缩放后确定鼠标在 HTML5 画布上的位置

Question

I'm working on developing some software in HTML5 which involves the use of canvases. There is one canvas in which I need to be able to zoom and allow the user to doodle on the canvas on mouse clicks. So far, I have gotten the zoom to work, with help from some examples I found. The problem is that after zooming, the mouse position on my drawing tool is out of whack. Before any zooming, I can draw just fine. Here is the code for the zoom:

//Zoom
        mainCanvas.onmousewheel = function(event) {
            var mousex = event.clientX - mainCanvas.offsetLeft;
            var mousey = event.clientY - mainCanvas.offsetTop;
            var wheel = event.wheelDelta / 120;
            //n or -n

            var zoom = 0;
            if(wheel < 0) {
                zoom = 1 / 2;
                if(currentzoom == 1)
                    return;
            } else {
                mousex = event.clientX - mainCanvas.offsetLeft;
                mousey = event.clientY - mainCanvas.offsetTop;
                zoom = 2;
                if(currentzoom == 32)
                    return;
            }
            currentzoom *= zoom;
            mainContext.translate(originx, originy);

            mainContext.scale(zoom, zoom);
            mainContext.translate(-(mousex / scale + originx - mousex / (scale * zoom ) ), -(mousey / scale + originy - mousey / (scale * zoom ) ));
            originx = (mousex / scale + originx - mousex / (scale * zoom ) );
            originy = (mousey / scale + originy - mousey / (scale * zoom ) );
            scale *= zoom;
            draw(mainContext, gridArray);
        }

Like I said, the zoom is not the actual problem, just the root of the problem. Here is the code which determines mouse position for the drawing tool:

//this function determines the mouse position relative to the canvas element
        function ev_canvas(ev) {
            if(ev.layerX || ev.layerX == 0) {//Firefox, IE
                ev._x = ev.layerX;
                ev._y = ev.layerY;
            } else if(ev.offsetX || ev.offsetX == 0) {//Opera
                ev._x = ev.offsetX;
                ev._y = ev.offsetY;
            }

            var func = tool[ev.type];
            if(func) {
                func(ev);
            }
        }

I'm sure that the problem lies in the latter block of code, but I'm not sure out to fix it. Any help would be appreciated.

Answer 1

I suspect this is a snippet of code that relies on some global variables like currentzoom

So, if I understand this question right, the problem lies in how you are using the canvas for both mouse control and zooming. I suspect that when you zoom, the DOM is placing the mouse cursor where it belongs on the picture as it was initially rendered. So if you zoom in 200% and place the mouse 100 pixels left of center of the canvas, the canvas will behave as though the mouse was 200 pixels to the left of center.

currentzoom = 1;
originX = 0;
originY = 0;

function ev_canvas(ev) {
        if(ev.layerX || ev.layerX == 0) {//Firefox, IE
            ev._x = ev.layerX * currentzoom / 1 - originX;
            ev._y = ev.layerY * currentzoom / 1 - originY;
        } else if(ev.offsetX || ev.offsetX == 0) {//Opera
            ev._x = ev.offsetX * currentzoom / 1 - originX;
            ev._y = ev.offsetY * currentzoom / 1 - originY;
        }

        var func = tool[ev.type];
        if(func) {
            func(ev);
        }
    }

The / 1 is preserved in case future users want to set a non "1" value to currentzoom .

Answer 2

For Joomla site you can use Mouse Over Zoom - it's a Joomla Extension that enables you to view bigger versions of the thumbnails from your Joomla webpage. Now you can browse images galleries with ease: just move the mouse cursor over thumbnails to view images in their full size without loading a new page. Download it here http://joomlaboat.com/mouse-over-zoom

Pictures are automatically resized if they don't fit the window. You can customize the extension there are a lot of additional options. In order to load transparent images, without background just use .png images.

