Rectangle Bounce collision detection - c++

Question

I've created a program using SDL in which a rectangle continuously collides within the walls of the program, but the collision checking is not working properly.
Here is the code:`
int main(int argc, char *argv[]){
//variable Initialization]
width = height = 45;
srcX = srcY = 0;
destY = destX = 0;
vlc = 1;
SDL_Init(SDL_INIT_VIDEO);
screen = SDL_SetVideoMode(640, 480, 32, SDL_SWSURFACE);
SDL_WM_SetCaption("Bouncing Balls","./ball.jpg");
backg = IMG_Load("./back.png");
ball = IMG_Load("./ball.jpg");
while (checkBounce){
//Increase velocity
destX += vlc;
destY += vlc;
//Collision Checking
if (destX < 0){
destX = 0;
vlc = -vlc;
destX += vlc;
}
if (destY < 0){
destY = 0;
vlc = -vlc;
destY += vlc;
}
if (destY + height > 480){
destY = 480 - height;
vlc = -vlc;
}
if (destX + width > 640){
destX = 640 - width;
vlc = -vlc;
}
if (SDL_PollEvent(&event)){
if (event.type == SDL_QUIT)
checkBounce = false;
}
//Applying Surfaces
applySurface(0, 0, backg, screen);
applyBall(srcX, srcY, destX, destY, width, height, ball, screen);
SDL_Flip(screen);
}
SDL_Quit();
return 0;
}
Here is the gif Image What is happening :Bouncing Rectangle.gif

Answer 1

I'm assuming that the expected result is for the rectangle to bounce off of the walls correctly?
You need to separate your velocity in to x and y components rather than using a single number. This is because velocity is two dimensional.
Your program is causing both the x and y components to become negative whenever a collision is detected. This causes the rectangle to bounce backwards along its path.
Here's an edit:
int main(int argc, char *argv[]){
//variable Initialization]
width = height = 45;
srcX = srcY = 0;
destY = destX = 0;
vlcX = 1;
vlcY = 1;
SDL_Init(SDL_INIT_VIDEO);
screen = SDL_SetVideoMode(640, 480, 32, SDL_SWSURFACE);
SDL_WM_SetCaption("Bouncing Balls","./ball.jpg");
backg = IMG_Load("./back.png");
ball = IMG_Load("./ball.jpg");
while (checkBounce){
//Increase velocity
destX += vlcX;
destY += vlcY;
//Collision Checking
if (destX < 0){
destX = 0;
vlcX = -vlcX;
destX += vlcX;
}
if (destY < 0){
destY = 0;
vlcY = -vlcY;
destY += vlcY;
}
if (destY + height > 480){
destY = 480 - height;
vlcY = -vlcY;
}
if (destX + width > 640){
destX = 640 - width;
vlcX = -vlcX;
}
if (SDL_PollEvent(&event)){
if (event.type == SDL_QUIT)
checkBounce = false;
}
//Applying Surfaces
applySurface(0, 0, backg, screen);
applyBall(srcX, srcY, destX, destY, width, height, ball, screen);
SDL_Flip(screen);
}
SDL_Quit();
return 0;
}