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• Client Side Prediction for a Look Vector

  - by Mike Sawayda

  So I am making a first person networked shooter. I am working on client-side prediction where I am predicting player position and look vectors client-side based on input messages received from the server. Right now I am only worried about the look vectors though. I am receiving the correct look vector from the server about 20 times per second and I am checking that against the look vector that I have client side. I want to interpolate the clients look vector towards the correct one that is server side over a period of time. Therefore no matter how far you are away from the servers look vector you will interpolate to it over the same amount of time. Ex. if you were 10 degrees off it would take the same amount of time as if you were 2 degrees off to be correctly lined up with the server copy. My code looks something like this but the problem is that the amount that you are changing the clients copy gets infinitesimally small so you will actually never reach the servers copy. This is because I am always calculating the difference and only moving by a percentage of that every frame. Does anyone have any suggestions on how to interpolate towards the servers copy correctly? if(rotationDiffY > ClientSideAttributes::minRotation) { if(serverRotY > clientRotY) { playerObjects[i]->collisionObject->rotation.y += (rotationDiffY * deltaTime); } else { playerObjects[i]->collisionObject->rotation.y -= (rotationDiffY deltaTime); } }

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• Acceleration Based Player Movement

  - by Mike Sawayda

  Ok, so I am making a first person shooter game and I am currently working on movement that looks and feels good. I want to incorporate acceleration based movement for the player so that he has to accelerate to max speed and decelerate to minimum speed. Acceleration will happen when you have the key pressed and deceleration will happen when you let go of that key. The problem is that there are some instances where you switch from moving forward to moving backward where no deceleration is needed because you could potentially be moving at double speed in the reverse if you did. Does anyone have a good implementation of how to accomplish acceleration based movement that works well?

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• How do I interpolate air drag with a variable time step?

  - by Valentin Krummenacher

  So I have a little game which works with small steps, however those steps vary in time, so for example I sometimes have 10 Steps/second and then I have 20 Steps/second. This changes automatically depending on how many steps the user's computer can take. To avoid inaccurate positioning of the game's player object I use y=v0*dt+g*dt^2/2 to determine my objects y-position, where dt is the time since the last step, v0 is the velocity of my object in the beginning of my step and g is the gravity. To calculate the velocity in the end of a step I use v=v0+g*dt what also gives me correct results, independent of whether I use 2 steps with a dt of for example 20ms or one step with a dt of 40ms. Now I would like to introduce air drag. For simplicity's sake I use a=k*v^2 where a is the air drag's acceleration (I am aware that it would usually result in a force, but since I assume 1kg for my object's mass the force is the same as the resulting acceleration), k is a constant (in this case I'm using 0.001) and v is the speed. Now in an infinitely small time interval a is k multiplied by the velocity in this small time interval powered by 2. The problem is that v in the next time interval would depend on the drag of the last which again depends on the v of the last interval and so on... In other words: If I use a=k*v^2 I get different results for my position/velocity when I use 2 steps of 20ms than when I use one step of 40ms. I used to have this problem for my position too, but adding +g*dt^2/2 to the formula for my position fixed the problem since it takes into account that the position depends on the velocity which changes slightly in every infinitely small time interval. Does something like that exist for air drag too? And no, I dont mean anything like Adding air drag to a golf ball trajectory equation or similar, for that kind of method only gives correct results when all my steps are the same. (I hope you can understand my intermediate english, it's not my main language so I would like to say sorry for all the silly mistakes I might have made in my question)

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• How to handle jumping up a slope in a runner game?

  - by you786

  In an 2D endless runner, what should happen when the player is running "too fast" up a slope and jumps? For example, in a "normal" case: .O. . __..O_____ . / . / O/ _/ If he is moving to the right slowly enough, he will jump upwards and land on the flat part of the surface. However, if he is moving too fast, the jump will have no effect as his forward motion will bring him back in contact with the slope before he can get high enough to pass over it. When the speed is sufficiently high, there will effectively be no jump. _________ / .O/ O/ _/ Are there any known ways to solve this issue? I know it's physically correct*, but are there techniques that other games use to overcome this in a reasonable manner? As a last resort I'll have to just remove all slopes that are too slanted. *If you constrain the player to never jumping backwards.

