SVG rounded corner

Answer 1

I've happened upon this problem today myself and managed to solve it by writing a small JavaScript function.

From what I can tell, there is no easy way to give a path element in an SVG rounded corners except if you only need the borders to be rounded, in which case the (CSS) attributes stroke, stroke-width and most importantly stroke-linejoin="round" are perfectly sufficient.

However, in my case I used a path object to create custom shapes with n corners that are filled out with a certain color and don't have visible borders, much like this:

I managed to write a quick function that takes an array of coordinates for an SVG path and returns the finished path string to put in the d attribute of the path html element. The resulting shape will then look something like this:

Here is the function:

/**
 * Creates a coordinate path for the Path SVG element with rounded corners
 * @param pathCoords - An array of coordinates in the form [{x: Number, y: Number}, ...]
 */
function createRoundedPathString(pathCoords) {
    const path = [];
    const curveRadius = 3;

    // Reset indexes, so there are no gaps
    pathCoords = pathCoords.slice();

    for (let i = 0; i < pathCoords.length; i++) {

      // 1. Get current coord and the next two (startpoint, cornerpoint, endpoint) to calculate rounded curve
      const c2Index = ((i + 1) > pathCoords.length - 1) ? (i + 1) % pathCoords.length : i + 1;
      const c3Index = ((i + 2) > pathCoords.length - 1) ? (i + 2) % pathCoords.length : i + 2;

      const c1 = pathCoords[i];
      const c2 = pathCoords[c2Index],
      const c3 = pathCoords[c3Index];

      // 2. For each 3 coords, enter two new path commands: Line to start of curve, bezier curve around corner.

      // Calculate curvePoint c1 -> c2
      const c1c2Distance = Math.sqrt(Math.pow(c1.x - c2.x, 2) + Math.pow(c1.y - c2.y, 2));
      const c1c2DistanceRatio = (c1c2Distance - curveRadius) / c1c2Distance;
      const c1c2CurvePoint = [
        ((1 - c1c2DistanceRatio) * c1.x + c1c2DistanceRatio * c2.x).toFixed(1),
        ((1 - c1c2DistanceRatio) * c1.y + c1c2DistanceRatio * c2.y).toFixed(1)
      ];

      // Calculate curvePoint c2 -> c3
      const c2c3Distance = Math.sqrt(Math.pow(c2.x - c3.x, 2) + Math.pow(c2.y - c3.y, 2));
      const c2c3DistanceRatio = curveRadius / c2c3Distance;
      const c2c3CurvePoint = [
        ((1 - c2c3DistanceRatio) * c2.x + c2c3DistanceRatio * c3.x).toFixed(1),
        ((1 - c2c3DistanceRatio) * c2.y + c2c3DistanceRatio * c3.y).toFixed(1)
      ];

      // If at last coord of polygon, also save that as starting point
      if (i === pathCoords.length - 1) {
        path.unshift('M' + c2c3CurvePoint.join(','));
      }

      // Line to start of curve (L endcoord)
      path.push('L' + c1c2CurvePoint.join(','));
      // Bezier line around curve (Q controlcoord endcoord)
      path.push('Q' + c2.x + ',' + c2.y + ',' + c2c3CurvePoint.join(','));
    }
    // Logically connect path to starting point again (shouldn't be necessary as path ends there anyway, but seems cleaner)
    path.push('Z');

    return path.join(' ');
}

You can determine the rounding strength by setting the curveRadius variable at the top. The default is 3 for a 100x100 (viewport) coordinate system, but depending on the size of your SVG, you may need to adjust this.