HTML5画布 - 旋转对象而不移动坐标

时间:2013-06-15 16:23:41

标签: javascript html5-canvas

我拥有的是: Tank

我想要的是旋转红色矩形,例如20度,但这是我最终得到的: Bad rotation

正如您所看到的,矩形完美旋转,但它已移动并且不再与黑色物体相匹配。

我的代码是:

context.save();
context.rotate(angle * Math.PI / 180); // in the screenshot I used angle = 20
context.translate(angle * 4, 2);
context.fillStyle = "red";
context.fillRect(x + 14, y - 16, 5, 16);
context.restore();

我想让矩形转动而不会移动它的位置。

感谢。

3 个答案:

答案 0 :(得分:50)

听起来你想围绕其中心点旋转矩形。

红色矩形是原始的,黄色矩形围绕中心点旋转。

enter image description here

要做到这一点,你需要在旋转前先将context.translate放到矩形的中心点。

// move the rotation point to the center of the rect

    ctx.translate( x+width/2, y+height/2 );

// rotate the rect

    ctx.rotate(degrees*Math.PI/180);

请注意,上下文现在处于旋转状态。

这意味着绘图位置[0,0]在视觉上为[x +宽度/ 2,y +高度/ 2]。

所以你必须在[-width / 2,-height / 2]处绘制旋转的矩形(而不是原始未旋转的x / y)。

// draw the rect on the transformed context
// Note: after transforming [0,0] is visually [-width/2, -height/2]
//       so the rect needs to be offset accordingly when drawn

    ctx.rect( -width/2, -height/2, width,height);

这是代码和小提琴:http://jsfiddle.net/m1erickson/z4p3n/

<!doctype html>
<html>
<head>
<link rel="stylesheet" type="text/css" media="all" href="css/reset.css" /> <!-- reset css -->
<script type="text/javascript" src="http://code.jquery.com/jquery.min.js"></script>

<style>
    body{ background-color: ivory; }
    canvas{border:1px solid red;}
</style>

<script>
$(function(){

    var canvas=document.getElementById("canvas");
    var ctx=canvas.getContext("2d");

    var startX=50;
    var startY=80;

    // draw an unrotated reference rect
    ctx.beginPath();
    ctx.rect(startX,startY,100,20);
    ctx.fillStyle="blue";
    ctx.fill();

    // draw a rotated rect
    drawRotatedRect(startX,startY,100,20,45);

    function drawRotatedRect(x,y,width,height,degrees){

        // first save the untranslated/unrotated context
        ctx.save();

        ctx.beginPath();
        // move the rotation point to the center of the rect
        ctx.translate( x+width/2, y+height/2 );
        // rotate the rect
        ctx.rotate(degrees*Math.PI/180);

        // draw the rect on the transformed context
        // Note: after transforming [0,0] is visually [x,y]
        //       so the rect needs to be offset accordingly when drawn
        ctx.rect( -width/2, -height/2, width,height);

        ctx.fillStyle="gold";
        ctx.fill();

        // restore the context to its untranslated/unrotated state
        ctx.restore();

    }

}); // end $(function(){});
</script>

</head>

<body>
    <canvas id="canvas" width=300 height=300></canvas>
</body>
</html>

答案 1 :(得分:9)

您可以使用一个简单的函数来替换翻译和旋转:

此功能可让您绘制一条从x和y开始的行,并以角度结束:

function lineToAngle(ctx, x1, y1, length, angle) {

    angle *= Math.PI / 180;

    var x2 = x1 + length * Math.cos(angle),
        y2 = y1 + length * Math.sin(angle);

    ctx.moveTo(x1, y1);
    ctx.lineTo(x2, y2);

    return {x: x2, y: y2};
}

用法:

lineToAngle(ctx, x, y, length, angle);

演示

var canvas = document.querySelector('canvas'),
    ctx = canvas.getContext('2d'),
    x = 100, y =50, length = 40, angle = 0, dlt = -2;

(function animate() {
    
    //clear
    ctx.clearRect(0, 0, canvas.width, canvas.height);
    
    ctx.beginPath();
    lineToAngle(ctx, x, y, length, angle);
    ctx.lineWidth = 10;
    ctx.stroke();

    angle += dlt;
    if (angle < -90 || angle > 0) dlt = -dlt;
    
    requestAnimationFrame(animate);
})();

function lineToAngle(ctx, x1, y1, length, angle) {

    angle *= Math.PI / 180;
    
    var x2 = x1 + length * Math.cos(angle),
        y2 = y1 + length * Math.sin(angle);
    
    ctx.moveTo(x1, y1);
    ctx.lineTo(x2, y2);

    return {x: x2, y: y2};
}
body {background-color:#555557}
canvas {background:#ddd;border:1px solid #000}
<canvas></canvas>

enter image description here

答案 2 :(得分:0)

在矩形的特定情况下,您可以只画一条线。

在 HTML5 画布中,线条的绘制与矩形完全一样,因此,您可以使用此功能:

let canvas = document.getElementById('myCanvas')

let ctx = canvas.getContext('2d')

function drawRotatedRect(ctx, x, y, width, height, angle) {
  angle *= Math.PI / 180
  ctx.lineWidth = height / 2
  ctx.beginPath()
  ctx.moveTo(x, y)
  ctx.lineTo(x + width * Math.cos(angle), y + width * Math.sin(angle))
  ctx.stroke()
}


drawRotatedRect(ctx, 30, 30, 200, 100, 20)
canvas {
  border: 1px solid black;
  width: 100%;
}
<!DOCTYPE html>
<html>
<head>
  <meta charset="UTF-8">
  <title>Rotated Rectangle</title>
</head>
<body>
  <canvas id='myCanvas'></canvas>
</body>
</html>