How to Draw Smooth Curves Through Multiple Points in HTML5 Canvas?

How to Draw Smooth Curves Using Multiple Points in Javascript HTML5 Canvas

In a drawing application, artists often need to create smooth curves that pass through multiple points. While HTML5 canvas provides line drawing functions like lineTo, quadraticCurveTo, and bezierCurveTo, using them in sequence for each group of points can lead to disjointed curves. This article presents a solution for drawing smooth curves through any number of points.

Midpoint Interpolation for Smooth Transitions

The key to drawing smooth curves is to avoid sharp angles where curves meet. This can be achieved by drawing curves between the midpoints of adjacent points. This ensures that the end points of each curve share a common control point, resulting in a smooth transition between curves.

Approximation Method

The following code snippet demonstrates the midpoint interpolation method:

<code class="js">// move to the first point
ctx.moveTo(points[0].x, points[0].y);

for (var i = 1; i < points.length - 2; i++) {
  var xc = (points[i].x + points[i + 1].x) / 2;
  var yc = (points[i].y + points[i + 1].y) / 2;
  ctx.quadraticCurveTo(points[i].x, points[i].y, xc, yc);
}

// curve through the last two points
ctx.quadraticCurveTo(points[i].x, points[i].y, points[i + 1].x, points[i + 1].y);</code>

In this code, we move to the first point, then iterate through the remaining points. For each point, we calculate the midpoint between the current point and the next point, and draw a quadratic curve to this midpoint. Finally, we draw a quadratic curve between the last two points.

Advantage of Approximation

While this method does not exactly follow through each point, it provides a visually smooth approximation. For most drawing applications, this level of accuracy is sufficient.

Further Reading

For a more precise method that passes through every point, refer to the following resource: http://www.cartogrammar.com/blog/actionscript-curves-update/.

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