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• Trying to make a game with C++, using lists to store bullets and enemies, but they are not erased

  - by XD_dued

  I've been trying to make a pretty simple space shooter game with C++, and I have been having a lot of trouble trying to use lists to store enemies and bullets. Basically I followed the post here almost exactly to store my bullets: SDL Bullet Movement I've done something similar to store my enemies. However, when I call bullets.erase(it++), for some reason the bullet is not erased. When the bullet movement is run for the next frame, it tries to re delete the bullet and segfaults the program. When I comment out the erase line, it runs fine, but the bullets are then never erased from the list... Is there any reason why the elements of the list aren't being deleted? I also set it up to print the number of elements in the list for every iteration, and it does not go down after deleting. Thanks! EDIT: Here's the specific code I'm using to store my enemies and having them act: std::list<Grunt*> doGrunts(std::list<Grunt*> grunts) { for(std::list<Grunt*>::iterator it = grunts.begin(); it != grunts.end();) { if((*it)->getHull() == 0) { delete * it; grunts.erase(it++); } else { (**it).doUnit(grunts, it); ++it; } } } Grunt is my enemy class, and the list grunts is a global variable. Is that my problem? When I pass the global into the function it becomes local? I assumed lists would be a reference type so thought this wouldn't be a problem. Sorry if this was a dumb question, I'm very new to C++, this is the first major thing I'm working on.

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• How to make a stack stable? Need help for an explicit resting contact scheme (2-dimensional)

  - by Register Sole

  Previously, I struggle with the sequential impulse-based method I developed. Thanks to jedediah referring me to this paper, I managed to rebuild the codes and implement the simultaneous impulse based method with Projected-Gauss-Seidel (PGS) iterative solver as described by Erin Catto (mentioned in the reference of the paper as [Catt05]). So here's how it currently is: The simulation handles 2-dimensional rotating convex polygons. Detection is using separating-axis test, with a SKIN, meaning closest points between two polygons is detected and determined if their distance is less than SKIN. To resolve collision, simultaneous impulse-based method is used. It is solved using iterative solver (PGS-solver) as in Erin Catto's paper. Error-correction is implemented using Baumgarte's stabilization (you can refer to either paper for this) using J V = beta/dt*overlap, J is the Jacobian for the constraints, V the matrix containing the velocities of the bodies, beta an error-correction parameter that is better be < 1, dt the time-step taken by the engine, and overlap, the overlap between the bodies (true overlap, so SKIN is ignored). However, it is still less stable than I expected :s I tried to stack hexagons (or squares, doesn't really matter), and even with only 4 to 5 of them, they would swing! Also note that I am not looking for a sleeping scheme. But I would settle if you have any explicit scheme to handle resting contacts. That said, I would be more than happy if you have a way of treating it generally (as continuous collision, instead of explicitly as a special state). Ideas I have tried: Using simultaneous position based error correction as described in the paper in section 5.3.2, turned out to be worse than the current scheme. If you want to know the parameters I used: Hexagons, side 50 (pixels) gravity 2400 (pixels/sec^2) time-step 1/60 (sec) beta 0.1 restitution 0 to 0.2 coeff. of friction 0.2 PGS iteration 10 initial separation 10 (pixels) mass 1 (unit is irrelevant for now, i modified velocity directly<-impulse method) inertia 1/1000 Thanks in advance! I really appreciate any help from you guys!! :) EDIT In response to Cholesky's comment about warm starting the solver and Baumgarte: Oh right, I forgot to mention! I do save the contact history and the impulse determined in this time step to be used as initial guess in the next time step. As for the Baumgarte, here's what actually happens in the code. Collision is detected when the bodies' closest distance is less than SKIN, meaning they are actually still separated. If at this moment, I used the PGS solver without Baumgarte, restitution of 0 alone would be able to stop the bodies, separated by a distance of ~SKIN, in mid-air! So this isn't right, I want to have the bodies touching each other. So I turn on the Baumgarte, where its role is actually to pull the bodies together! Weird I know, a scheme intended to push the body apart becomes useful for the reverse. Also, I found that if I increase the number of iteration to 100, stacks become much more stable, though the program becomes so slow. UPDATE Since the stack swings left and right, could it be something is wrong with my friction model? Current friction constraint: relative_tangential_velocity = 0

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• Voronoi regions of a (convex) polygon.