Answer 3

Just try this out:

<!DOCTYPE HTML>
<html>
    <head>
    <script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.3.2/jquery.js"></script>
        <style>
            body {
                margin: 0px;
                padding: 0px;
            }

            #wrapper {
                position: relative;
                border: 1px solid #9C9898;
                width: 578px;
                height: 200px;
            }

            #buttonWrapper {
                position: absolute;
                width: 30px;
                top: 2px;
                right: 2px;
            }

            input[type =
            "button"] {
                padding: 5px;
                width: 30px;
                margin: 0px 0px 2px 0px;
            }
        </style>
        <script>
            function draw(scale, translatePos){
                var canvas = document.getElementById("myCanvas");
                var context = canvas.getContext("2d");

                // clear canvas
                context.clearRect(0, 0, canvas.width, canvas.height);

                context.save();
                context.translate(translatePos.x, translatePos.y);
                context.scale(scale, scale);
                context.beginPath(); // begin custom shape
                context.moveTo(-119, -20);
                context.bezierCurveTo(-159, 0, -159, 50, -59, 50);
                context.bezierCurveTo(-39, 80, 31, 80, 51, 50);
                context.bezierCurveTo(131, 50, 131, 20, 101, 0);
                context.bezierCurveTo(141, -60, 81, -70, 51, -50);
                context.bezierCurveTo(31, -95, -39, -80, -39, -50);
                context.bezierCurveTo(-89, -95, -139, -80, -119, -20);
                context.closePath(); // complete custom shape
                var grd = context.createLinearGradient(-59, -100, 81, 100);
                grd.addColorStop(0, "#8ED6FF"); // light blue
                grd.addColorStop(1, "#004CB3"); // dark blue
                context.fillStyle = grd;
                context.fill();

                context.lineWidth = 5;
                context.strokeStyle = "#0000ff";
                context.stroke();
                context.restore();
            }

            window.onload = function(){
                var canvas = document.getElementById("myCanvas");

                var translatePos = {
                    x: canvas.width / 2,
                    y: canvas.height / 2
                };

                var scale = 1.0;
                var scaleMultiplier = 0.8;
                var startDragOffset = {};
                var mouseDown = false;

                // add button event listeners
                document.getElementById("plus").addEventListener("click", function(){
                    scale /= scaleMultiplier;
                    draw(scale, translatePos);
                }, false);

                document.getElementById("minus").addEventListener("click", function(){
                    scale *= scaleMultiplier;
                    draw(scale, translatePos);
                }, false);

                // add event listeners to handle screen drag
                canvas.addEventListener("mousedown", function(evt){
                    mouseDown = true;
                    startDragOffset.x = evt.clientX - translatePos.x;
                    startDragOffset.y = evt.clientY - translatePos.y;
                });

                canvas.addEventListener("mouseup", function(evt){
                    mouseDown = false;
                });

                canvas.addEventListener("mouseover", function(evt){
                    mouseDown = false;
                });

                canvas.addEventListener("mouseout", function(evt){
                    mouseDown = false;
                });

                canvas.addEventListener("mousemove", function(evt){
                    if (mouseDown) {
                        translatePos.x = evt.clientX - startDragOffset.x;
                        translatePos.y = evt.clientY - startDragOffset.y;
                        draw(scale, translatePos);
                    }
                });

                draw(scale, translatePos);
            };



            jQuery(document).ready(function(){
               $("#wrapper").mouseover(function(e){
                  $('#status').html(e.pageX +', '+ e.pageY);
               }); 
            })  
        </script>
    </head>
    <body onmousedown="return false;">
        <div id="wrapper">
            <canvas id="myCanvas" width="578" height="200">
            </canvas>
            <div id="buttonWrapper">
                <input type="button" id="plus" value="+"><input type="button" id="minus" value="-">
            </div>
        </div>
        <h2 id="status">
        0, 0
        </h2>
    </body>
</html>

Works perfect for me with mouse movement track.. Njoy!!!

Answer 4

I got a better script to get mouse position on the canvas:

function writeMessage(canvas, message) {
        var context = canvas.getContext('2d');
        context.clearRect(0, 0, canvas.width, canvas.height);
        context.font = '18pt Calibri';
        context.fillStyle = 'black';
        context.fillText(message, 10, 25);
      }
      function getMousePos(canvas, evt) {
        var rect = canvas.getBoundingClientRect(), root = document.documentElement;

        // return relative mouse position
        var mouseX = evt.clientX - rect.top - root.scrollTop;
        var mouseY = evt.clientY - rect.left - root.scrollLeft;
        return {
          x: mouseX,
          y: mouseY
        };
      }

      window.onload = function() {
        var canvas = document.getElementById('myCanvas');
        var context = canvas.getContext('2d');

        canvas.addEventListener('mousemove', function(evt) {
          var mousePos = getMousePos(canvas, evt);
          var message = "Mouse position: " + mousePos.x + "," + mousePos.y;
          writeMessage(canvas, message);
        }, false);
      };

Try it out, comment if any issues..