单击 canvas 中的行

Question

如果点击 canvas 中的行，是否可以获取信息？

var canvas;
var x = 200;
var y = 200;
var dx=4;
var dy=4;
var d = 5;
var width, height;

function init() {
    canvasE = document.getElementById('game');
    canvas= canvasE.getContext('2d');
    width = canvasE.width;
    height = canvasE.height;
    canvasE.addEventListener("click", onClick, false);
    drawCircle();
}

function drawCircle() {
 clear();
    
 canvas.arc(x-1,y-1,d,0,Math.PI*2,true);
 canvas.beginPath();
 canvas.arc(x-1,y-1,d,0,Math.PI*2,true);
 canvas.closePath();
 canvas.fill(); 
    
 canvas.beginPath();
 canvas.moveTo(20,20);
 canvas.lineTo(150,100);
 canvas.stroke();
    
 //check if click coords within current path
 if(canvas.isPointInPath(x,y)) {
   document.getElementById("inPath").innerHTML = "yes";
 } else {
   document.getElementById("inPath").innerHTML = "no";
 }
    
 canvas.closePath();
 
}

function clear() {
    canvas.fillStyle="#ffffff";
    canvas.fillRect(0,0,width,height);
    canvas.fillStyle="#ff0000";
    canvas.strokeRect(0,0,width,height);
}

function onClick(e) {
    var element = canvasE;
    var offsetX = 0, offsetY = 0

        if (element.offsetParent) {
      do {
        offsetX += element.offsetLeft;
        offsetY += element.offsetTop;
      } while ((element = element.offsetParent));
    }

    x = e.pageX - offsetX;
    y = e.pageY - offsetY;
    
    drawCircle();
    document.getElementById("info").value = "x: " + x + ", y: " + y;
}

init();

canvas, input {
    margin: 10px;
    text-align: center;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js"></script>
<input id="info" type="text" readonly>
<span id="inPath">No</span>
<canvas id="game" width="500" height="500"></canvas>

谢谢

Answer 1

基于JavaScript eyedropper (tell color of pixel under mouse cursor)，注意每个像素表示为数组data中的4个元素，即data[0] = red of (0,0), data[1] = (0,0) 的绿色，data[2] = (0,0) 的蓝色，data[3] = (0,0) 的 alpha，data[4] = (0,1) 的红色，.. .

var canvas;
var x = 200;
var y = 200;
var dx = 4;
var dy = 4;
var d = 5;
var width, height;

function init() {
  canvasE = document.getElementById('game');
  canvas = canvasE.getContext('2d');
  width = canvasE.width;
  height = canvasE.height;
  canvasE.addEventListener("click", onClick, false);
  drawCircle();
}

function drawCircle() {
  clear();

  canvas.arc(x - 1, y - 1, d, 0, Math.PI * 2, true);
  canvas.beginPath();
  canvas.arc(x - 1, y - 1, d, 0, Math.PI * 2, true);
  canvas.closePath();
  canvas.fill();

  canvas.beginPath();
  canvas.moveTo(20, 20);
  canvas.lineTo(150, 100);
  canvas.stroke();

  //check if click coords within current path
  if (canvas.isPointInPath(x, y)) {
    document.getElementById("inPath").innerHTML = "yes";
  } else {
    document.getElementById("inPath").innerHTML = "no";
  }

  canvas.closePath();

}

function clear() {
  canvas.fillStyle = "#ffffff";
  canvas.fillRect(0, 0, width, height);
  canvas.fillStyle = "#ff0000";
  canvas.strokeRect(0, 0, width, height);
}

function onClick(e) {
  var element = canvasE;
  var offsetX = 0,
    offsetY = 0

  if (element.offsetParent) {
    do {
      offsetX += element.offsetLeft;
      offsetY += element.offsetTop;
    } while ((element = element.offsetParent));
  }

  x = e.pageX - offsetX;
  y = e.pageY - offsetY;

  document.getElementById("info").value = "x: " + x + ", y: " + y;

  function pixelIndex(x, y) {
    return 4 * (y * canvasE.width + x);
  }

  // data is an array where each 4 elements define a pixed, these are red,green,blue,alpha
  var data = canvas.getImageData(0, 0, canvasE.width, canvasE.height).data;
  var pixelIndexStart = pixelIndex(x, y);
  // only need the first three to see if there's something drawn there
  var red = 255 - data[pixelIndexStart + 0];
  var green = 255 - data[pixelIndexStart + 1];
  var blue = 255 - data[pixelIndexStart + 2];
  var hasLine = red + green + blue > 0;
  document.getElementById("info").value = 'has line: ' + hasLine;
  
  drawCircle();
}

init();

canvas,
input {
  margin: 10px;
  text-align: center;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js"></script>
<input id="info" type="text" readonly>
<span id="inPath">No</span>
<canvas id="game" width="500" height="500"></canvas>

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