freeCodeCamp/guide/english/canvas/paths/index.md

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Paths

Paths in Canvas

Paths are the building block of drawing in Canvas. A path is just a shape. Then, the shape can be drawn on by either stroking it or filling it.

We can create use paths with beginPath, fill, and stroke.

ctx.beginPath();
ctx.rect(0, 0, 100, 100);
ctx.fillStyle="black";
ctx.fill();

This creates a black square in the upper-right corner of the canvas. We can change strokes and fills with strokeStyle and fillStyle, which both take CSS-like color strings. We can also use lineWidth to make strokes thicker.

ctx.beginPath();
ctx.moveTo(0,0);
ctx.lineTo(300,150);
ctx.moveTo(0, 100);
ctx.lineTo(300, 250);

ctx.strokeStyle="red";
ctx.lineWidth=2;
ctx.stroke();

There are four basic drawing functions: rect, moveTo, lineTo, and arc.

• rect(x, y, width, height) adds a rectangle created from (x,y) of dimensions (width, height) to the path.
• moveTo(x,y) moves the pen to a point on the canvas
• lineTo(x,y) moves the pen to a point on the canvas, then adds a line between the new point and the old point to the path.
• arc(x, y, r, ti, tf) adds an arc (a portion of a circle) at (x,y), of radius r, and from angle ti to tf to the path.

Note that these functions add to the working path. They don't create new paths.

//example 1
ctx.beginPath();
ctx.rect(0, 0, 50, 50);
ctx.rect(100, 100, 50, 50);
ctx.fill();

//example 2
ctx.beginPath();
ctx.rect(0, 0, 50, 50);
ctx.beginPath();
ctx.rect(100, 100, 50, 50);
ctx.fill();

The first example will draw two squares, while the second will only draw one since beginPath discarded the first rectangle on the old working path.

This fact leads to a common mistake in making canvas animations.

var x = 0;
var y = 0;
function draw() {
  ctx.clearRect(0, 0, canvas.width, canvas.height);

  ctx.rect(x, y, 50, 50);
  ctx.fill();
  x+=1;

  window.requestAnimationFrame(draw);
}

window.requestAnimationFrame(draw);

The above animation, which is supposed to make a square move across the screen, instead creates a long black bar. The reason is that beginPath is not called inside the draw loop.

To read more about animation, see the Animation page.

More Information:

• MDN Canvas API