HTML5画布 - 旋转对象而不移动坐标 [英] HTML5 canvas - rotate object without moving coordinates
问题描述
我拥有的是：
What I have is this:
我想要的是旋转红色矩形，例如20度，但这是我最终得到的：
What I want is to rotate the red rectangle e.g. 20 degrees, but this is what I end up with:
如您所见，矩形完美旋转，但它已移动并且不再与黑色物体相匹配。
As you can see, the rectangle is rotated perfectly, but it's moved and doesn't match the black object anymore.
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从其位置移动。
I want to make the rectangle turn without moving from its position.
谢谢。
推荐答案
听起来你想围绕它的中心点旋转矩形。 / strong>
Sounds like you want to rotate the rect around its centerpoint.
红色矩形是原始的，黄色矩形围绕中心点旋转。
Red rectangle is original, Yellow rectangle is rotated around the centerpoint.
要做到这一点，你需要先<$旋转前c $ c> context.translate 到rect的中心点。
To do that you need to first context.translate to the rect's centerpoint before rotating.
// 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。
Note that the context is now in its rotated state.
这意味着绘图位置[0,0]在视觉上为[x +宽度/ 2，y +高度/ 2]。
That means drawing position [0,0] is visually at [ x+width/2, y+height/2 ].
所以你必须画画[-width / 2，-height / 2]处的旋转矩形（不是原始未旋转的x / y）。
So you must draw the rotated rect at [ -width/2, -height/2 ] (not at the original unrotated 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>
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