How Can You Smoothly Connect Multiple Points Using JavaScript?

Using JavaScript to Draw Smooth Curves Through Multiple Points

In drawing applications, it's essential to be able to connect multiple points smoothly. However, using the native JavaScript line drawing functions (lineTo, quadraticCurveTo, and bezierCurveTo) provides a limited approach, resulting in kinks or disjoint curves. This article explores a method to create smooth curves through a series of points using an approximation technique.

The Problem with Disjoint Curves

When using separate "curveTo" functions for each pair of points, the resulting curve exhibits discontinuities at the end points. This is because each curve segment is only influenced by its own specific control points.

An Approximation to Smoothness

The proposed solution involves connecting the curves to the midpoints between the subsequent sample points. This creates more continuity at the end points, as the endpoint of one curve becomes a control point for the next.

Drawing Code

To implement this approximation, follow these steps:

1. Move to the first point using moveTo().
2. Iterate through the points, excluding the last two.
3. For each point pair (i, i 1), calculate the midpoint (xc, yc).
4. Use quadraticCurveTo() to draw a curve from the current point to the midpoint, effectively transitioning to a new curve starting from the midpoint.
5. Draw a curve through the last two points using quadraticCurveTo().

Code Example

Here is a code snippet that demonstrates this approach:

<code class="javascript">const canvas = document.querySelector("canvas");
const ctx = canvas.getContext("2d");
const points = [
  { x: 50, y: 50 },
  { x: 180, y: 100 },
  { x: 75, y: 120 },
  { x: 40, y: 40 },
];

// Move to the first point
ctx.moveTo(points[0].x, points[0].y);

// Iterate through the remaining points
for (var i = 1; i < points.length - 2; i++) {
  // Calculate the midpoint
  var xc = (points[i].x + points[i + 1].x) / 2;
  var yc = (points[i].y + points[i + 1].y) / 2;
  // Draw a curve from the current point to the midpoint
  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
);

// Stroke the curve
ctx.stroke();</code>

This code demonstrates the smooth curve that can be drawn through multiple points using this approximation technique.

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