如何在Javascript&amp ;; 2d中在对象前面绘制圆圈HTML5画布

问题描述

我正在努力解决这个问题。我有一个在画布上随机移动的箭头。它平稳旋转并向下一个X / Y位置移动。大。如何让我的圆形物体始终出现在箭头前面;就好像箭头在它的尖端上握住圆圈一样。

I'm struggling with the maths on this one. I have an arrow that moves randomly around the canvas. It rotates smoothly and moves towards the next X/Y position. Great. How can I make my circle object always appear just in front of the arrow; like as if the arrow is holding the circle on it's tip.

这是我的代码：

player.prototype.draw = function (w, h, ctx) {
    ctx.beginPath();
    ctx.save();
    ctx.fillStyle = '#E50000';

    if (this.XPercent !== 0 && this.YPercent !== 0) {
        // ** moves at correct angle (no rotation involved)

        // Get the difference between the points
        var tx = this.nextXPercent - this.XPercent;
        var ty = this.nextYPercent - this.YPercent;

        // Add the x and y areas then get the square root
        var dist = Math.sqrt(tx * tx + ty * ty);
        var newX = (tx / dist) * 0.1;
        var newY = (ty / dist) * 0.1;
        this.XPercent = this.XPercent + (isNaN(newX) ? 0 : newX);
        this.YPercent = this.YPercent + (isNaN(newY) ? 0 : newY);
    }

    var currentX = (w / 100) * this.XPercent;
    var currentY = (h / 100) * this.YPercent;

    ctx.translate(currentX - (playerWidth / 2), currentY);
    currentX = (playerWidth / 2);
    currentY = 0;

    // now rotate smoothly
    ctx.rotate(this.getRotation2(w, h, ctx) * Math.PI / 180);

    ctx.moveTo(currentX, currentY);
    ctx.lineTo(currentX - playerWidth, currentY - (playerHeight / 2));
    ctx.lineTo(currentX - playerWidth, currentY + (playerHeight / 2));
    ctx.fill();

    ctx.restore();
    ctx.closePath();
};

上面的箭头移动而没有旋转。我的轮换方法如下：

The above moves the arrow around with no rotation. My rotation method is as follows:

player.prototype.getRotation2 = function (w, h, ctx) {
    var y2 = this.nextYPercent;
    var y1 = this.YPercent.toFixed(0);
    var x2 = this.nextXPercent;
    var x1 = this.XPercent.toFixed(0);

    if (x1 == x2 && y2 == y1) {
        return this.angle;
    }

    var Xdiff = y2 - y1;
    var Ydiff = x2 - x1;

    this.angle %= 360;
    var rads = Math.atan2(y2 - y1, x2 - x1);
    var targetAngle = rads * (180 / Math.PI);
    targetAngle = (targetAngle + 360) % 360;

    if (this.angle != targetAngle) {
        var netAngle = (this.angle - targetAngle + 360) % 360;
        var delta = Math.min(Math.abs(netAngle - 360), netAngle, 5);

        var sign = (netAngle - 180) >= 0 ? 1 : -1;
        this.angle += sign * delta + 360;
        this.angle %= 360;

        if (this.hasBall) {
            // This is where we should position the circle at correct angle
            gameBall.XPercent = this.XPercent + ???
            gameBall.YPercent = this.YPercent + ???
        }
    }

    return this.angle;
};

推荐答案

继续使用箭头和向右在箭头的末尾画出球。

Just continue using the transformation you have for the arrow and go "right" to draw the ball at the end of the arrow.

我建议将绘图和转换操作分开。这样你就可以单向绘制箭头，而不必担心它的位置或旋转。可以使用标志作为方法的参数来添加球。

I would suggest separating the drawing and transformation operations. This way you can draw the arrow is a single way without worrying about its location or rotation. The ball can be added using a flag as argument to the method.

示例

var ctx = c.getContext("2d"), radius = 7;

function drawShape() {
  // Always draw with (0,0) as basis, this will rotate at tail.
  // If rotate at head, just offset the values so head is at (0,0)
  ctx.beginPath();
  ctx.moveTo(0, 0);
  ctx.lineTo(50, 0);
  ctx.moveTo(50, 0);
  ctx.lineTo(40, -5);
  ctx.lineTo(40, 5);
  ctx.fill();

  // add circle at tip
  ctx.moveTo(60 + radius, 0);
  ctx.arc(60, 0, radius, 0, 6.28);
  ctx.stroke();
}

// just to add action for the demo
window.onmousemove = function(e) {
  var rect = c.getBoundingClientRect(),
      x = e.clientX - rect.left,
      y = e.clientY - rect.top,
      dx = x - c.width*0.5,
      dy = y - c.height*0.5,
      a = Math.atan2(dy, dx);

  ctx.clearRect(0,0,c.width,c.height)
  ctx.translate(c.width*0.5, c.height*0.5);
  ctx.rotate(a);
  drawShape();
  ctx.setTransform(1,0,0,1,0,0);
}

<canvas id=c></canvas>

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