Canvas 用边框画线

Question

I need to draw a line and give it a border.

I tried to draw two lines, one 5px and above 3px

But that doesn't exactly seem like a real border

 const ctx = canvas.getContext('2d'); const path = new Path2D(); ctx.strokeStyle = "black"; ctx.lineWidth = 5; path.moveTo(40, 40); path.lineTo(50, 35); path.lineTo(60, 40); ctx.stroke(path); ctx.strokeStyle = "red"; ctx.lineWidth = 3; path.moveTo(40, 40); path.lineTo(50, 35); path.lineTo(60, 40); ctx.stroke(path);

 <canvas id=canvas width=100 height=100></canvas>

Is there a better way to draw a border for a line?

Answer 1

Try setting the "endCap" on the outer line:

const ctx = canvas.getContext('2d');
const path = new Path2D();

ctx.strokeStyle = "black";
ctx.lineWidth = 5;
ctx.lineCap = "butt"; // butt  round  square <-- other options

path.moveTo(40, 40);
path.lineTo(50, 35);
path.lineTo(60, 40);

ctx.stroke(path);

ctx.strokeStyle = "red";
ctx.lineWidth = 3;

path.moveTo(40, 40);
path.lineTo(50, 35);
path.lineTo(60, 40);

ctx.stroke(path);

See: https://developer.mozilla.org/en-US/docs/Web/API/CanvasRenderingContext2D/lineCap

Answer 2

You could try with shadow:

 const ctx = canvas.getContext('2d'); function drawPath(path) { ctx.lineWidth = 3; ctx.strokeStyle = "red"; ctx.shadowColor = 'black'; for (i = 0; i <= 360; i += 10) { a = i * Math.PI / 180 ctx.beginPath(); ctx.shadowOffsetX = 4 * Math.sin(a) ctx.shadowOffsetY = 4 * Math.cos(a) ctx.stroke(path); } } const path = new Path2D(); path.moveTo(20, 40); path.lineTo(50, 35); path.lineTo(80, 40); path.lineTo(80, 80); path.lineTo(160, 80); drawPath(path);

 <canvas id=canvas width=200 height=100></canvas>

The idea is to draw with a slightly different offset all-around your path object... and that same logic can be used for images:

https://raw.githack.com/heldersepu/hs-scripts/master/HTML/canvasOutline.html