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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.

Tile based collision detection, object falls through tiles

I am having a problem with collision detection on a tile based level at lower framerates. I have a platform game built in Java with the LibGdx game engine. At a framerate of 60 the game runs fine, but if I try it a 30 FPS the character will fall through tiles when it comes down from a jump.
I figured out that the character moved too fast. I already added something to check if there are any tiles that the character already passed, see "// 1" to "// 1 end" in the comments. I don't think it really helped because the problem still occurs.
Falling through the tiles seems to happen when the character hits corners of tiles, although I am not sure about it. It does not happen on a flat ground. Here is a picture of the problem (left is wrong, right is how it should be):
Again, the problem only happens at lower framerates. I am not sure what I have to change in my code. What am I missing in my code? Or do I have to use a different algorithm?
Here are the most important parts of the collision detection code. collisionY check collisions on the y-axis, collisionX on the x-axis. CheckTiles(checkX) helps finding the tiles that are should be checked (checkX is true if x-axis is checked, if false y-axis is checked):
protected boolean collisionY(Rectangle rect) {
int[] bounds = checkTiles(false);
Array<Rectangle> tiles = world.getTiles(bounds[0], bounds[1], bounds[2], bounds[3]);
rect.y += velocity.y;
if(velocity.y < 0 ) {
grounded = false;
}
for (Rectangle tile : tiles) {
if (rect.overlaps(tile)) {
if (velocity.y > 0) {
this.setY(tile.y - this.getHeight());
}
else {
// 1 Check if there are tiles above
Rectangle r = null;
int i = 1;
Rectangle r1 = null;
do {
r1 = r;
r = world.getTile(tile.x, tile.y + i);
i++;
} while (r != null);
if(r1 != null) {
this.setY(r1.y + r1.height);
}
// 1 end
else {
this.setY(tile.y + tile.height);
}
hitGround();
}
return true;
}
}
}
protected boolean collisionX(Rectangle rect) {
int[] bounds = checkTiles(true);
Array<Rectangle> tiles = world.getTiles(bounds[0], bounds[1], bounds[2], bounds[3]);
rect.x += velocity.x;
for (Rectangle tile : tiles) {
if (rect.overlaps(tile)) {
return true;
}
}
return false;
}
protected int[] checkTiles(boolean checkX) {
int startX, startY, endX, endY;
if(checkX) {
if (velocity.x > 0) {
startX = endX = (int) (this.getX() + this.getWidth() + velocity.x);
}
else {
startX = endX = (int) (this.getX() + velocity.x);
}
startY = (int) (this.getY());
endY = (int) (this.getY() + this.getHeight());
}
else {
if (velocity.y > 0) {
startY = endY = (int) (this.getY() + this.getHeight() + velocity.y); //
}
else {
startY = endY = (int) (this.getY() + velocity.y);
}
startX = (int) (this.getX());
endX = (int) (this.getX() + this.getWidth());
}
return new int[]{startX, startY, endX, endY};
}

The "Check if tile above" code will only be run if A) rect.overlaps(tile) was true, and B) velocity.y > 0 was false. I suspect in the case you care about, this code is simply not being executed. The character is either not overlapping the tile, so the check for tiles above doesn't occur, or its velocity is not such that the check occurs. However, I don't completely understand how the velocity works (how do up and down correspond to positive and negative values?) and I'm not familiar with this game engine.
I would try a different approach, though. Instead of first moving the character (which is what I think the line rect.y += velocity.y; does), and then trying to check if it has gone too far or passed through a tile, I would take the direction in which it's moving (velocity.y) and look for the first tile that it would hit going that way. If there is one, then place the character on that tile. If there's nothing in its way for velocity.y units in that direction, then it gets to move the whole distance in this time slice.

Java / LWJGL Pong AI issue

I am struggling with what must be a basic concept, but can you have a look at my issue?
I have the code where: ai moves the player bat, HEIGHT = total height of Display, and batHeight is the size of the pong paddle/bat:
public void ai(int bally, int HEIGHT, int batHeight) {
if (bally < this.y + ySize / 2) {
if (this.y <= 0) {
System.out.println("Upper Bound");
y = 0;
} else {
y -= 2;
}
}
if (bally > this.y + ySize / 2) {
if (this.y >= HEIGHT - batHeight) {
System.out.println("Lower Bounds");
y = HEIGHT - batHeight;
} else {
y += 2;
}
}
}
The above does exactly what I want it to do. Pong Bat moves up, and when it hits the top of the screen, it prints the console line, and stops the Bat. Exactly the same happens at the bottom of the screen. It prints the console, and stops the bat. It does this every time with no issues.
Now, if I modify the code slightly:
public void ai(int bally, int HEIGHT, int batHeight) {
if (bally < this.y + ySize / 2) {
if (this.y <= 0) {
System.out.println("Upper Bound");
y = 0;
} else {
if(rand.nextInt(2)+1 == 1){
y -= 2;
}else{
y -=3;
}
}
}
if (bally > this.y + ySize / 2) {
if (this.y >= HEIGHT - batHeight) {
System.out.println("Lower Bounds");
y = HEIGHT - batHeight;
} else {
y += 2;
}
}
}
It iterates once, stopping at the top bound, but then it loses itself, and forgets the bounds and the bat moves off the screen. I have Console printing the Bat y position, and it tracks with no issue, accurately displaying its y co-ord, but after the first iteration, it goes to negative y and greater that screen height.
I did have the theory that you cannot nest a IF inside an ELSE statement, so i tried moving it around so that it read:
if(this.y != 0){
if(rand.nextInt(2) + 1 == 1){
//move the paddle at speed 1
} else {
//move paddle at speed 2
}
}else{
//do not move the paddle
}
But that made no difference.
The idea behind the code was to add some chance for the AI bat. Sometimes its fast, and other times it is slower.
Thanks in advance,

