Question

I’m trying to use 2 canvases, I want them both to accept an image dropped into them and use mouseclicks to manipulate the images. Here is the code I’m working on.

I have it where if I only have the eventlisteners for the ‘droppedImage’ canvas it works ok, as soon as I put in handlers for the ‘changeImage’ canvas it sees the 2nd canvas and reports the coordinates as an extension of the 1st canvas and it no longer displays the the RGB values or change the background of the 3rd canvas ‘selectedColour’

Posting this as I head to bed, if anyone gives answers or requires further info I will pop on tomorrow before work to update.

window.onload = function() {
    var droppedImage = document.getElementById("droppedImage"),
        ctx = droppedImage.getContext("2d");
    droppedImage.addEventListener("mouseup", mpos);
        // init event handlers
    droppedImage.addEventListener("dragenter", dragEnter, false);
    droppedImage.addEventListener("dragexit", dragExit, false);
    droppedImage.addEventListener("dragover", dragOver, false);
    droppedImage.addEventListener("drop", drop, false);

        var changeImage = document.getElementById("changeImage"),
        ctx = changeImage.getContext("2d");
    changeImage.addEventListener("mouseup", mpos);
    // init event handlers
    changeImage.addEventListener("dragenter", dragEnter, false);
    changeImage.addEventListener("dragexit", dragExit, false);
    changeImage.addEventListener("dragover", dragOver, false);
    changeImage.addEventListener("drop", drop, false);

    var selectedColour = document.getElementById("selectedColour");

function dragEnter(evt) {
    evt.stopPropagation();
    evt.preventDefault();
}

function dragExit(evt) {
    evt.stopPropagation();
    evt.preventDefault();
}

function dragOver(evt) {
    evt.stopPropagation();
    evt.preventDefault();
}

function drop(evt) {
    evt.stopPropagation();
    evt.preventDefault();

    var files = evt.dataTransfer.files;
    var count = files.length;

    // Only call the handler if 1 or more files was dropped.
    if (count >0)   
        importImage(files);
}




function mpos(e) {
    var cX = 0,
        cY = 0;

    if (event.pageX || event.pageY) {
        cX = event.pageX;
        cY = event.pageY;
    }
    else {
        cX = event.clientX + document.body.scrollLeft + document.documentElement.scrollLeft;
        cY = event.clientY + document.body.scrollTop + document.documentElement.scrollTop;
    }

    cX -= droppedImage.offsetLeft;
    cY -= droppedImage.offsetTop;

   // ctx.fillRect(cX, cY, 2, 2);

   alert("X co-ord : "+ cX +", Y co-ord : "+ cY);
   var imageData = ctx.getImageData(cX, cY, 1, 1);
   alert("Pixel 1: "+ imageData.data[0]+", "+imageData.data[1]+", "+ imageData.data[2]+", "+ imageData.data[3]);
   selectedColour.style.backgroundColor = "rgb("+imageData.data[0]+","+imageData.data[1]+","+imageData.data[2]+")";

}
}


function importImage(files) {

    var file = files[0];
    var reader = new FileReader;
    reader.onloadend = handleReaderLoadEnd; 
    reader.readAsDataURL(file);



function handleReaderLoadEnd(evt){     


    var canvas = document.getElementsByTagName('canvas')[0];
    var ctx = canvas.getContext('2d');
    var img = new Image;
        img.src = event.target.result;          
        img.onload = function() {       
        width = img.width;
        height = img.height;        
        var scaleX, scaleY, scale;
        var scaledWidth, scaledHeight;
        scaleX = width / canvas.width;
        scaleY = height / canvas.height;
        scale = scaleX > scaleY ? scaleX : scaleY;
        scaledWidth = width / scale;
        scaledHeight = height / scale;
        ctx.clearRect(0,0, canvas.width, canvas.height);
        ctx.drawImage(img, (canvas.width - scaledWidth) / 2, (canvas.height - scaledHeight) / 2, scaledWidth, scaledHeight);

        }
    }


}

Answer 1

Theoretically, if you have two sets of canvases and add event listeners to both of them then both sets of event listeners will fire at the same time, by design. Better to have one canvas that has event listeners attached, then detect the mouse co-ordinates and manipulate the image that is being clicked. Remember that you can’t have click handlers on drawn sprites inside the canvas, only on the canvas itself; you check whether the mouse co-ordinates are inside the drawn sprites.

In your scenario I would simply duplicate the image that was dropped in and pass that to the second canvas element. I’m assuming that you’re doing some kind of two-up image editing application; in this case I would have one ‘drop zone’ on the left for the image to go in, and then it would appear, editable on the right hand side. And then have click handlers only on one canvas. I have also got some more thoughts on this in this thread.