Question

问题：当我旋转添加的图像时，图像不会像我希望的那样居中。我一直在画布上方的白色区域上方。

尝试使用16：9比例图像，旋转后你会看到我的问题。这将用于移动，因此图片的宽度可以是最大640。

HTML：

<div id="files">
    <input type="file" id="imageLoader" name="imageLoader" />
</div>
<canvas id="imageCanvas"></canvas>
<canvas id="preview"></canvas>
        <input type="button" id="saveButton" value="Save" />
<div id="rotate">
        <button type="button" id="rotateLeft">Rotate -90°</button>
        <button type="button" id="rotateRight">Rotate 90°</button>
    </div>

JS：

 var imageLoader = document.getElementById('imageLoader');
    imageLoader.addEventListener('change', handleImage, false);
    var image = "";
    var canvas = document.getElementById('imageCanvas');
    var ctx = canvas.getContext("2d");
    var fileName = "";
    var angleInDegrees = 0;
    function handleImage(e) {
        var reader = new FileReader();
        reader.onload = function(event) {
            var img = new Image();
            img.onload = function() {
                canvas.width = img.width;
                canvas.height = img.height;
                ctx.drawImage(img, 0,0);
                fileName = img.name;
                image = img;
            };
            img.src = event.target.result;
        };
        reader.readAsDataURL(e.target.files[0]);
    }

    function drawRotated(degrees) {
        ctx.clearRect(0, 0, canvas.width, canvas.height);
        ctx.save();
        ctx.translate(canvas.width / 2, canvas.height / 2);
        ctx.rotate(degrees * Math.PI / 180);
        ctx.drawImage(image, -image.width / 2, -image.width / 2);
        ctx.restore();
    }

    $("#rotateRight").click(function () {
        angleInDegrees += 90;
        drawRotated(angleInDegrees);
    });

    $("#rotateLeft").click(function () {
        angleInDegrees -= 90;
        drawRotated(angleInDegrees);
    });

jsfiddle.net/dJ5BH

这是我的新小提琴:)如果我希望它最大640宽度怎么办？

Answer 1

你的drawImage中有一个拼写错误：

ctx.drawImage(image, -image.width / 2, -image.height / 2);

除非您的图像始终是方形，否则您必须调整画布大小以允许旋转的图像适合。

演示如何调整画布大小：http://jsfiddle.net/m1erickson/3E99A/

如果您不想更改画布大小，则会剪裁图像。此外，如果图像是横向的，则必须在drawimage中翻转宽度/高度：

// flip width/height when the image is in landscape orientation

ctx.drawImage(image, -image.height/2, -image.width/2);

关于如何调整画布大小的代码：

<!doctype html>
<html>
<head>
<link rel="stylesheet" type="text/css" media="all" href="css/reset.css" /> <!-- reset css -->
<script type="text/javascript" src="http://code.jquery.com/jquery.min.js"></script>
    <style>
        #canvas{
          border: 1px solid green;
        }
    </style>
    <script>
    $(function(){
         var canvas=document.getElementById("canvas");
         var ctx=canvas.getContext("2d");
         var imgWidth=200;
         var imgHeight=300;
         var size={width:imgWidth, height:imgHeight};
         var rotation=0;
         var deg2Rad=Math.PI/180;
         var count1=0;
         var count2=0;

          var img=new Image();
          img.onload=function(){
              imgWidth=img.width;
              imgHeight=img.height;
              size={width:imgWidth, height:imgHeight};
              draw();
          }
          img.src="https://dl.dropboxusercontent.com/u/139992952/stackoverflow/card.png";

      function draw(){
         canvas.width=size.width;
         canvas.height=size.height; 

         // calculate the centerpoint of the canvas
         var cx=canvas.width/2;
         var cy=canvas.height/2;
         var info=document.getElementById("info");
         info.innerHTML="canvas size: "+(count1++)+": "+cx+" / "+cy;

         // draw the rect in the center of the newly sized canvas
         ctx.clearRect(0,0,canvas.width,canvas.height);
         ctx.fillStyle="rgba(216,216,150,1.0)";
         ctx.translate(cx,cy);
         ctx.rotate(rotation * deg2Rad);
         ctx.drawImage(img,-imgWidth/2,-imgHeight/2);
      }

      document.getElementById("rotate").addEventListener("click", resizeClicked, false);

      function resizeClicked(e){
        rotation+=30;
        newSize(imgWidth,imgHeight,rotation);
        draw();
      }

      function newSize(w,h,a){
        var rads=a*Math.PI/180;
        var c = Math.abs(Math.cos(rads));
        var s = Math.abs(Math.sin(rads));
        size.width = h * s + w * c;
        size.height = h * c + w * s ;
      }

    });  // end onload
    </script>
</head>
<body>
    <button id="rotate">Rotate with resize</button>
    <p id=info></p>
    <canvas id="canvas" width=400 height=400></canvas>
</body>
</html>

Answer 2

尝试更改此行：

ctx.drawImage(image, -image.width / 2, -image.width / 2);

为：

ctx.drawImage(image, -image.width / 2, -image.height / 2);
                                              ^^^^^^

或宽度将用作旋转两个轴的基础。

另请参阅此答案，了解旋转的一种方法，并根据旋转将画布调整到图像：
Rotate original Image with jQuery or JavaScript

在画布内旋转图像时遇到一些问题

2 个答案: