如何旋转 HTML5 画布的现有内容?

问题描述

有没有办法通过 Javascript 来旋转 HTML5 画布的现有内容?我知道可以旋转将要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.

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.

缩放现有画布内容的相同问题...

Same question to scale the existing canvas content...

我知道 getImageData/putImageData 提供了转换像素数组的潜力，但它太慢且效率低下.

I know getImageData/putImageData provide a potential to transform the pixel array, but it's just too slow and inefficient.

推荐答案

使用临时画布非常容易.

It's pretty easy to do with a temp canvas.

现场演示

现场演示动画(只是为了见鬼)

Live Demo Animated (just for the heck of it)

上面的例子画了 2 个盒子，然后在 0,0 到 200,200 之间旋转和缩放

The above example draws 2 boxes, then rotates and scales from 0,0 to 200,200

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();

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