  - by Xavura

  I'm looking to add circle-polygon collisions to my Separating Axis Theorem collision detection. The metanet software tutorial (http://www.metanetsoftware.com/technique/tutorialA.html#section3) on SAT, which I discovered in the answer to a question I found when searching, talks about voronoi regions. I'm having trouble finding material on how I would calculate these regions for an arbitrary convex polygon and aleo how I would determine if a point is in one + which. The tutorial does contain source code but it's a .fla and I don't have Flash unfortunately.

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• box2d and constant movement

  - by Arnas

  i'm developing a game with a top down view, the players body is a circle. To move the character you need to tap on the screen and it moves to the spot. To achieve this i'm saving the coordinate of the touch and call a method every frame which applies linear velocity to the body with a vector of the direction the body should go _body->SetLinearVelocity(b2Vec2((a.x - currPos.x)/SPEED_RATIO,(size.height - a.y - currPos.y)/SPEED_RATIO)); //click position - current position, screen height - click position (since the y axis is flipped, (0,0) is in the bottom left ) - current position = vector of the direction we want to go now the problem with this is that the body slows down until it finally stops when getting closer to the point we want it to go, since the closer we are to that point the lenght of the vector gets smaller. Besides that i've read that it's bad practice to set linear velocity in box2d and i should use apply force instead, but that way the forces would add up and overshoot the target where it's supposed to stop. So what i'm asking is how to move a box2d body to a coordinate in constant speed.

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• How to calculate new velocities between resting objects (AABB) after accelerations?

  - by Tiedye

  lately I have been trying to create a 2D platformer engine in C++ with Direct2D. The problem I am currently having is getting objects that are resting against each other to interact correctly after accelerations like gravity have been applied to them. Right now I can detect collisions and respond to them correctly (I think) and when objects collide they remember what other objects they're resting against so objects can be pushed by other objects (note that there is no bounce in any collisions so when objects collide they are guaranteed to become resting until something else happens). Every time the simulation advances, the acceleration for objects is applied to their velocities (for example vx += ax * t, where t is time elapsed since last advancement). After these accelerations are applied, I want to check if any objects that are resting against each other are moving at different speeds than their counterparts (as different objects can have different accelerations) and depending on that difference either unlink the two objects so they are no longer resting, or even out their velocities so they are moving at the same speed once again. I am having trouble creating an algorithm that can do this across many resting objects. Here's a diagram to help explain my problem

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• Determining the angle to fire a shot when target and shooter moves, and bullet moves with shooter velocity added in

