javascript - 如何获取放置在 Canvas 上的图像的点击事件

Question

我已经创建了在 Canvas 上显示多个图像的程序。现在我想使用放置在 Canvas 上的鼠标单击事件获取该图像数据。我的 JavaScript 代码是 -

var canvas = document.getElementById("myCanvas");
ctx = canvas.getContext("2d");
canvas.width = 720;
canvas.height = 480;

//I got images data in one array 
var imageobj = new Array();
for (var d=0;d<calloutImageArray.length;d++)
{
    imageobj[d] = new Image();
    (function(d)
     imageobj[d].src = sitePath+"ATOM/chapter1/book/"+calloutImageArray[d];
     imageobj[d].onload = function() 
      {
        ctx.drawImage(imageobj[d], calloutImageArrayX[d], calloutImageArrayY[d],calloutImageArrayW[d], calloutImageArrayH[d]);
      }; 
   })(d);
}

Answer 1

相当容易完成:

• 使用 canvas.addEventListener

  监听 mousedown 事件

• 在 mousedown 时，检查鼠标是否在任何图像内。

• 获取鼠标下图像的图像数据。

示例代码:

function handleMousedown(e){

    // tell the browser we're handling this event
    e.preventDefault();
    e.stopPropagation();

    // get the mouse position
    var mouseX=e.clientX-BBoffsetX;
    var mouseY=e.clientY-BBoffsetY;

    // loop through each image and see if mouse is under
    var hit=-1;
    for(var i=0;i<imageobj.length;i++){
        var x=calloutImageArrayX[i];
        var y=calloutImageArrayY[i];
        var w=calloutImageArrayW[i];
        var h=calloutImageArrayH[i];
        if(mouseX>=x && mouseX<=x+w && mouseY>=y && mouseY<=y+h){
            hit=i;
        }
    }

    // you clicked the image with index==hit
    // so get its image data
    if(hit>=0){
        var imageData=ctx.getImageData(
            calloutImageArrayX[hit],
            calloutImageArrayY[hit],
            calloutImageArrayW[hit],
            calloutImageArrayH[hit]);

        // now do your thing with the imageData!

    }
}

示例代码和演示:

var $results=document.getElementById('results');

var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");
var cw=canvas.width;
var ch=canvas.height;
var BB,BBoffsetX,BBoffsetY;
function setBB(){
  BB=canvas.getBoundingClientRect();
  BBoffsetX=BB.left;
  BBoffsetY=BB.top;
}
setBB();
window.onscroll=function(e){ setBB(); }


var imageobj=[];
var calloutImageArrayX=[10,125,10,125];
var calloutImageArrayY=[10,10,150,150];
var calloutImageArrayW=[];
var calloutImageArrayH=[];


// put the paths to your images in imageURLs[]
var imageURLs=[];  
imageURLs.push("https://dl.dropboxusercontent.com/u/139992952/multple/character3.png");
imageURLs.push("https://dl.dropboxusercontent.com/u/139992952/multple/character2.png");
imageURLs.push("https://dl.dropboxusercontent.com/u/139992952/multple/character1.png");
imageURLs.push("https://dl.dropboxusercontent.com/u/139992952/stack1/avatar.png");

// the loaded images will be placed in imageobj[]
var imagesOK=0;
startLoadingAllImages(imagesAreNowLoaded);

// Create a new Image() for each item in imageURLs[]
// When all images are loaded, run the callback (==imagesAreNowLoaded)
function startLoadingAllImages(callback){

  // iterate through the imageURLs array and create new images for each
  for (var i=0; i<imageURLs.length; i++) {
    // create a new image an push it into the imageobj[] array
    var img = new Image();
    img.crossOrigin='anonymous';
    // Important! By pushing (saving) this img into imageobj[],
    //     we make sure the img variable is free to
    //     take on the next value in the loop.
    imageobj.push(img);
    // when this image loads, call this img.onload
    img.onload = function(){ 
      // this img loaded, increment the image counter
      imagesOK++; 
      // if we've loaded all images, call the callback
      if (imagesOK>=imageURLs.length ) {
        callback();
      }
    };
    // notify if there's an error
    img.onerror=function(){alert("image load failed");} 
    // set img properties
    img.src = imageURLs[i];
  }      
}

// All the images are now loaded
// Do drawImage & fillText
function imagesAreNowLoaded(){

  // the imageobj[] array now holds fully loaded images
  // the imageobj[] are in the same order as imageURLs[]

  // add widths & heights to the appropriate arrays
  for(var i=0;i<imageobj.length;i++){
    calloutImageArrayW.push(imageobj[i].width);
    calloutImageArrayH.push(imageobj[i].height);
  }

  // listen for mousedown events
  canvas.onmousedown=handleMousedown;

  draw();

}


function draw(){
  ctx.clearRect(0,0,cw,ch);
  for(var d=0;d<imageobj.length;d++){
    ctx.drawImage(imageobj[d], 
                  calloutImageArrayX[d], calloutImageArrayY[d],
                  calloutImageArrayW[d], calloutImageArrayH[d]);
  }    
}        



function handleMousedown(e){

  // tell the browser we're handling this event
  e.preventDefault();
  e.stopPropagation();

  // get the mouse position
  var mouseX=e.clientX-BBoffsetX;
  var mouseY=e.clientY-BBoffsetY;

  // loop through each image and see if mouse is under
  var hit=-1;
  for(var i=0;i<imageobj.length;i++){
    var x=calloutImageArrayX[i];
    var y=calloutImageArrayY[i];
    var w=calloutImageArrayW[i];
    var h=calloutImageArrayH[i];
    if(mouseX>=x && mouseX<=x+w && mouseY>=y && mouseY<=y+h){
      hit=i;
    }
  }

  // you clicked the image with index==hit
  // so get its image data
  if(hit>=0){
    var imageData=ctx.getImageData(
      calloutImageArrayX[hit],
      calloutImageArrayY[hit],
      calloutImageArrayW[hit],
      calloutImageArrayH[hit]);

    var pos;
    switch(hit){
      case 0:pos='top-left';break;
      case 1:pos='top-right';break;
      case 2:pos='bottom-left';break;
      case 3:pos='bottomRight';break;
    }
    $results.innerHTML='You clicked the '+pos+' image.<br>This imageData has this length: '+imageData.data.length;

  }

}

body{ background-color: ivory; }
canvas{border:1px solid red;}

<h4 id="results">Click on an image</h4>
<canvas id="canvas" width=300 height=300></canvas>