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Change Ball Direction after collision with another ball

I need to change Ball Direction after collision with another ball or with a edge of the window.
I managed to do something like that:
y += yMove;
x += xMove;
//if the ball moves to the right edge of the window, turn around.
if(x > width - size)
{
x = width - size;
xMove *= -1;
if (xMove > 0) {
xSpeed = xMove + (Math.random() * (1));
}
if (xMove <= 0) {
xSpeed = xMove - (Math.random() * (1));
}
if (yMove > 0) {
ySpeed = yMove + (Math.random() * (1));
}
if (yMove <= 0) {
ySpeed = yMove - (Math.random() * (1));
}
}
And same for another edges.
I'm trying to use same method for changing direction of balls after they collide with each other, but it's just not working / it's weird. Can anyone help me?

When balls collide, make vector connecting ball centers (N) and normalize it (uN)
Components of velocities parallel to N (normal) are exchanged (due to impulse law)
Components of velocities perpendicular to N (tangential) remain the same
To get components in given local system, use scalar and cross product:
V1t = dot(V1, uN)
V2t = dot(V2, uN)
V1n = cross(V1, uN)
V2n = cross(V2, uN)
after collision
V1t' = V2t
V2t' = V1t
V1n' = V1n
V2n' = V2n
To return into global system (I did not checked signs thoroughly):
V1x = V1t` * uN.X + V2n` * uN.Y
V1y = -V1t` * uN.Y + V2n` * uN.X
(This is essentially dot and cross products again, but I expanded expressions to show different bases)
Note that this approach is like to ball-edge collision, when N is normal to the border and you reverse only one component of velocity vector.

For your BouncingBall class, you can have a method like flipDirection(), but you can have a finer directional control by splitting it into 2 methods which filps the direction of the ball vertically and horizontally.
class BouncingBall{
public void horizontalFlip(){
moveX *= -1;
}
public void verticalFlip(){
moveY *= -1;
}
//To have move control over each direction, you can have a method for each direction.
public void moveNorth(){
moveY = Math.abs(moveY) * -1;
}
public void moveSouth(){
moveY = Math.abs(moveY);
}
public void moveWest(){
moveX = Math.abs(moveX) * -1;
}
public void mpveEast(){
moveX = Math.abs(moveX);
}
}
Depending on how you want the ball to bounce off. In a simple bounce off, the balls can bounce towards 4 possible directions:
North West
North East
South West
South East
The direction of the ball to bounce off will be relative to the position of the ball it is colliding with and you do not want 2 collided balls which move in the same direction to switch direction just because they collided. Hence you need to check the positions of the 2 balls, and flipDirection() becomes insufficinet to achieve that.
if(b1.intersects(b2)){
if(b1.getX() < b2.getX()){ // b1 above b2
b1.moveNorth();
b2.moveSouth();
}
else{
b1.moveSouth();
b2.moveNorth();
}
if(b1.getY() < b2.getY()){ // b1 at left side of b2
b1.moveWest();
b2.moveEast();
}
else{
b1.moveEast();
b2.moveWest();
}
}
For example, to change direction when hitting the walls on the left and right:
if(ball.getPosX() <= 0 || ball.getPosX() >= PNL_WIDTH-Ball.SIZE)
ball.horizontalReverse();
Same logic for verticalReverse.

Collision Detection between player and tiles almost works

I have been stuck on making some collision detection in my game (its kind of like Terraria) for a while but i made this code and... well, it kind of works. It works if the collision is above or on the left of the player, but if the collision is on the right, or below, instead of bouncing back, the player accelerates through the blocks until there is empty space. Here is the code that i made:
private void checkCollision() {
for(int x = (int) (xpos-1); x <= xpos+1; x++){
if(x < 0 || x > main.mw-1) continue;
for(int y = (int) (ypos-2); y <= ypos+1; y++){
if(y < 0 || y > main.mh-1) continue;
if(main.map[x][y] == null) continue;
if(!main.map[x][y].solid) continue;
if(main.map[x][y].blocktype == -1) continue;
double distance = Math.sqrt((xpos-x)*(xpos-x) + (ypos-y)*(ypos-y));
if(distance > 1.0){
continue;
}else{
double x_overlap = Math.max(0, Math.min(xpos + 16, x + 16) - Math.max(xpos, x));
double y_overlap = Math.max(0, Math.min(ypos + 32, y + 16) - Math.max(ypos, y));
double overlapArea = x_overlap * y_overlap;
if(overlapArea > 0){
if(x_overlap > y_overlap){
yblock += y_overlap/2;
}
if(x_overlap < y_overlap){
xblock += x_overlap/2;
}
//guessing i need to do something here to make player
go other way if block is on other side
}
}
}
}
}
So how would i make the player bounce back if the block that he is colliding with is on the right or below. Also is there any way i can make this smoother - right now the player be bouncing all over the place. Thanks! :)

What you want to do is keep track of the player's location, and if the location after moving is out of bounds you can reset the player's position to be right on the edge of the limit.
That's how I dealt with collision detection, I answered another question similar to this one though some folk decided to downvote the answer, go figure.

Add or subtract to an angle to reach onther angle the shortest way

I have encountered some issues regarding angles. I have an angle A and another angle B, I want to animate A the shortest way so that it reaches B. The first confusion for me is that angles go from 0 to 180, and 0 to -180. Not sure what the pros of that is. Anyway, I will give a for instance:
float a = -35;
float b = 90;
For each update I want to either add 1 or subtract 1 degree from a, until it reaches b, and I want to make sure it goes the shortest way.
Here's my code, which seems to be working. But it does not seem very efficient:
b += 360;
if (b > a) {
if (b - a < 180) {
a += 1;
} else {
a -= 1;
}
} else {
if (a - b < 180) {
a -= 1;
} else {
a += 1;
}
}
Is there a better/easier way to do it?