  - by Azaral

  I saw this question: Predicting enemy position in order to have an object lead its target and followed the link in the answer to stack overflow. In the stack overflow page I used the 2nd answer, the one that is a large mathematical derivation. My situation is a little different though. My first question though is will the answer provided in the stack overflow page even work to begin with, assuming the original circumstances of moving target and stationary shooter. My situation is a little different than that situation. My target moves, the shooter moves, and the bullets from the shooter start off with the velocities in x and y added to the bullets' x and y velocities. If you are sliding to the right, the bullets will remain in front of you as you move so as long as your velocity remains constant. What I'm trying to do is to get the enemy to be able to determine where they need to shoot in order to hit the player. Unless the player and enemy is stationary, the velocity from the ship adding to the velocity of the bullets will cause a miss. I'd rather like to prevent that. I used the formula in the stack overflow answer and did what I thought were the appropriate adjustments. I've been banging at this for the last four hours and I just can't make it click. It is probably something really simple and boneheaded that I am missing (that seems to be a lot of my problems lately). Here is the solution presented from the stack overflow answer: It boils down to solving a quadratic equation of the form: a * sqr(x) + b * x + c == 0 Note that by sqr I mean square, as opposed to square root. Use the following values: a := sqr(target.velocityX) + sqr(target.velocityY) - sqr(projectile_speed) b := 2 * (target.velocityX * (target.startX - cannon.X) + target.velocityY * (target.startY - cannon.Y)) c := sqr(target.startX - cannon.X) + sqr(target.startY - cannon.Y) Now we can look at the discriminant to determine if we have a possible solution. disc := sqr(b) - 4 * a * c If the discriminant is less than 0, forget about hitting your target -- your projectile can never get there in time. Otherwise, look at two candidate solutions: t1 := (-b + sqrt(disc)) / (2 * a) t2 := (-b - sqrt(disc)) / (2 * a) Note that if disc == 0 then t1 and t2 are equal. If there are no other considerations such as intervening obstacles, simply choose the smaller positive value. (Negative t values would require firing backward in time to use!) Substitute the chosen t value back into the target's position equations to get the coordinates of the leading point you should be aiming at: aim.X := t * target.velocityX + target.startX aim.Y := t * target.velocityY + target.startY Here is my code, after being corrected by Sam Hocevar (thank you again for your help!). It still doesn't work. For some reason it never enters the section of code inside the if(disc = 0) (obviously because it is always less than zero but...). However, if I plug the numbers from my game log on the enemy and player positions and velocities it outputs a valid firing solution. I have looked at the code side by side a couple of times now and I can't find any differences. There has got to be something simple I'm missing here. If someone else could look at this code and determine what is going on here I'd appreciate it. I know it's not going through that section because if it were, shouldShoot would become true and the enemy would be blasting away at the player. This section calls the function in question, CalculateShootHeading() if(shouldMove) { UseEngines(); } x += xVelocity; y += yVelocity; CalculateShootHeading(); if(shouldShoot) { ShootWeapons(); } UpdateWeapons(); This is CalculateShootHeading(). This is inside the enemy class so x and y are the enemy's x and y and the same with velocity. One output from my game log gives Player X = 2108, Player Y = -180.956, Player X velocity = 10.9949, Player Y Velocity = -6.26017, Enemy X = 1988.31, Enemy Y = -339.051, Enemy X velocity = 1.81666, Enemy Y velocity = -9.67762, 0 enemy projectiles. The output from the console tester is Bullet position = 2210.49, -239.313 and Player Position = 2210.49, -239.313. This doesn't make any sense. The only thing that could be different is the code or the input into my function in the game and I've checked that and I don't think that it is wrong as it's updated before this and never changed. float const bulletSpeed = 30.f; float const dx = playerX - x; float const dy = playerY - y; float const vx = playerXVelocity - xVelocity; float const vy = playerYVelocity - yVelocity; float const a = vx * vx + vy * vy - bulletSpeed * bulletSpeed; float const b = 2.f * (vx * dx + vy * dy); float const c = dx * dx + dy * dy; float const disc = b * b - 4.f * a * c; shouldShoot = false; if (disc >= 0.f) { float t0 = (-b - std::sqrt(disc)) / (2.f * a); float t1 = (-b + std::sqrt(disc)) / (2.f * a); if (t0 < 0.f || (t1 < t0 && t1 >= 0.f)) { t0 = t1; } if (t0 >= 0.f) { float shootx = vx + dx / t0; float shooty = vy + dy / t0; heading = std::atan2(shooty, shootx) * RAD2DEGREE; } shouldShoot = true; }

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• Projectile Rotation

  - by Alex

  I'm trying to add a projectile system like the projectiles in Realm Of The Mad God. (YouTube it to see what I mean) These projectiles seem to move according to their rotation perfectly and can have nearly any rotation. They also have near perfect hitboxing. What's the maths behind this? My Game works on an integer-based coordinate system, but at the moment projectiles can only shoot either 0, 45, 90, 135, 180, 225, 270 and 315 degrees.

