如何使用HTML5和JavaScript在鼠标单击事件上创建图像

Question

I an newbie in html5, I want to create an a dot like image on click event. I try to use context but unfortunately not working. The code that i am using is

 document.addEventListener("DOMContentLoaded", init, false);

      function init()
      {
        var canvas = document.getElementById("options");
        canvas.addEventListener("mousedown", getPosition, false);
      }

      function getPosition(event)
      {
        var x = new Number();
        var y = new Number();
        var canvas = document.getElementById("options");

        if (event.x != undefined && event.y != undefined)
        {
          x = event.x;
          y = event.y;
        }
        else // Firefox method to get the position
        {
          x = event.clientX + document.body.scrollLeft +
              document.documentElement.scrollLeft;
          y = event.clientY + document.body.scrollTop +
              document.documentElement.scrollTop;
        }

        x -= canvas.offsetLeft;
        y -= canvas.offsetTop;

        alert("x: " + x + "  y: " + y);
           var b_canvas = document.getElementById("b");
    var b_context = b_canvas.getContext("2d");
    b_context.fillRect(50, 25, 150, 100);

Please suggest where I am doing mistake. Give your suggestions

Answer 1

Not sure if that's the problem, but to draw a dot on canvas, you can start a path, draw a completely closed arc (from 0 rad to 2pi rad) and fill that:

     b_context.beginPath();
     b_context.arc(x, y, 5 , 0,  2 * Math.PI,true);
     b_context.fillStyle = "blue";
     b_context.fill();

Further it is important to set width and height directly on canvas and not in css to prevent distortion. (just found that out myself ;) ) Complete example fiddle: http://jsfiddle.net/wHsMJ/ (click inside the rectangle on the right to test)