javascript - 如何在鼠标移动时在 Canvas 上绘制圆

我试图通过鼠标移动在画布上画一个圆，但是没有成功。我的代码如下：

var damages_canvas = document.getElementById('damages-area');
var ctx_damages = damages_canvas.getContext('2d');
var circle_rect = damages_canvas.getBoundingClientRect();
var circle_drag = false;
var circle_canvas_update = true;
var circles = [];
var markerColor = "#f00";

damages_canvas.addEventListener('mousedown', drawCircleMouseDown, false);
damages_canvas.addEventListener('mouseup', drawCircleMouseUp, false);
damages_canvas.addEventListener('mousemove', drawCircleMouseMove, false);

requestAnimationFrame(updateCircleCanvas);

function updateCircleCanvas(){
    if(circle_canvas_update){
        drawCircleCanvas();
        circle_canvas_update = false;
    }

    requestAnimationFrame(updateCircleCanvas);
}

function drawCircle(circle){
    var startX = circle.startX,
        endX = circle.endX,
        startY = circle.startY,
        endY = circle.endY;

    ctx_damages.clearRect(0,0,damages_canvas.width,damages_canvas.height);
    ctx_damages.beginPath();
    ctx_damages.moveTo(startX, startY + (endY-startY)/2);
    ctx_damages.bezierCurveTo(startX, startY, endX, startY, endX, startY + (endX-startY)/2);
    ctx_damages.bezierCurveTo(endX, endY, startX, endY, startX, startY + (endY-startY)/2);
    ctx_damages.closePath();
    ctx_damages.stroke();
}

function drawCircleCanvas(){
    ctx_damages.setTransform(1,0,0,1,0,0);
    ctx_damages.clearRect(0, 0, damages_canvas.width, damages_canvas.height);
    ctx_damages.fillStyle = "transparent"; // Color
    ctx_damages.strokeStyle = markerColor; // Color
    circles.forEach(drawCircle)
}

function drawCircleMouseDown(e){
    circle = {
        startX : e.offsetX - circle_rect.left,
        startY : e.offsetY - circle_rect.top,
        endX : 10,
        endY : 10
    };
    circle_drag = true;
    circles.push(circle);
    circle_canvas_update = true;
}

function drawCircleMouseUp(){
    circle_drag = false;
    circle_canvas_update = true;
}

function drawCircleMouseMove(e){
    if(circle_drag){
        circle.endX = e.offsetX - circle_rect.left;
        circle.endY = e.offsetY - circle_rect.top;
        circle_canvas_update = true;
    }
}

#damages-area {background:#f9f9f9}

<canvas id="damages-area" width=600 height=600></canvas>

我想画一个圆，而不是省略号或其他任何东西。但就我而言，我得到了一些奇怪的东西。

Here is also the fiddle.

知道我该怎么办吗？

最佳答案

使用arc在画布中绘制圆弧。对于圆start和end角将为0和2*Math.PI

var damages_canvas = document.getElementById('damages-area');
var ctx_damages = damages_canvas.getContext('2d');
var circles = [];
var markerColor = "#f00";
var offsetX = damages_canvas.offsetLeft;
var offsetY = damages_canvas.offsetTop;
var startX;
var startY;
var isMouseDown = false;
var circle, radius;

damages_canvas.addEventListener('mousedown', drawCircleMouseDown, false);
damages_canvas.addEventListener('mouseup', drawCircleMouseUp, false);
damages_canvas.addEventListener('mousemove', drawCircleMouseMove, false);

function Circle(startX, startY) {
  this.startX = startX;
  this.startY = startY;
  this.radius;
  this.draw = function() {
    ctx_damages.beginPath();
    ctx_damages.arc(this.startX, this.startY, this.radius, 0, 2 * Math.PI);
    ctx_damages.strokeStyle = markerColor;
    ctx_damages.stroke();
  }
}

function drawCircleMouseDown(e) {
  startX = parseInt(e.clientX - offsetX);
  startY = parseInt(e.clientY - offsetY);
  isMouseDown = true;
  circle = new Circle(startX, startY);
  circles.push(circle);
}

function drawCircleMouseUp() {
  isMouseDown = false;
  circle = null;
}

function drawCircleMouseMove(e) {
  if (!isMouseDown) {
    return;
  }
  mouseX = parseInt(e.clientX - offsetX);
  mouseY = parseInt(e.clientY - offsetY);
  circle.radius = getDistance(startX, startY, mouseX, mouseY);
  ctx_damages.clearRect(0, 0, damages_canvas.width, damages_canvas.height);
  circles.forEach(function(circ) {
    circ.draw();
  });
}

function getDistance(p1X, p1Y, p2X, p2Y) {
  return Math.sqrt(Math.pow(p1X - p2X, 2) + Math.pow(p1Y - p2Y, 2))
}

#damages-area {background:#f9f9f9}

<canvas id="damages-area" width=600 height=600></canvas>

关于javascript - 如何在鼠标移动时在 Canvas 上绘制圆，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/49006488/