Your code from far away looks like this:
for a given time:
if the ball is below the paddle {
if the paddle is below the screen, put it back
else move it down 2 or 3 units
}
if the ball is above the paddle {
if the paddle is above the screen, put it back
else move it up 2 units
}
Imagine the case where the ball is at y = 1 and the paddle is at y = 2. The first if statement will be triggered (1 < 2), the paddle is not outside (2 > 0), so it moves down 2 or 3 units. Let's say 3, for argument's sake. Now, paddle is at y = -1, the ball is still at y = 1. Now, the condition for the second big if is true! So we enter it: the paddle's not above, and we move it up two units. Now, the paddle is at y = 1...
It is clear that it should not have entered the second loop. So, stick an else in front of it, because it should only ever enter one :)

Java: rotating image so that it points at the mouse cursor

I want the player image to point towards the mouse cursor. I use this code to get the postion of the mouse cursor:
private int cursorX = MouseInfo.getPointerInfo().getLocation().x;
private int cursorY = MouseInfo.getPointerInfo().getLocation().y;
Note: The default player image points upwards

You'll have to use trigonometry in order to calculate the angle of rotation. For that you'll first need to obtain the location of the image and the cursor. I cannot tell you how to get the position for the image as this may vary. For this example (adapted from here), I'll assume imageX and imageY are the x and y positions of your image:
float xDistance = cursorX - imageX;
float yDistance = cursorY - imageY;
double rotationAngle = Math.toDegrees(Math.atan2(yDistance, xDistance));

To find the angle from a coordinate (0,0) to another coordinate (x,y), we can use the trigonometric function tan^-1(y/x).
Java's Math class specifies a static method atan2 which acts as a tan^-1 function (also known as "arctangent", hence "atan") and returns the angle in radians. (There is a method atan which takes one argument. See the linked Javadoc.)
In order to find the angle in degrees from the coordinate of your "player" to the coordinate of the mouse cursor, (I'll assume this "player" you make mention of has x and y coordinates), we need to do something like this:
double theta = Math.atan2(cursorY - player.getY(), cursorX - player.getX());
It is also of note that an angle of zero radians would indicate that the mouse is directly to the right of the player. You mention that the "default player image" points upwards; if you mean that before rotation, your image faces upward for the player, it would be more conventional to geometry and the Java implementation of atan2 to have your player face right "by default".

Though this was asked two years ago...
If you need the mouse to keep updating the mouse position in the window, see mouseMotionListener. The current you use to get the mouse position is relative to the whole screen. Just keep that in mind.
Otherwise, here is a method I use,
public double angleInRelation(int x1, int y1, int x2, int y2) {
// Point 1 in relation to point 2
Point point1 = new Point(x1, y1);
Point point2 = new Point(x2, y2);
int xdiff = Math.abs(point2.x - point1.x);
int ydiff = Math.abs(point2.y - point1.y);
double deg = 361;
if ( (point2.x > point1.x) && (point2.y < point1.y) ) {
// Quadrant 1
deg = -Math.toDegrees(Math.atan(Math.toRadians(ydiff) / Math.toRadians(xdiff)));
} else if ( (point2.x > point1.x) && (point2.y > point1.y) ) {
// Quadrant 2
deg = Math.toDegrees(Math.atan(Math.toRadians(ydiff) / Math.toRadians(xdiff)));
} else if ( (point2.x < point1.x) && (point2.y > point1.y) ) {
// Quadrant 3
deg = 90 + Math.toDegrees(Math.atan(Math.toRadians(xdiff) / Math.toRadians(ydiff)));
} else if ( (point2.x < point1.x) && (point2.y < point1.y) ) {
// Quadrant 4
deg = 180 + Math.toDegrees(Math.atan(Math.toRadians(ydiff) / Math.toRadians(xdiff)));
} else if ((point2.x == point1.x) && (point2.y < point1.y)){
deg = -90;
} else if ((point2.x == point1.x) && (point2.y > point1.y)) {
deg = 90;
} else if ((point2.y == point1.y) && (point2.x > point1.x)) {
deg = 0;
} else if ((point2.y == point2.y) && (point2.x < point1.x)) {
deg = 180;
}
if (deg == 361) {
deg = 0;
}
return deg;
}
In words, you get the angle of each of the θs as shown in the picture below and check if x or y are 0 and make a special case for that.
The origin is the middle of the picture and each of the points (marked with a hand-drawn cross) is where the mouse position is.