Losing a bouncing ball's position

I'm trying to draw a Bouncing ball using WIN32 GUI, so i define a 2d vector for ball position and a rectangle map with my screen size

struct pos {
    float x;
    float y;
};
RECT maprect = {0, 0, 800, 600};
pos ballpos;
ballpos.x=300;
ballpos.y=300;

(of course im using a class for 2dvec and it's more than this)

and i draw a ellipse

Ellipse(backbufferDC, (int)ballpos.x-45, (int)ballpos.y-45,
        (int)ballpos.x+45, (int)ballpos.y+45);

now to make my ellipse bounce i use this code in in my message loop and it works fine:

bool balldown = false; 
if (ballpos.y > maprect.bottom-40) {
    balldown = true;
}
else if (ballpos.y < maprect.top+300) {
    balldown = false;
}

if (ballpos.y > maprect.bottom-40) {
    balldown = true ;
}
else if(ballpos.y < maprect.top+300) {
    balldown = false;
}

if (!balldown) {
    vel+=1;
    ballpos.y +=3;
}
else {
    ballpos.y-=3;
}


Ellipse(bbdc, (int)tankpos.x-45, (int)tankpos.y-45,
        (int)tankpos.x+45, (int)tankpos.y+45);

but to make it look more realistic i decided to change the ball velocity on the move so i end up with this code

float vel;
if (ballpos.y > maprect.bottom-40) {
    balldown = true ;
}
else if (ballpos.y < maprect.top+300) {
    balldown = false ;
}

if (!balldown) {
    vel+=0.5f;
    ballpos.y +=vel;
}
else {
    vel-=0.5f;
    ballpos.y-=vel;
}



Ellipse(bbdc, (int)tankpos.x-45, (int)tankpos.y-45,
        (int)tankpos.x+45, (int)tankpos.y+45);

and it looks much better now, but the problem is the ball ju开发者_运维百科st bounces once, then the second time it goes into the ground and disappears!

if (!balldown) {
    vel+=0.5f;
    ballpos.y +=vel;
}
else {
    vel-=0.5f;
    ballpos.y-=vel;
}

This logic is wrong. Gravity always causes acceleration in the same direction, down.

What you should do is handle collision. An elastic collision with a massive wall could look like

vel = -vel;

So in the end you have

vel -= 0.5f;
ballpos.y += vel;
if (ballpos.y <= floory) {
    vel = -vel;
    ballpos.y = 2*floory - ballpos.y;
}

 else {
        vel-=0.5f;
        ballpos.y-=vel;
      }

You might be getting the negative values here.