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• Damageable ground similar to pocket tanks or archanists [closed]

  - by XenElement

  Possible Duplicate: Implementing a 2D destructible landscape (like Worms) A really cool feature in both the iPhone game pocket tanks and the online jagex game archanists is ground which can be blown up. When a projectile collides with the ground, an area equal to the blast radius which overlaps the ground is removed. It's strictly two dimensional, but it makes the experience that much more dynamic since you can dig a hole under your opponents or yourself. How is this implemented?

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• How to detect and collide two elastic line segments?

  - by Tautrimas

  There are 4 moving physical nodes in 3D space. They are paired with two elastic line segments / strings (1 <- 2; 3 <- 4). Part I: How to detect the collision of two segments? Part II: On the moment of collision, fifth node is created at the intersection point and here you have the force-based graph. 5-th node (bend point) can slide among the strings as in a real world. Given the new coordinates of 4 nodes, how to calculate the position of the 5-th node on the next frame? I assume string force on the nodes to be F = -k * x where x is the string length. All I came up to is that the force between 5 and 1 equals 5 and 2 (the same with 3 and 4). What are the other properties?.

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• How do I implement deceleration for the player character?

  - by tesselode

  Using delta time with addition and subtraction is easy. player.speed += 100 * dt However, multiplication and division complicate things a bit. For example, let's say I want the player to double his speed every second. player.speed = player.speed * 2 * dt I can't do this because it'll slow down the player (unless delta time is really high). Division is the same way, except it'll speed things way up. How can I handle multiplication and division with delta time? Edit: it looks like my question has confused everyone. I really just wanted to be able to implement deceleration without this horrible mass of code: else if speed > 0 then speed = speed - 20 * dt if speed < 0 then speed = 0 end end if speed < 0 then speed = speed + 20 * dt if speed > 0 then speed = 0 end end end Because that's way bigger than it needs to be. So far a better solution seems to be: speed = speed - speed * whatever_number * dt

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• Why is the MaskBit maxed out

  - by CStreel

  Hi there for some reason the maskbit of my b2FixtureDef is being maxxed out and im not sure why Here is the declaration of the items that are used in the game enum PhysicBits { PB_NONE = 0x0000, PB_PLAYER = 0x0001, PB_PLATFORM = 0x0002 }; Basically what i want is the player to run along a surface is not slow down (i set platform & player friction to 0.0f) I then setup my Contact Listener to print out the connections (currently only have 1 platform and 1 player) Player Fixture Def b2FixtureDef fixtureDef; fixtureDef.shape = &groundBox; fixtureDef.density = 1.0f; fixtureDef.friction = 0.0f; fixtureDef.filter.categoryBits = PB_PLAYER; fixtureDef.filter.maskBits = PB_PLATFORM; Platform Fixture Def b2FixtureDef fixtureDef; fixtureDef.shape = &groundBox; fixtureDef.density = 1.0f; fixtureDef.friction = 0.0f; fixtureDef.filter.categoryBits = PB_PLATFORM; fixtureDef.filter.maskBits = PB_PLAYER; Now correct me if im wrong but these are saying the following: Player Collides with Platform Platform Collides with Player Here is the printout of the fixtures colliding with each other ******** <-- Indicates new Contact Platform ContactA: 2 MaskA: 1 ------ Player ContactB: 1 MaskB: 2 ******** <-- Indicates new Contact Platform ContactA: 2 MaskA: 1 ------ Player ContactB: 1 MaskB: 65535 ******** <-- Indicates new Contact Platform ContactA: 1 MaskA: 65535 ------ Player ContactB: 1 MaskB: 65535 Here is where i am confused. On the second & third contact the player maskBit is set to 65535 when it should be 2 and there are 3 contacts when i am sure at most there should only be 2. I've been trying to figure this out for hours and i can't understand why it is doing this. I would be very grateful is someone could shine some light on this for me UPDATE: **I printed out the class of the contacting objects. For some reason it seems to do the following: First Contact: Correct Result. Second Contact: Player b2Fixture Obtains a new maskBit. Third Contact: Platform b2Fixture appears to be set to the same as the Player b2Fixture. It would seem I have a memory race condition i think**

