Waterlining function for QGIS

waterlines.py

from qgis.core import *

@qgsfunction(args='auto', group='Custom')
def waterlines(inc, start, nb_max, grow_ratio, feature, parent):
    """
    Creates repeated buffered polygons
    for a waterlining effect like on old engraved maps.
    <h2>Arguments</h2>
    <table>
        <tr>
            <td>inc</dt>
            <td>Distance increment between each line, in current "units". Negative for inner lines, positive for outer.</td>
        </tr>
        <tr>
            <td>start</dt>
            <td>Distance where to draw the first line.</td>
        </tr>
        <tr>
            <td>nb_max</dt>
            <td>Max number of lines. If -1, fills the entire space.</td>
        </tr>
        <tr>
            <td>grow_ratio</dt>
            <td>Multiplication factor applied at each step. 1 to get equally spaced lines, >1 for increasing distance each time.</td>
        </tr>
    </table>
    <h2>Example usage:</h2>
    <ul>
      <li>waterlines(-50, 0, -1, 1) -> equally spaced lines, spaced by 50, filling the entire polygon.</li>
      <li>waterlines(50, 0, 10, 1.25) -> 10 lines, growing from the polygon, spaced by 50 in the first iteration, then growing by a 1.25 factor.</li>
    </ul>
    """
    geom = feature.geometry()
    if geom.type() != QgsWkbTypes.PolygonGeometry:
        return NULL
    dist = start
    lines = []
    nb_loops = 500 if nb_max < 0 else nb_max    # use 500 as default max to prevent infinite loops
    while nb_loops > 0:
        line = geom.buffer(dist, 8)
        if line.isNull() or line.isEmpty() or not line.isGeosValid():
            break
        lines.append(line)
        dist = (dist + inc) * grow_ratio
        nb_loops -= 1
    return QgsGeometry.collectGeometry(lines)