在HTML5画布上绘制图像

Question

I wanted to draw images on the canvas but the image will only appear at the position when I click on the canvas. I have also learn that element.getBoundingClientRect method which will be able to allow me to get the offsets of the canvas. I wanted to combine them, so when click on any part of the canvas the image will appear at the same location and the X and Y offsets will be display out too.

Currently my image will be display onload, how do I change it when I Onclick on the canvas the image will the appear at the same position. Second thing is that at the same time the X and Y offsets position of the image will be display out too.

Answer 1

Modify your JS code like this:

var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");

$results=$("#results");
$dotposition=$("#dotposition");
var position = {x: 0, y: 0};

function handleMouseMove(e) {
    e.preventDefault();
    e.stopPropagation();

    // You could calc the offsets once if you know
    // the user will not scroll the canvas
    var BB=canvas.getBoundingClientRect();
    var offsetX=BB.left;
    var offsetY=BB.top;

    mouseX=parseInt(e.clientX-offsetX);
    mouseY=parseInt(e.clientY-offsetY);
    $results.text("Mouse position: "+mouseX+" / "+mouseY);
    position.x = mouseX;
    position.y = mouseY;
}

function handleMouseClick(e) {
    //Clearing the canvas before redraw, remove this line if you want to keep prev images
    ctx.clearRect ( 0 , 0 , canvas.width , canvas.height );

    ctx.drawImage(img,position.x - img.width / 2,position.y - img.height / 2);
    $dotposition.text("Dot position: "+position.x+" / "+position.y);
}

var img = new Image();
img.onload = function(){
    canvas.onmousemove = handleMouseMove;
    canvas.onclick = handleMouseClick;
};
img.src="https://dl.dropboxusercontent.com/s/7vlxwy1156wnhcw/redFood.png";

http://jsfiddle.net/ZH4KW/1/