javascript 如何旋转 HTML5 画布的现有内容？
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How to rotate the existing content of HTML5 canvas?
提问by Tony T
Is there a way to rotate the existing content of HTML5 canvas by Javascript? I know it's possible to rotate an image that will be drawn on to canvas, but I want to rotate the content that has been drawn on to canvas, for example, a 200x200 corner of a 400x400 canvas, or any specific region of an existing canvas.
有没有办法通过Javascript旋转HTML5画布的现有内容？我知道可以旋转将绘制到画布上的图像，但我想旋转已绘制到画布上的内容，例如，400x400 画布的 200x200 角，或现有画布的任何特定区域.
Same question to scale the existing canvas content...
缩放现有画布内容的相同问题......
I know getImageData/putImageData provide a potential to transform the pixel array, but it's just too slow and inefficient.
我知道 getImageData/putImageData 提供了转换像素阵列的潜力，但它太慢且效率低下。
回答by Loktar
It's pretty easy to do with a temp canvas.
使用临时画布很容易做到。
Live Demo Animated(just for the heck of it)
现场演示动画（只是为了它）
The above example draws 2 boxes, then rotates and scales from 0,0 to 200,200
上面的例子绘制了 2 个盒子，然后从 0,0 到 200,200 旋转和缩放
var canvas = document.getElementById("canvas"),
ctx = canvas.getContext("2d");
canvas.width = canvas.height = 400;
// fill the canvas black, and draw 2 boxes
ctx.fillStyle = "#000";
ctx.fillRect(0,0,400,400);
ctx.fillStyle = "rgb(255,0,0)";
ctx.fillRect(10,10,190,190);
ctx.fillStyle = "rgb(255,255,0)";
ctx.fillRect(250,250,90,90);
// Create a temp canvas to store our data (because we need to clear the other box after rotation.
var tempCanvas = document.createElement("canvas"),
tempCtx = tempCanvas.getContext("2d");
tempCanvas.width = canvas.width;
tempCanvas.height = canvas.height;
// put our data onto the temp canvas
tempCtx.drawImage(canvas,0,0,canvas.width,canvas.height);
// Append for debugging purposes, just to show what the canvas did look like before the transforms.
document.body.appendChild(tempCanvas);
// Now clear the portion to rotate.
ctx.fillStyle = "#000";
ctx.fillRect(0,0,200,200);
ctx.save();
// Translate (190/2 is half of the box we drew)
ctx.translate(190/2, 0);
// Scale
ctx.scale(0.5,0.5);
// Rotate it
ctx.rotate(45*Math.PI/180);
// Finally draw the image data from the temp canvas.
ctx.drawImage(tempCanvas,0,0,200,200,10,10,190,190);
ctx.restore();
回答by Jonas
If you first want to draw on a canvasand then rotate it for use on e.g. corners, you can to that when you "clone" the canvas or by using CSS.
如果您首先想在 a 上绘制canvas然后旋转它以用于例如角落，则可以在“克隆”画布或使用 CSS 时做到这一点。
Examples
例子
Get the first canvas element:
获取第一个画布元素：
var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
draw on it:
画在它上面：
ctx.fillStyle = 'blue';
ctx.fillRect(0,0, 25, 5);
ctx.fill();
ctx.fillStyle = 'red';
ctx.fillRect(25, 0, 25, 5);
ctx.fill();
clone it to another canvas (that is rotated by CSS):
将其克隆到另一个画布（由 CSS 旋转）：
var ctx2 = document.getElementById("canvas2").getContext("2d");
ctx2.drawImage(canvas, 0,0);
or rotate the canvas while you "clone" it:
或在“克隆”它时旋转画布：
var ctx3 = document.getElementById("canvas3").getContext("2d");
ctx3.rotate(Math.PI/2);
ctx3.translate(0,-50);
ctx3.drawImage(canvas, 0,0);
here is the CSS for rotating it:
这是用于旋转它的 CSS：
#canvas2 {
-webkit-transform:rotate(90deg);
-moz-transform:rotate(90deg);
-o-transform:rotate(90deg);
-ms-transform:rotate(90deg);
}
Here is the full example:
这是完整的示例：
<!DOCTYPE html>
<html>
<head>
<script>
window.onload = function() {
var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
ctx.fillStyle = 'blue';
ctx.fillRect(0,0, 25, 5);
ctx.fill();
ctx.fillStyle = 'red';
ctx.fillRect(25, 0, 25, 5);
ctx.fill();
var ctx2 = document.getElementById("canvas2").getContext("2d");
ctx2.drawImage(canvas, 0,0);
var ctx3 = document.getElementById("canvas3").getContext("2d");
ctx3.rotate(Math.PI/2);
ctx3.translate(0,-50);
ctx3.drawImage(canvas, 0,0);
}
</script>
<style>
#canvas2 {
-webkit-transform:rotate(90deg);
-moz-transform:rotate(90deg);
-o-transform:rotate(90deg);
-ms-transform:rotate(90deg);
}
</style>
</head>
<body>
<canvas id="canvas" width="50" height="50"></canvas>
<canvas id="canvas2" width="50" height="50"></canvas>
<canvas id="canvas3" width="50" height="50"></canvas>
</body>
</html>
