CanvasRenderingContext2D 翻转变换 [英] CanvasRenderingContext2D flip transformation

问题描述

我找到了

这是示例代码和演示:

var canvas=document.getElementById("canvas");var ctx=canvas.getContext("2d");var img=new Image();img.onload=开始；img.src="https://dl.dropboxusercontent.com/u/139992952/multple/marioRunningRight.png";函数开始(){//原来的ctx.drawImage(img,200,200);//水平翻转ctx.translate(200,0);ctx.scale(-1,1);ctx.drawImage(img,0,200);ctx.setTransform(1,0,0,1,0,0);//垂直翻转ctx.translate(0,200);ctx.scale(1,-1);ctx.drawImage(img,200,0);ctx.setTransform(1,0,0,1,0,0);//标签ctx.font='18px verdana';ctx.textAlign='center';ctx.fillText('原始',275,375);ctx.fillText('scale(-1,1)',125,375);ctx.fillText('水平翻转',125,395);ctx.fillText('scale(1,-1)',275,20);ctx.fillText('垂直翻转',275,40);}

I found this CanvasRenderingContext2D and i played around a little bit with it. I was able to scale and to rotate my Image using this context:

crop: function () {
    var canvas = document.createElement("canvas");
    var context = canvas.getContext("2d");

    canvas.width = this.options.width / this.scale;
    canvas.height = this.options.height / this.scale;

    var currWidth = this.imgWidth * this.scale;
    var currHeight = this.imgHeight * this.scale;
    var correctX = (currWidth - this.imgWidth) / 2;
    var correctY = (currHeight - this.imgHeight) / 2;
    var sourceX = (this.posX - correctX) / this.scale;
    var sourceY = (this.posY - correctY) / this.scale;

    context.translate(sourceX, sourceY);
    context.translate(this.imgWidth / 2, this.imgHeight / 2);
    context.rotate(this.rotate * Math.PI / 180);
    context.drawImage(this.imgFull, this.imgWidth / 2 * -1, this.imgHeight / 2 * -1);

    this.options.modal.remove();
    this.promise.resolve(canvas);
},

Unfortunately i could not find any function to flip the canvas vertically or horizontally. In code i thought i could do something like:

if(self.flipV) {
    context.rotateY(180);
}

if(self.flipH) {
    context.rotateX(180);
}

But i could not find any methods for rotating on the y- or x-axis. Is there any way i could perform my flip transformation here?

解决方案

Although it's not obvious, you can flip using the context.scale method.

Flip horizontally with context.scale(-1,1) and flip vertically with context.scale(1,-1).

Just like with rotation, you must set the rotation point using context.translate before doing your context.scale.

Here's example code and a Demo:

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

var img=new Image();
img.onload=start;
img.src="https://dl.dropboxusercontent.com/u/139992952/multple/marioRunningRight.png";
function start(){

  // original
  ctx.drawImage(img,200,200);

  // horizontal flip
  ctx.translate(200,0);
  ctx.scale(-1,1);
  ctx.drawImage(img,0,200);
  ctx.setTransform(1,0,0,1,0,0);

  // vertical flip
  ctx.translate(0,200);
  ctx.scale(1,-1);
  ctx.drawImage(img,200,0);
  ctx.setTransform(1,0,0,1,0,0);

  // labels
  ctx.font='18px verdana';
  ctx.textAlign='center';
  ctx.fillText('Original',275,375);
  ctx.fillText('scale(-1,1)',125,375);
  ctx.fillText('Horizontal Flip',125,395);
  ctx.fillText('scale(1,-1)',275,20);
  ctx.fillText('Vertical Flip',275,40);
}

<canvas id="canvas" width=400 height=425></canvas>

  

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