So you want the shortest route from a to b.
Since we are looking at a difference lets subtract:
float d = a-b;
If the value of the result is greater than 180 then we want to subtract 360.
if (d > 180) {
d -= 360;
} else if (d<-180) {
d += 360;
}
Now d is the total distance to travel. You can compare that with 0 to see which way to go. You can also do nice things like move further the larger d is. For example to make it always move 10% of the way (note that this series will never end as it will constantly approach by smaller and smaller amounts so you need to cope with that scenario):
a += d/10;
You also need to consider frame rate if you want a smooth animation.
If you work out tpf (time per frame) as a floating point fraction of a second.
long frameTook = lastFrame - System.currentTimeMillis();
long lastFrame = System.currentTimeMillis();
float tpf = frameTook / 1000;
You can now do a constant animation (where degreesPerFrame is the speed of animation) using:
float move = degreesPerFrame * tpf;
Check we aren't going to move past the destination, if we are just move to it.
if (move > FastMath.abs(d)) {
a = b;
} else {
if (d>0) {
a+=move;
} else {
a-=move;
}
}

Collision Detection in Box2D

So I'm using Box2D for collision detection in a game. I have a tilemap that contains information on the terrain: for now it's just a char[][] that has either road or grass. Now, at the start of each level I wanted to create rectangles to describe the different terrains, but I wanted these rectangles to be optimized and apparently that takes quite an algorithm.
My first approach was to create an individual terrain for EVERY tile in the map at the start of the level. The FPS was reduced to 5.
My second idea was to simply create the different rectangles for terrains as the player moved along the map, deleting the rectangles that were out of view. Although it would still be a lot of rectangles, it would be considerably less.
I haven't attempted the second method yet, but I want to know: is there any easy way for me to efficiently perform collision detection against terrain with a large tilemap?
Thanks.

Try combining tiles. For example, if you have 16 rectangular collision volumes for 16 tiles like so...
* * * *
* * * *
* * * *
* * * *
You can obviously combine these tiles into one large rectangle.
Now, things get more difficult if you have tiles in a weird arrangement, maybe like this...
**---
****-
*--**
-*-*-
I just recently solved this problem in my game using a quad tree and sweep and prune. (Sweep and prune isn't strictly necessary, its an optimization.)
Quad tree partitions your square tiles into bigger rectangles, then you iterate over the rectangles the quad tree produces, and combine them if they have the same width, then iterate over them again and combine them by similar heights. Repeat until you can't combine them anymore, then generate your collision volumes.
Here's a link to a question I asked about a more optimal reduction. I probably won't implement this as it sounds difficult, and my current approach is working well.
Some code:
do {
lastCompressSize = currentOutput;
this.runHorizontalCompression(this.output1, this.output2);
this.output1.clear();
this.runVerticalCompression(this.output2, this.output1);
this.output2.clear();
currentOutput = this.output1.size;
iterations += 1;
}while (lastCompressSize > currentOutput);
public void runHorizontalCompression(Array<SimpleRect> input,
Array<SimpleRect> output) {
input.sort(this.xAxisSort);
int x2 = -1;
final SimpleRect newRect = this.rectCache.retreive();
for (int i = 0; i < input.size; i++) {
SimpleRect r1 = input.get(i);
newRect.set(r1);
x2 = newRect.x + newRect.width;
for (int j = i + 1; j < input.size; j++) {
SimpleRect r2 = input.get(j);
if (x2 == r2.x && r2.y == newRect.y
&& r2.height == newRect.height) {
newRect.width += r2.width;
x2 = newRect.x + newRect.width;
input.removeIndex(j);
j -= 1;
} else if (x2 < r2.x)
break;
}
SimpleRect temp = this.rectCache.retreive().set(newRect);
output.add(temp);
}
}
public void runVerticalCompression(Array<SimpleRect> input,
Array<SimpleRect> output) {
input.sort(this.yAxisSort);
int y2 = -1;
final SimpleRect newRect = this.rectCache.retreive();
for (int i = 0; i < input.size; i++) {
SimpleRect r1 = input.get(i);
newRect.set(r1);
y2 = newRect.y + newRect.height;
for (int j = i + 1; j < input.size; j++) {
SimpleRect r2 = input.get(j);
if (y2 == r2.y && r2.x == newRect.x
&& r2.width == newRect.width) {
newRect.height += r2.height;
y2 = newRect.y + newRect.height;
input.removeIndex(j);
j -= 1;
} else if (y2 < r2.y)
break;
}
SimpleRect temp = this.rectCache.retreive().set(newRect);
output.add(temp);
}
}

Blocking light on a tiled level

I'm making a tiled(tiles size is 16px) level scrolling game in Java.
Right now I'm dealing with the lighting system.
I calculated the light gradient(as shown on the picture) with this code for each light(yellow blocks and tiles):
visMap = new int[level.getWidth() * level.getHeight()];
int lighted = 0;
for (int x = 0; x < level.getWidth(); x++) {
for (int y = 0; y < level.getHeight(); y++) {
double xd = (this.x >> 4) - x;
double yd = (this.y >> 4) - y;
double distance = Math.sqrt(xd * xd + yd * yd);
double p = power * 1.0;
double bright = p - distance;
visMap[x + y * level.getWidth()] = (int) (bright * power);
}
}
And now I'm trying to make the block somehow block the light(like in real life).
Is there a good method for this?
Thank's in advance,
Zaplik
The Picture: click

Spread light recursively. Decrease with each level of recursion the light intensity with the appropriated amount. Keep also track of the direction light is moving. Once you hit an obstacle, stop that branch of the recursion.