GUIDs but I'm unsure about their security for these purposes. Here are the 4 consecutive UUIDGEN outputs: c44dc549-5d92-4330-b451-b29a87848993 d56d4c8d-bfba-4b95-8332-e86d7f204c1c 63cdf958-9d5a-4b63-ae65-74e4237888ea 6fd09369-0fbd-456d-9c06-27fef4c8eca5 Here are 4 of them by Guid.NewGuid(): 0652b193-64c6-4c5e-ad06-9990e1ee3791 374b6313-34a0-4c28-b336-bb2ecd879d0f 3c5a345f-3865-4420-a62c-1cdfd2defed9 5b09d7dc-8546-4ccf-9c85-de0bf4f43bf0

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• Calculating terrain height in 3d-space

  - by Jonas B

  Hi I'm diving into 3d programming a bit and am currently learning by writing a procedural terrain generator that generates terrain based on a heightmap. I would also want to implement some physics and my first attempt at terrain collision was by simply checking the current position vs the heightmap. This however wont work well against small objects as you'd have to calculate the height by taking the heightdifference of the nearest vertices of the object and doing this every colision check is pretty slow. Beleive me I tried googling for it but there's simply so much crap and millions of blogs posting ripped-of newbie tutorials everywhere with basically no real information on the subject, I can't find anything that explains it or even names any generally used techniques. I'm not asking for code or a complete solution, but if anyone knows a particular technique good for calculating a high-res heightmap out of the already generated and smoothed terrain I would be very happy as I could look into it further when I know what I'm looking for. Thanks

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• jquery ui position fails with margin

  - by oliwel

  Hi, I am trying to use jQuery to position an inpage div (Context-Menu) relative to a clicked tablerow on a webpage. This works fine until I add an offset with either "left" or "margin-left" to the outer container. Using padding-left works.... The relevant code is: $('#supermenu').position({ my: "left top", at: "left top", of: $(event.target).closest('tr'), collision: "fit" }); The caller looks like: <tr onclick="getMenu(event)"><td>.... What happens: The left margin is added "twice" to the X Offset, so the Menu is not aligned to the left border of the table but somewhere right of it. The offset from the border is exactly the offset from the outer container. Anybody can shed some light? Oliver

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• How can I call python module inside versioned package folder?

  - by Yanhua

  I need write python codes which run inside a host application. The python codes should be deployed under a specific folder of the host application. I must put my entry python module under the root of the specific folder. And I want put all my other python codes and c/c++ dll under a sub folder, I prefer to name the sub folder like XXX-1.0, the number is the version of my python codes. The entry python module is just simple call a python module under the sub-folder. By this way different version python codes can be deployed together without collision. May I know it is possible or not? Thanks.

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• Eval IronPython Scripts during ASP.NET Web Request; Static Engine or Not

  - by Josh Pearce

  I would like to create an ASP.NET MVC web application which has extensible logic that does not require a re-build. I was thinking of creating a filter which had an instance of the IronPython engine. What I would like to know is: how much overhead is there in creating a new engine during each web request, and would it be a better idea to keep a static engine around? However, if I were to keep a single static engine around, what are the issues I might run into as far as locking and script scope? Is it possible to have multiple scopes in the same IropPython engine so I don't get variable collision and security issues between web requests?

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• XNA - Pong Clone - Reflecting ball when it hits a wall?

  - by toleero

  Hi guys, I'm trying to make the ball bounce off of the top and bottom 'Walls' of my UI when creating a 2D Pong Clone. This is my Game.cs public void CheckBallPosition() { if (ball.Position.Y == 0 || ball.Position.Y >= graphics.PreferredBackBufferHeight) ball.Move(true); else ball.Move(false); if (ball.Position.X < 0 || ball.Position.X >= graphics.PreferredBackBufferWidth) ball.Reset(); } At the moment I'm using this in my Ball.cs public void Move(bool IsCollidingWithWall) { if (IsCollidingWithWall) { Vector2 normal = new Vector2(0, 1); Direction = Vector2.Reflect(Direction,normal); this.Position += Direction; Console.WriteLine("WALL COLLISION"); } else this.Position += Direction; } It works, but I'm using a manually typed Normal and I want to know how to calculate the normal of the top and bottom parts of the screen?

