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copied!<p>I once needed something similar than you. I needed to test two paths for similarity. Therefore I created a path from a list of points (I hope you don't need a more complex path since this solution would become extremely more difficult for general QPaintingPaths). This path is constructed using a given "tolerance", this is your <code>selectionMargin</code>.</p> <p>The function returns a QPainterPath which "draws a region around the given polyline". This region can then be filled and would result in the same image as drawing the original polyline using a pen width of <code>tolerance</code> using round cap and round join options.</p> <p>You can also, and this is what you want to do, check if a given point is contained in this path. Note that <code>QPainterPath::contains</code> checks for a point to lie within the closed region defined by the path. E.g., this closed region is empty for a single line segment and a triangle for two line segments, so this is not what you want if you use <code>contains</code> directly on your path (as I mentioned in the 3rd comment to your question).</p> <pre><code>QPainterPath intersectionTestPath(QList<QPointF> input, qreal tolerance) { //will be the result QPainterPath path; //during the loop, p1 is the "previous" point, initially the first one QPointF p1 = input.takeFirst(); //begin with a circle around the start point path.addEllipse(p1, tolerance, tolerance); //input now starts with the 2nd point (there was a takeFirst) foreach(QPointF p2, input) { //note: during the algorithm, the pair of points (p1, p2) // describes the line segments defined by input. //offset = the distance vector from p1 to p2 QPointF offset = p2 - p1; //normalize offset to length of tolerance qreal length = sqrt(offset.x() * offset.x() + offset.y() * offset.y()); offset *= tolerance / length; //"rotate" the offset vector 90 degrees to the left and right QPointF leftOffset(-offset.y(), offset.x()); QPointF rightOffset(offset.y(), -offset.x()); //if (p1, p2) goes downwards, then left lies to the left and //right to the right of the source path segment QPointF left1 = p1 + leftOffset; QPointF left2 = p2 + leftOffset; QPointF right1 = p1 + rightOffset; QPointF right2 = p2 + rightOffset; //rectangular connection from p1 to p2 { QPainterPath p; p.moveTo(left1); p.lineTo(left2); p.lineTo(right2); p.lineTo(right1); p.lineTo(left1); path += p; //add this to the result path } //circle around p2 { QPainterPath p; p.addEllipse(p2, tolerance, tolerance); path += p; //add this to the result path } p1 = p2; } //This does some simplification; you should use this if you call //path.contains() multiple times on a pre-calculated path, but //you won't need this if you construct a new path for every call //to path.contains(). return path.simplified(); } </code></pre>

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