[英]Canvas rotate from bottom center image angle?
如何从底部中心角度旋转带有canvas html5元素的图像?
<html>
<head>
<title>test</title>
<script type="text/javascript">
function startup() {
var canvas = document.getElementById('canvas');
var ctx = canvas.getContext('2d');
var img = new Image();
img.src = 'player.gif';
img.onload = function() {
ctx.translate(185, 185);
ctx.rotate(90 * Math.PI / 180);
ctx.drawImage(img, 0, 0, 64, 120);
}
}
</script>
</head>
<body onload='startup();'>
<canvas id="canvas" style="position: absolute; left: 300px; top: 300px;" width="800" height="800"></canvas>
</body>
</html>
不幸的是,这似乎使它从图像的左上角旋转。 任何想法?
编辑:最后,对象(太空飞船)必须像时钟指针一样旋转,就好像它在向右/向左旋转一样。
首先,您必须平移到想要旋转的点。 在这种情况下,图像尺寸为64 x120。要围绕底部中心旋转,您需要转换为32、120。
ctx.translate(32, 120);
这会将您带到图像的底部中心。 然后旋转画布:
ctx.rotate(90 * Math.PI/180);
其中旋转90度。
然后,当您绘制图像时,请尝试以下操作:
ctx.drawImage(img, -32, -120, 64, 120);
? 那样有用吗?
正确的答案当然是文森特的答案。
我对这种旋转/平移的东西感到有些困惑,我认为我的小实验可能会很有趣:
<html>
<head>
<title>test</title>
<script type="text/javascript">
canvasW = canvasH = 800;
imgW = imgH = 128;
function startup() {
var canvas = document.getElementById('canvas');
var ctx = canvas.getContext('2d');
// Just to see what I do...
ctx.strokeRect(0, 0, canvasW, canvasH);
var img = new Image();
img.src = 'player.png';
img.onload = function() {
// Just for reference
ctx.drawImage(img, 0, 0, 128, 128);
ctx.drawImage(img, canvasW/2 - imgW/2, canvasH/2 - imgH/2, 128, 128);
mark(ctx, "red");
// Keep current context (transformations)
ctx.save();
// Put bottom center at origin
ctx.translate(imgW/2, imgH);
// Rotate
// Beware the next translations/positions are done along the rotated axis
ctx.rotate(45 * Math.PI / 180);
// Mark new origin
mark(ctx, "red");
// Restore position
ctx.translate(-imgW/2, -imgH);
ctx.drawImage(img, 0, 0, imgW, imgH);
mark(ctx, "green");
// Draw it an wanted position
ctx.drawImage(img, canvasW/2, canvasH/3, imgW, imgH);
// Move elsewhere:
ctx.translate(canvasW/2, canvasH/2);
ctx.drawImage(img, 0, 0, imgW, imgH);
mark(ctx, "blue");
ctx.restore();
}
}
function mark(ctx, color) {
ctx.save();
//~ ctx.fillStyle = color;
//~ ctx.fillRect(-2, -2, 4, 4);
ctx.strokeStyle = color;
ctx.strokeRect(0, 0, imgW, imgH);
ctx.restore();
}
</script>
</head>
<body onload='startup();'>
<canvas id="canvas" style="position: absolute; left: 300px; top: 300px;" width="800" height="800"></canvas>
</body>
</html>
我的绊脚石是很难定位旋转的图形,因为它是沿着旋转的轴进行的:要么我们必须做一些trigo数学以在原始原点上绘画,要么我们必须在辅助的隐藏画布上绘制然后在其上绘制目标之一。
除非有人有更好的主意?
当需要旋转从指定点(中心)开始的文本时,我已经使用了此功能?
ctx.save();
ctx.translate(x,y);
ctx.rotate(degree*Math.PI/180);
ctx.translate(-x,-y);
ctx.fillText(text,x,y);
ctx.stroke();
ctx.restore();
<html>
<head>
<title>Canvas Pinball flippers by stirfry</title>
<script type="application/x-javascript">
/*THIS SCRIPT ADAPTED BY STIRFRY. SOURCE TEETHGRINDER no warranty or liability implied or otherwise. use at your own risk. No credit required. Enjoy.stirfry.thank(you)*/
var img = new Image();
//img.src = "flipper.gif";//right
img.src="http://i39.tinypic.com/k1vq0x.gif"
var img2 = new Image();
//img2.src = "flipper2.gif";//left
img2.src ="http://i42.tinypic.com/14c8wht.gif"
var gAngle = 0;
gAngle = 60;
stop = false;
inertia = .8;
vel = 10;
k = 0.1;
function drawagain(){
gAngle = 60;
stop = false;
inertia = .8;
vel = 10;
k = 0.1;
draw()
}
function draw(){
var ctx = document.getElementById('canvas').getContext('2d');
ctx.save();
vel = ( vel * inertia ) + ( -gAngle * k );
gAngle += vel;
ctx.fillStyle = 'rgb(255,255,255)';
ctx.fillRect (0, 0, 600, 600);
ctx.translate(380, 480); //location of the system
ctx.rotate( gAngle * Math.PI / 180 );//rotate first then draw the flipper
ctx.drawImage(img, -105, -16);
ctx.restore();
ctx.save();
ctx.translate(120, 480); //location of the system
ctx.rotate( -1*gAngle * Math.PI / 180 );//rotate first then draw the flipper
ctx.drawImage(img2, -18, -16);
ctx.restore();
if( !stop )
setTimeout(draw, 30);
}
</script>
<style type="text/css">
body { margin: 20px; font-family: arial,verdana,helvetica; background: #fff;}
h1 { font-size: 140%; font-weight:normal; color: #036; border-bottom: 1px solid #ccc; }
canvas { border: 2px solid #000; float: left; margin-right: 20px; margin-bottom: 20px; }
pre { float:left; display:block; background: rgb(238,238,238); border: 1px dashed #666; padding: 15px 20px; margin: 0 0 10px 0; }
.gameLayer {position: absolute; top: 0px; left: 0px;}
#scoreLayer {font-family: arial; color: #FF0000; left: 10px; font-size: 70%; }
#windowcontainer {position:relative; height:300px;}
</style>
</head>
<body onload="draw()">
<div id="windowcontainer">
<canvas id="canvas" width="500" height="500"></canvas>
<INPUT VALUE="flip" TYPE=BUTTON onClick="drawagain();"><br/>
</div>
</body>
</html>
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