放大一个点(使用缩放和平移) [英] Zoom in on a point (using scale and translate)

问题描述

我希望能够放大 HTML 5 画布中鼠标下方的点，例如放大 Google 地图.我怎样才能做到这一点?

I want to be able to zoom in on the point under the mouse in an HTML 5 canvas, like zooming on Google Maps. How can I achieve that?

推荐答案

终于解决了:

const zoomIntensity = 0.2;

const canvas = document.getElementById("canvas");
let context = canvas.getContext("2d");
const width = 600;
const height = 200;

let scale = 1;
let originx = 0;
let originy = 0;
let visibleWidth = width;
let visibleHeight = height;


function draw(){
    // Clear screen to white.
    context.fillStyle = "white";
    context.fillRect(originx, originy, width/scale, height/scale);
    // Draw the black square.
    context.fillStyle = "black";
    context.fillRect(50, 50, 100, 100);

    // Schedule the redraw for the next display refresh.
    window.requestAnimationFrame(draw);
}
// Begin the animation loop.
draw();

canvas.onwheel = function (event){
    event.preventDefault();
    // Get mouse offset.
    const mousex = event.clientX - canvas.offsetLeft;
    const mousey = event.clientY - canvas.offsetTop;
    // Normalize mouse wheel movement to +1 or -1 to avoid unusual jumps.
    const wheel = event.deltaY < 0 ? 1 : -1;

    // Compute zoom factor.
    const 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，正如@Tata​​rize指出，是计算轴位置，使得缩放点(鼠标指针) 缩放后保持在同一位置.

The key, as @Tatarize pointed out, is to compute the axis position such that the zoom point (mouse pointer) remains in the same place after the zoom.

本来鼠标在距离角落mouse/scale，我们希望鼠标下的点在缩放后保持在同一个位置，但这是在mouse/new_scale 远离角落.因此，我们需要移动 origin(角的坐标)来解决这个问题.

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 *= zoom

剩下的代码需要应用缩放并转换到绘制上下文，使其原点与画布角重合.

The remaining code then needs to apply the scaling and translate to the draw context so it's origin coincides with the canvas corner.

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