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• SDL Bullet Movement

  - by Code Assasssin

  I'm currently working on my first space shooter, and I'm in the process of making my ship shoot some bullets/lasers. Unfortunately, I'm having a hard time getting the bullets to fly vertically. I'm a total noob when it comes to this so you might have a hard time understanding my code :/ // Position Bullet Function projectilex = x + 17; projectiley = y + -20; if(keystates[SDLK_SPACE]) { alive = true; } And here's my show function if(alive) { if(frame == 2) { frame = 0; } apply_surface(projectilex,projectiley,ShootStuff,screen,&lazers[frame]); frame++; projectiley + 1; } I'm trying to get the bullet to fly vertically... and I have no clue how to do that. I've tried messing with the y coordinate but that makes things worse. The laser/bullet just follows the ship :( How would I get it to fire at the starting position and keep going in a vertical line without it following the ship? int main( int argc, char* args[] ) { Player p; Timer fps; bool quit = false; if( init() == false ) { return 1; } //Load the files if( load_files() == false ) { return 1; } clip[ 0 ].x = 0; clip[ 0 ].y = 0; clip[ 0 ].w = 30; clip[ 0 ].h = 36; clip[ 1 ].x = 31; clip[ 1 ].y = 0; clip[ 1 ].w = 39; clip[ 1 ].h = 36; clip[ 2 ].x = 71; clip[ 2 ].y = 0; clip[ 2 ].w = 29; clip[ 2 ].h = 36; lazers [ 0 ].x = 0; lazers [ 0 ].y = 0; lazers [ 0 ].w = 3; lazers [ 0 ].h = 9; lazers [ 1 ].x = 5; lazers [ 1 ].y = 0; lazers [ 1 ].w = 3; lazers [ 1 ].h = 7; while( quit == false ) { fps.start(); //While there's an event to handle while( SDL_PollEvent( &event ) ) { p.handle_input(); //If a key was pressed //If the user has Xed out the window if( event.type == SDL_QUIT ) { //Quit the program quit = true; } } //Scroll background bgX -= 8; //If the background has gone too far if( bgX <= -GameBackground->w ) { //Reset the offset bgX = 0; } p.move(); apply_surface( bgX, bgY,GameBackground, screen ); apply_surface( bgX + GameBackground->w, bgY, GameBackground, screen ); apply_surface(0,0, FullHealthBar,screen); p.shoot(); p.show(); //Apply the message //Update the screen if( SDL_Flip( screen ) == -1 ) { return 1; } SDL_Flip(GameBackground); if( fps.get_ticks() < 1000 / FRAMES_PER_SECOND ) { SDL_Delay( ( 1000 / FRAMES_PER_SECOND ) - fps.get_ticks() ); } } //Clean up clean_up(); return 0; }

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• How to draw a texture to a MS Terrain object - Farseer

  - by Brad

  I'm using Farseer to make a game in XNA and I can't seem to figure this out. I've decided to use MSTerrain for making my game's terrain because I wanted destructible terrain and MSTerrain seemed like the best bet. Unfortunately, I'm stumped on how to actually show the terrain. When I generate the terrain it's visible in debug view, but MSTerrain does not have a Draw method, so I'm wondering how it is supposed to be drawn to the screen? Is it worth pursuing? I'm starting to think that MSTerrain is more trouble than it's worth, is there another better way to do this with bodies? I appreciate any help you can give. Thanks.

