放大一个点（使用缩放和平移）

Question

我希望能够放大HTML 5画布中鼠标下方的点，例如Google Maps上的。我该如何实现？

Answer 1

终于解决了：

var zoomIntensity = 0.2;



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

var context = canvas.getContext("2d");

var width = 600;

var height = 200;



var scale = 1;

var originx = 0;

var originy = 0;

var visibleWidth = width;

var visibleHeight = height;





function draw(){

    // Clear screen to white.

    context.fillStyle = "white";

    context.fillRect(originx,originy,800/scale,600/scale);

    // Draw the black square.

    context.fillStyle = "black";

    context.fillRect(50,50,100,100);

}

// Draw loop at 60FPS.

setInterval(draw, 1000/60);



canvas.onwheel = function (event){

    event.preventDefault();

    // Get mouse offset.

    var mousex = event.clientX - canvas.offsetLeft;

    var mousey = event.clientY - canvas.offsetTop;

    // Normalize wheel to +1 or -1.

    var wheel = event.deltaY < 0 ? 1 : -1;



    // Compute zoom factor.

    var zoom = Math.exp(wheel*zoomIntensity);



    // Translate so the visible origin is at the context's origin.

    context.translate(originx, originy);



    // Compute the new visible origin. Originally the mouse is at a

    // distance mouse/scale from the corner, we want the point under

    // the mouse to remain in the same place after the zoom, but this

    // is at mouse/new_scale away from the corner. Therefore we need to

    // shift the origin (coordinates of the corner) to account for this.

    originx -= mousex/(scale*zoom) - mousex/scale;

    originy -= mousey/(scale*zoom) - mousey/scale;



    // Scale it (centered around the origin due to the trasnslate above).

    context.scale(zoom, zoom);

    // Offset the visible origin to it's proper position.

    context.translate(-originx, -originy);



    // Update scale and others.

    scale *= zoom;

    visibleWidth = width / scale;

    visibleHeight = height / scale;

}


<canvas id="canvas" width="600" height="200"></canvas>

关键是计算轴位置，以便缩放点（鼠标指针）在缩放后保持在同一位置。

最初，鼠标mouse/scale与角之间有一段距离，我们希望鼠标下方的点在缩放后保持在同一位置，但这是mouse/new_scale远离角的地方。因此，我们需要移动origin（角的坐标）以解决此问题。

originx -= mousex/(scale*zoom) - mousex/scale;
originy -= mousey/(scale*zoom) - mousey/scale;
scale *= zoom

然后，其余代码需要应用缩放并转换到绘制上下文，以便其原点与画布角重合。