javascript - HTML5 Canvas globalAlpha 不工作

Question

我需要在没有 clearRect 方法的情况下使用透明创建平滑的绘图线，我尝试使用:globalAlpha 和 StrokeStyle 与 rgba，如下所示:

ctx.StrongStyle = "rgba( redChannel, greenChannel, blueChannel, AlphaChannel)";

但这两种方法都不起作用。我如何在没有 clearRect 方法的情况下绘制透明线。虽然我在每次绘制 globalAlpha 之前使用 clearRect 工作，但我需要在没有它的情况下工作。

我的代码示例:

var el = document.getElementById('c');
var ctx = el.getContext('2d');

ctx.lineWidth = 10;
ctx.lineJoin = ctx.lineCap = 'round';
ctx.globalAlpha = "0.2";
//ctx.strokeStyle = "rgba(255, 0, 0, 150)";
ctx.strokeStyle = "red";

var isDrawing, points = [ ];

el.onmousedown = function(e) {
  isDrawing = true;
  points.push({ x: e.clientX, y: e.clientY });
};

el.onmousemove = function(e) {
  if (!isDrawing) return;

  //ctx.clearRect(0, 0, ctx.canvas.width, ctx.canvas.height);
  points.push({ x: e.clientX, y: e.clientY });

  ctx.beginPath();
  ctx.moveTo(points[0].x, points[0].y);
  for (var i = 1; i < points.length; i++) {
    ctx.lineTo(points[i].x, points[i].y);
  }
  ctx.stroke();
  ctx.closePath();
  
};

el.onmouseup = function() {
  isDrawing = false;
  points.length = 0;
};

canvas { border: 1px solid #ccc }

<canvas id="c" width="500" height="300"></canvas>

Answer 1

每次鼠标移动时，您都会重新绘制整个路径，因此即使将 globalAlpha 值设置为 0.2，分层效果也会使线条显示为实心。

选项 1:使用clearRect清除路径onmousemove然后重绘；

选项 2:仅绘制路径的最后一段，但重叠可见:

var el = document.getElementById('c');
var ctx = el.getContext('2d');

ctx.lineWidth = 10;
ctx.lineJoin = ctx.lineCap = 'round';
ctx.globalAlpha = "0.2";
//ctx.strokeStyle = "rgba(255, 0, 0, 150)";
ctx.strokeStyle = "red";

var isDrawing, points = [ ];

el.onmousedown = function(e) {
  isDrawing = true;
  points.push({ x: e.clientX, y: e.clientY });
};

el.onmousemove = function(e) {
  if (!isDrawing) return;

  //ctx.clearRect(0, 0, ctx.canvas.width, ctx.canvas.height);
  points.push({ x: e.clientX, y: e.clientY });

  ctx.beginPath();

  //draw just the last segment
  if(points.length>1) {
      ctx.moveTo(points[points.length-2].x, points[points.length-2].y);
      ctx.lineTo(points[points.length-1].x, points[points.length-1].y);
  }
  ctx.stroke();
  ctx.closePath();
  
};

el.onmouseup = function() {
  isDrawing = false;
  points.length = 0;
};

canvas { border: 1px solid #ccc }

<canvas id="c" width="500" height="300"></canvas>

选项 3:设置 canvas 元素的不透明度

var el = document.getElementById('c');
var ctx = el.getContext('2d');

ctx.lineWidth = 10;
ctx.lineJoin = ctx.lineCap = 'round';
//ctx.globalAlpha = "0.2";
//ctx.strokeStyle = "rgba(255, 0, 0, 150)";
ctx.strokeStyle = "red";

var isDrawing, points = [ ];

el.onmousedown = function(e) {
  isDrawing = true;
  points.push({ x: e.clientX, y: e.clientY });
};

el.onmousemove = function(e) {
  if (!isDrawing) return;

  //ctx.clearRect(0, 0, ctx.canvas.width, ctx.canvas.height);
  points.push({ x: e.clientX, y: e.clientY });

  ctx.beginPath();
  ctx.moveTo(points[0].x, points[0].y);
  for (var i = 1; i < points.length; i++) {
    ctx.lineTo(points[i].x, points[i].y);
  }
  ctx.stroke();
  ctx.closePath();
  
};

el.onmouseup = function() {
  isDrawing = false;
  points.length = 0;
};

canvas { border: 1px solid #ccc; opacity:0.2; }

<canvas id="c" width="500" height="300"></canvas>