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• Grid collision - finding the location of an entity in each box

  - by Gregg1989

  I am trying to implement grid-based collision in a 2d game with moving circles. The canvas is 400x400 pixels. Below you can see the code for my Grid class. What I want it to do is check inside which box the entities are located and then run a collision check if there are 2 or more entities in the same box. Right now I do not know how to find the position of an entity in a specific box. I know there are many tutorials online, but I haven't been able to find an answer to my question, because they are either written in C/C++ or use the 2d array approach. Code snippets and other help is greatly appreciated. Thanks. public class Grid { ArrayList<ArrayList<Entity>> boxes = new ArrayList<>(); double boxSize = 40; double boxesAmount = 10; ... ... public void checkBoxLocation(ArrayList<Entity> entities) { for (int i = 0; i < entities.size(); i++) { // Get top left coordinates of each entity double entityLeft = entities.get(i).getLayoutX() - entities.get(i).getRadius(); double entityTop = entities.get(i).getLayoutY() + entities.get(i).getRadius(); // Divide coordinate by box size to find the approximate location of the entity for (int j = 0; j < boxesAmount; j++) { //Select each box if ((entityLeft / boxSize <= j + 0.7) && (entityLeft / boxSize >= j)) { if ((entityTop / boxSize <= j + 0.7) && (entityTop / boxSize >= j)) { holdingBoxes.get(j).add(entities.get(i)); System.out.println("Entity " + entities.get(i) + " added to box " + j); } } } } } }

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• How to configure Bullet for LookAt?

  - by AllCoder

  I'm having problems positioning Bullet objects. I am doing: ToolVec3 origin = ToolVec3( obj_posx, obj_posy, obj_posz ); ToolVec3 vmod = ToolVec3( object_sizex / 2.0f, object_sizey / 2.0f, object_sizez / 2.0f ); btTransform shapeTransform = btTransform::getIdentity(); shapeTransform.setOrigin( btVector3(origin.x+vmod.x, origin.y+vmod.y, origin.z+vmod.z) ); btDefaultMotionState* myMotionState = new btDefaultMotionState(shapeTransform); btRigidBody::btRigidBodyConstructionInfo rbInfo(mass,myMotionState,m_collisionShapes[2],localInertia); btRigidBody* body = new btRigidBody(rbInfo); I then do: btCollisionObject* colObj = m_dynamicsWorld->getCollisionObjectArray()[i]; btRigidBody* body = btRigidBody::upcast(colObj); if(body && body->getMotionState()) { btDefaultMotionState* myMotionState = (btDefaultMotionState*)body->getMotionState(); myMotionState->m_graphicsWorldTrans.getOpenGLMatrix(m); } else { colObj->getWorldTransform().getOpenGLMatrix(m); } And after obtaining the matrix m, I paste it as model matrix. I am observing few things: I must add some weird "size / 2" to object's position, to have it drawed normally, I have following "up" look at vector defined: "0.0f, -1.0f, 0.0f" – basically, Y grows up, Z grows forward (to monitor), BUT – x grows LEFT, I think there is some conflict with the X direction.. I cannot obtain consistent positioning having world setup like this How to configure this in Bullet? Why the weird + size/2 requirement?

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• How to make an object fly out of a slingshot?

  - by Deza

  At the moment I'm improvising a slingshot, the user can click and drag the projectile and let go. The force on the object is calculated by getting the distance between the vector of the slingshots two forks and the vector between where the user pulled it. However this will always result in a positive number and will not take into account the angle of the object relative to that of the slingshot. How can I make it fly out of the slingshot correctly?

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• Farseer circle hangs where it's spawned

