如何使用画布90°逆时针旋转从div渲染的.png? [英] How to rotate .png rendered from div with canvas 90° ccw?

问题描述

首先让我说我是使用Canvas元素的新手.我已经设法通过jQuery和html2canvas.js将div渲染为png，但是我想将png逆时针旋转90度.

我假设我需要在canvas阶段执行此操作，然后再将其输出为png，以便即使在下载图片后也能保持持久性-我该怎么做?

这是我目前正在执行的操作(减去旋转)

html2canvas($("#portrait_back_div"), {
    onrendered: function (canvas) {
        //document.getElementById('canvas').appendChild(canvas);
        var data = canvas.toDataURL('image/png');

        var image = new Image();
        image.src = data;
        document.getElementById('portrait_back_png').appendChild(image);
        //$( "#portrait_back_div" ).hide();
    }
});

即使这似乎是一个基本问题，我也很感谢我能提供的任何帮助. 这个小提琴基本上以与您要查看的项目相同的原理工作.

解决方案

您必须创建一个新的画布，然后平移到中心以将原点移动到中心(此处用作旋转的枢轴)，旋转然后绘制图像. /p>

最后从旋转的画布中提取图像.

示例

 html2canvas($("#portrait_back_div"), {
  onrendered: function (canvas) {

    // create intermediate canvas
    var rotCanvas = document.createElement("canvas");

    // swap width and height
    rotCanvas.width = canvas.height;
    rotCanvas.height = canvas.width;

    // get context
    var rctx = rotCanvas.getContext("2d");

    // translate to center (rotation pivot)
    rctx.translate(rotCanvas.width * 0.5, rotCanvas.height * 0.5);

    // rotate -90° (CCW)
    rctx.rotate(-Math.PI * 0.5);

    // draw image offset so center of image is on top of pivot
    rctx.drawImage(canvas, -canvas.width * 0.5, -canvas.height * 0.5);

    // extract image from rotate canvas
    var data = rotCanvas.toDataURL('image/png');

    var image = new Image();
    image.src = data;
    document.getElementById('portrait_back_png').appendChild(image);
    //$( "#portrait_back_div" ).hide();
  }
});
 

 // proof-of-concept example
var img = new Image;
img.onload = function() {
  var rotCanvas = document.createElement("canvas");
  var rctx = rotCanvas.getContext("2d");
  rotCanvas.width = this.height;
  rotCanvas.height = this.width;
  rctx.translate(rotCanvas.width * 0.5, rotCanvas.height * 0.5);
  rctx.rotate(-Math.PI * 0.5);
  rctx.drawImage(this, -this.width * 0.5, -this.height * 0.5);
  document.body.appendChild(this);
  document.body.appendChild(rotCanvas)
};
img.src = "//i.imgur.com/YqeJUhl.jpg?1"; 

 <h4>Original on top, rotated canvas at bottom</h4> 

Let me start off by saying that I'm new to working with the Canvas element. I've managed to get a div to render as a png with jQuery and html2canvas.js, but I'd like to rotate the png 90 degrees counter-clockwise.

I assume I need to do this in the canvas stage, before the outputting it as a png for it to be persistent even when the image is downloaded – how would I go about and accomplish that?

This is what I've got at the moment, which is working (minus the rotation)

html2canvas($("#portrait_back_div"), {
    onrendered: function (canvas) {
        //document.getElementById('canvas').appendChild(canvas);
        var data = canvas.toDataURL('image/png');

        var image = new Image();
        image.src = data;
        document.getElementById('portrait_back_png').appendChild(image);
        //$( "#portrait_back_div" ).hide();
    }
});

I appreciate any help I can get, even though this might seem like a basic question. This fiddle basically works on the same principles as my project it you want to take a look.

解决方案

You would have to create a new canvas, then translate to center to move origin to center (here functions as pivot for rotation), rotate then draw image.

Finally extract image from the rotated canvas.

Example

html2canvas($("#portrait_back_div"), {
  onrendered: function (canvas) {

    // create intermediate canvas
    var rotCanvas = document.createElement("canvas");

    // swap width and height
    rotCanvas.width = canvas.height;
    rotCanvas.height = canvas.width;

    // get context
    var rctx = rotCanvas.getContext("2d");

    // translate to center (rotation pivot)
    rctx.translate(rotCanvas.width * 0.5, rotCanvas.height * 0.5);

    // rotate -90° (CCW)
    rctx.rotate(-Math.PI * 0.5);

    // draw image offset so center of image is on top of pivot
    rctx.drawImage(canvas, -canvas.width * 0.5, -canvas.height * 0.5);

    // extract image from rotate canvas
    var data = rotCanvas.toDataURL('image/png');

    var image = new Image();
    image.src = data;
    document.getElementById('portrait_back_png').appendChild(image);
    //$( "#portrait_back_div" ).hide();
  }
});

// proof-of-concept example
var img = new Image;
img.onload = function() {
  var rotCanvas = document.createElement("canvas");
  var rctx = rotCanvas.getContext("2d");
  rotCanvas.width = this.height;
  rotCanvas.height = this.width;
  rctx.translate(rotCanvas.width * 0.5, rotCanvas.height * 0.5);
  rctx.rotate(-Math.PI * 0.5);
  rctx.drawImage(this, -this.width * 0.5, -this.height * 0.5);
  document.body.appendChild(this);
  document.body.appendChild(rotCanvas)
};
img.src = "//i.imgur.com/YqeJUhl.jpg?1";

<h4>Original on top, rotated canvas at bottom</h4>

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