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• Simple network gaming, client-server architecture planning.

  - by michal

  Hi, I'm coding simple game which I plan to make multiplayer (over the network) as my university project. I'm considering two scenarios for client-server communication: The physics (they're trivial! I should call it "collision tests" in fact :) ) are processed on server machine only. Therefore the communication looks like Client1->Server: Pressed "UP" Server->Clients: here you go, Client1 position is now [X,Y] Client2->Server: Pressed "fire" Server->Clients: Client1 hit Client2, make Client2 disappear! server receives the event and broadcasts it to all the other clients. Client1->Server: Pressed "UP" Server->Clients: Client1 pressed "UP", recalculate his position!! [Client1 receives this one as well!] Which one is better? Or maybe none of them? :)

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• create a dataset by using modulo division method

  - by ayoom

  create a dataset with 101 integers. Use the modulo division method of hashing to store the random data values into hash tables with table sizes of 7, 51, and 151. Use the linear probing and quadratic method of collision resolution. Print out the tables after the data values have been stored. Search for 10 different values in each of the three hash tables, counting the number of comparisons necessary. Print out the number of comparisons necessary in each case, in tabular form.

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• Parallel Processing Simulation in Javascript

  - by le_havre

  Hello, I'm new to JavaScript so forgive me for being a n00b. When there's intensive calculation required, it more than likely involves loops that are recursive or otherwise. Sometimes this may mean having am recursive loop that runs four functions and maybe each of those functions walks the entire DOM tree, read positions and do some math for collision detection or whatever. While the first function is walking the DOM tree, the next one will have to wait its for the first one to finish, and so forth. Instead of doing this, why not launch those loops-within-loops separately, outside the programs, and act on their calculations in another loop that runs slower because it isn't doing those calculations itself? Retarded or clever? Thanks in advance!

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• namespacing large javascript like jquery

  - by frenchie

  I have a very large javascript file: it's over 9,000 lines. The code looks like this: var GlobalVar1 = ""; var GlobalVar2 = null; function A() {...} function B(SomeParameter) {...} I'm using the google compiler and the global variables and functions get renamed a,b,c... and there's a good change that there might be some collision later with some outside code. What I want to do is have my code organized like the jquery library where everything is accessible with $. Is there a way to namespace my code so that everything is behind a # character for example. I'd like to have this to call my code: #.GlobalVar #.functionA(SomeParameter) How can I do this? Thanks.

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• How do you resolve the common collsision between type name and object name?

  - by Catskul

  Since the convention is to capitalize the first letter of public properties, the old c++ convention of initial capital for type names, and initial lowercase for non-type names does not prevent the classic name collision class FooManager { public BarManager BarManager { get; set; } // Feels very wrong. // Recommended naming convention? public int DoIt() { return Foo.Blarb + Foo.StaticBlarb; // 1st and 2nd Foo are two // different symbols } } class BarManager { public int Blarb { get; set; } public static int StaticBlarb { get; set; } } It seems to compile, but feels so wrong. Is there a recommend naming convention to avoid this?

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• How long can a hash left out in the open be considered safe?

  - by Xeoncross

  If I were to leave a SHA2 family hash out on my website - how long would it be considered safe? How long would I have before I could be sure that someone would find a collision for it and know what was hashed? I know that the amount of time would be based on the computational power of the one seeking to break it. It would also depend on the string length, but I'm curious just how secure hashes are. Since many of us run web-servers we constantly have to be prepared for the day when someone might make it all the way to the database which stores the user hashes. So, move the server security out of the way and then what do you have? This is a slightly theoretical area for many of the people I have talked with, so I would love to actually have some more information about average expectations for cracking.

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• Getting object coordinates from camera

  - by user566757

  I've implemented a camera in Java using a position vector and three direction vectors so I can use gluLookAt(); moving around in `ghost mode' works fine enough, but I want to add collision detection. I can't seem to figure out how to transform my position vector to coordinates in which OpenGL draws my objects. A rough sketch of my drawing loop is this: glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); glLoadIdentity(); camera.setView(); drawer.drawTheScene(); I'm at a loss of how to proceed; looking at the ModelView matrix between calls and my position vector, I haven't found any kind of correlation.

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