  - by necrosmash

  I'm currently trying to simply spawn a circle in Farseer. However, it's stuck wherever I spawn it! The game is updating fine, as I can see the circle spinning in place when I spawn it because of how I currently have gravity set up (following code from Game1.cs): // Initialise the screen center for use with // the Level class screenCenter = new Vector2(graphics.GraphicsDevice.Viewport.Width / 2f, graphics.GraphicsDevice.Viewport.Height / 2f); world = new World(new Vector2(20, 20)); currentLevel = new Level1(screenCenter, circleSprite, groundSprite, ref world); Level1 constructor: public Level1(Vector2 screenCenter, Texture2D circleSprite, Texture2D groundSprite, ref World world) { player = new Player(ref world, screenCenter, circleSprite); ground = new Ground(ref world, screenCenter, groundSprite); listLevelItems = new List<LevelItem>(); listLevelItems.Add(player); listLevelItems.Add(ground); } Player constructor: public Player(ref World world, Vector2 screenCenter, Texture2D sprite) { setSprite(sprite); setPosition((screenCenter / MeterInPixels) + new Vector2(0f, 0f)); playerBody = BodyFactory.CreateCircle(world, 96f / (2f * MeterInPixels), 1f, playerPosition); getBody().BodyType = BodyType.Dynamic; // Ball bounce and friction getBody().Restitution = 0.3f; getBody().Friction = 0.5f; } If I use a breakpoint and change the playerBody position while the game is halted, the ball does move, but stays fixed in its new location. Any help would be greatly appreciated.

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• Elastic Collision Formula in Java

  - by Shijima

  I'm trying to write a Java formula based on this tutorial: 2-D elastic collisions without Trigonometry. I am in the section "Elastic Collisions in 2 Dimensions". In step 1, it mentions "Next, find the unit vector of n, which we will call un. This is done by dividing by the magnitude of n". My below code represents the normal vector of 2 objects (I'm using a simple array to represent the normal vector), but I am not really sure what the tutorial means by dividing the magnitude of n to get the un. int[] normal = new int[2]; normal[0] = ball2.x - ball1.x; normal[1] = ball2.y - ball1.y; Can anyone please explain what un is, and how I can calculate it with my array in Java?

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• Simulating water droplets on a window

  - by skyuzo

  How do I simulate water droplets realistically falling, gathering, and flowing down a window? For example, see http://www.youtube.com/watch?v=4jaGyv0KRPw. In particular, I want to simulate how smaller droplets merge together to form larger droplets that have enough weight to oppose the surface tension and flow downward, leaving a trail of water. I'm aware of fluid simulation, but how would it be applied in this situation?

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• How to determine which thrusters to turn on to rotate the ship?

  - by migimunz

  The configuration of the ship changes dynamically, so I have to determine which thruster to turn on when I want to rotate the ship clockwise or counter clockwise. The thrusters are always axis aligned with the ship (never at an angle) and are either on or off. Here's one of the possible setups: What I've tried so far is to visualize the firing vector and the direction vector to the center of mass of the ship: Unfortunately, I didn't get very far with that.

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• Collider2D and Rigidbody2D, how do they work?

  - by user42646

  I have been learning JavaScript and Unity for a week now. I learned how to make Cube as a Ground and another Cube as a player and I used this code to make the Player Cube move forward and backward and jumping var walkspeed: float = 5.0; var jumpheight: float = 250.0; var grounded = false; function Update() { rigidbody.freezeRotation = true; if (Input.GetKey("a")) transform.Translate(Vector3(-1, 0, 0) * Time.deltaTime * walkspeed); if (Input.GetKey("d")) transform.Translate(Vector3(1, 0, 0) * Time.deltaTime * walkspeed); if (Input.GetButton("Jump")) { Jump(); } } function OnCollisionEnter(hit: Collision) { grounded = true; } function Jump() { if (grounded == true) { rigidbody.AddForce(Vector3.up * jumpheight); grounded = false; } } I also learned how to make a character hit box. how to make a sprite and animation. pretty much the basic stuff. Couple of days ago I created simple ground in Photoshop and a simple character and imported them to Unity3D. Whenever I use my code above the character keeps falling from the scene. Like the character has nothing to stand on. After thinking about it it make sense because I really didn't make anything to make the player character understand that he should stand on something so I started reading about this issue and I realized that there is something called Collider2D and Rigidbody2D. Now I'm really stuck here I just don't know what to do. I applied the rigibody2d to my character picture and the Collider2D to the ground picture but whenever I play the project the gravity makes my character falls down. This is my question: How can I make the rigibody2d object realize that it shouldn't fall if there is a Collider2D object under it? So when I jump it's going to jump and the gravity going to bring it back to the ground.

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