html5 乒乓球(碰撞检测)实例二技巧

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var ctx;
var canvas;
var ball_x=10;
var ball_y=10;
var ball_radius=10;
var ball_vx=10;
var ball_vy=8;
var wall_x=30;
var wall_y=40;
var wall_width=30;
var wall_height=60;
var box_x=0;
var box_y=0;
var box_width=300;
var box_height=300;
var bound_left=box_x+ball_radius;
var bound_right=box_x+box_width-ball_radius;
var bound_top=box_y+ball_radius;
var bound_bottom=box_y+box_height-ball_radius;
var unit=10;
function intersect(sx, sy, fx, fy, cx, cy, rad)
{
var dx;
var dy;
var t;
var rt;
dx = fx – sx;
dy = fy – sy;
t = 0.0 – (((sx – cx) * dx + (sy – cy) * dy) / (dx * dx + dy * dy));
if (t < 0.0)
{
t = 0.0;
}
else if (t > 1.0)
t = 1.0;
var dx1 = (sx + t * dx) – cx;
var dy1 = (sy + t * dy) – cy;
var rt = dx1 * dx1 + dy1 * dy1;
if (rt < rad * rad)
return true;
else
return false;
}
function move_ball()
{
ball_x=ball_x+ball_vx;
ball_y=ball_y+ball_vy;
if(ball_x<bound_left)
{
ball_x=bound_left;
ball_vx=-ball_vx;
}
if(ball_x>bound_right)
{
ball_x=bound_right;
ball_vx=-ball_vx;
}
if(ball_y<bound_top)
{
ball_y=bound_top;
ball_vy=-ball_vy;
}
if(ball_y>bound_bottom)
{
ball_y=bound_bottom;
ball_vy=-ball_vy;
}
//撞到上边
if(intersect(wall_x,wall_y,wall_x+wall_width,wall_y+wall_height,ball_x,ball_y,ball_radius))
{
ball_y=wall_y-ball_radius;
ball_vy=-ball_vy;
}
//撞到左边
if(intersect(wall_x,wall_y,wall_x,wall_y+wall_height,ball_x,ball_y,ball_radius))
{
ball_x=wall_x-ball_radius;
ball_vx=-ball_vx;
}
//撞到右边
if(intersect(wall_x+wall_width,wall_y,wall_x+wall_width,wall_y+wall_height,ball_x,ball_y,ball_radius))
{
ball_x=wall_x+wall_width+ball_radius;
ball_vx=-ball_vx;
}
//撞到下边
if(intersect(wall_x,wall_y+wall_height,wall_x+wall_width,wall_y+wall_height,ball_x,ball_y,ball_radius))
{
ball_y=wall_y+wall_height+ball_radius;
ball_vy=-ball_vy;
}
}
function move_wall(ev)
{
var keyCode;
if(event==null)
{
keyCode=window.event.keyCode;
window.event.preventDefault();
}
else
{
keyCode=event.keyCode;
event.preventDefault();
}
switch(keyCode)
{
case 37://left;
wall_x-=unit;
if(wall_x<bound_left)
wall_x=bound_left;
break;
case 38://up
wall_y-=unit;
if(wall_y<bound_top)
wall_y=bound_top;
break;
case 39://right
wall_x+=unit;
if(wall_x+wall_width>bound_right)
wall_x=bound_right-wall_width;
break;
case 40://down
wall_y+=unit;
if(wall_y+wall_height>bound_bottom)
wall_y=bound_bottom-wall_height;
break;
default:
break;
}
}
function draw_all()
{
ctx.beginPath();
ctx.clearRect(box_x,box_y,box_width,box_height);
ctx.fillStyle=”rgb(255,0,0)”;
//ctx.lineWidth=ball_radius;
ctx.arc(ball_x,ball_y,ball_radius,0,Math.PI*2,true);
ctx.fill();//note
ctx.fillStyle=”rgb(0,0,0)”;
ctx.fillRect(wall_x,wall_y,wall_width,wall_height);
ctx.strokeRect(box_x,box_y,box_width,box_height);
}
function init()
{
canvas=document.getElementById(‘canvas’);
ctx=canvas.getContext(‘2d’);
draw_all();
setInterval(draw_all,100);
setInterval(move_ball,50);
window.addEventListener(‘keydown’,move_wall,false);//note
}
难点

小球和砖块的碰撞检测以及碰撞处理
将砖块分解为4条线段
分别对小球和每条线段进行碰撞检测。

小球和线段的碰撞检测在另一篇文章http://www.liuzhongwei.com/html5/93997.html中有介绍。