缩放与画布 [英] Zooming with canvas

问题描述

在测试应用程序中，我有一个带有一个简单矩形的画布。方法 draw 每100ms调用一次。

In a test application i have a canvas with a simple rectangle on it. The method draw is called every 100ms.

你可以从代码中看到，我使用Mousewheel缩放一切。现在发生的是，一切都是按比例缩放，但是当矩形是10px，10px，我有鼠标正上方，矩形在缩放后不再在鼠标下。 （这当然是正确的，因为所有单位都放大到。

as you can see from the code i'm using the Mousewheel to scale everything. What happens now is, that everything is scaled, but i.e. when the rectangle is at 10px,10px and i have the mouse right over it the rectangle is not under the mouse anymore after scaling. (Which is of course right because all units are scaled up to.

但是我想要的是，mouseposition是缩放操作的中心谷歌地图，所以在缩放之前的鼠标下的内容，在鼠标后面以及我做了几个attemps与翻译，但我不能弄清楚，如何做。

But what i want to is, that the mouseposition is the "center of the zooming action" like in google maps so the content which is under the mouse before scaling, is under the mouse afterwards as well. I made a few attemps with translating, but i can't figure out, how to do that.

提前感谢。

以下是我的代码：

 <script type="text/javascript">
        var scroll = 0;
        var scale = 1.0;


                    /** This is high-level function.
         * It must react to delta being more/less than zero.
         */
        function handle(delta) {

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

                scroll = delta;
                if(scroll > 0)
                {
                    scale += 0.2;
                }
                if(scroll < 0)
                {
                    scale -= 0.2;
                }


        }

        /** Event handler for mouse wheel event.
         */
        function wheel(event){
                var delta = 0;
                if (!event) /* For IE. */
                        event = window.event;
                if (event.wheelDelta) { /* IE/Opera. */
                        delta = event.wheelDelta/120;
                } else if (event.detail) { /** Mozilla case. */
                        /** In Mozilla, sign of delta is different than in IE.
                         * Also, delta is multiple of 3.
                         */
                        delta = -event.detail/3;
                }
                /** If delta is nonzero, handle it.
                 * Basically, delta is now positive if wheel was scrolled up,
                 * and negative, if wheel was scrolled down.
                 */
                if (delta)
                        handle(delta);
                /** Prevent default actions caused by mouse wheel.
                 * That might be ugly, but we handle scrolls somehow
                 * anyway, so don't bother here..
                 */
                if (event.preventDefault)
                        event.preventDefault();
            event.returnValue = false;
        }

        /** Initialization code. 
         * If you use your own event management code, change it as required.
         */
        if (window.addEventListener)
                /** DOMMouseScroll is for mozilla. */
                window.addEventListener('DOMMouseScroll', wheel, false);
        /** IE/Opera. */
        window.onmousewheel = document.onmousewheel = wheel;

        var drawX = 0;
        var drawY = 0;
        var overX = 0;
        var overY = 0;

         function startCanvas()
         {
             var myCanvas = document.getElementById("myCanvas");
             var ctx = myCanvas.getContext("2d");
             ctx.canvas.width  = window.innerWidth;
             ctx.canvas.height = window.innerHeight;                 
             setInterval(draw,100);
         }

         function draw()
         {
            var canvas = document.getElementById("myCanvas");
            var ctx = canvas.getContext("2d");

            ctx.clearRect(0,0,window.innerWidth,window.innerHeight);
            ctx.save();
            ctx.scale(scale,scale);
            ctx.fillRect(drawX,drawY,20,20);
            //ctx.translate(-scale,-scale);
            ctx.restore();
            ctx.font="20pt Arial";  
            ctx.fillText(scale+":"+drawX,0,150);                
         }

         function canvasClick(event)
         {
            console.log(event.layerX+"/"+scale);
            drawX = event.layerX/scale;
            drawY = event.layerY/scale;
         }

         function canvasOver(event)
         {
            console.log("over");
            overX = event.layerX;
            overY = event.layerY;
         }

    </script>

推荐答案

这实际上是一个非平凡的数学问题，作为缩放点

It's actually a non-trivial math question, usually known as a "zoom point"

查看

Take a look at here where another canvas user wanted to do the same thing and found a way.

<canvas id="canvas" width="800" height="600"></canvas>
<script type="text/javascript">
var canvas = document.getElementById("canvas");
var context = canvas.getContext("2d");
var scale = 1;
var originx = 0;
var originy = 0;

function draw(){
    context.fillStyle = "white";
    context.fillRect(originx,originy,800/scale,600/scale);
    context.fillStyle = "black";
    context.fillRect(50,50,100,100);
}
setInterval(draw,100);

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

    var zoom = 1 + wheel/2;

    context.translate(
        originx,
        originy
    );
    context.scale(zoom,zoom);
    context.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;
}

</script>

这篇关于缩放与画布的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！