Fixed miter joins generated by the GL2 engine stroker.

The stroker generated two points for each miter join, one on the
convex side of the join, and one on the concave side. For sharp
joins between curved segments, the point on the concave side
could end up poking out of the stroke. This was fixed by
generating one point on the convex side only.

Reviewed-by: Trond

1 files changed, 36 insertions, 17 deletions

diff --git a/src/opengl/gl2paintengineex/qtriangulatingstroker_p.h b/src/opengl/gl2paintengineex/qtriangulatingstroker_p.h
index 412e239..fd2972c 100644
--- a/src/opengl/gl2paintengineex/qtriangulatingstroker_p.h
+++ b/src/opengl/gl2paintengineex/qtriangulatingstroker_p.h
@@ -301,23 +301,42 @@ void QTriangulatingStroker::join(const qreal *pts)
     case Qt::BevelJoin:
         break;
     case Qt::MiterJoin: {
-        int p1 = m_vertices.size() - 6;
-        int p2 = m_vertices.size() - 2;
-        QLineF line(m_vertices.at(p1), m_vertices.at(p1+1),
-                    m_vertices.at(p2), m_vertices.at(p2+1));
-        QLineF nextLine(m_cx - m_nvx, m_cy - m_nvy,
-                        pts[0] - m_nvx, pts[1] - m_nvy);
-
-        QPointF isect;
-        if (line.intersect(nextLine, &isect) != QLineF::NoIntersection
-            && QLineF(line.p2(), isect).length() <= m_miter_limit) {
-            // The intersection point mirrored over the m_cx, m_cy point
-            m_vertices.add(m_cx - (isect.x() - m_cx));
-            m_vertices.add(m_cy - (isect.y() - m_cy));
-
-            // The intersection point
-            m_vertices.add(isect.x());
-            m_vertices.add(isect.y());
+        // Find out on which side the join should be.
+        int count = m_vertices.size();
+        float prevNvx = m_vertices.at(count - 2) - m_cx;
+        float prevNvy = m_vertices.at(count - 1) - m_cy;
+        float xprod = prevNvx * m_nvy - prevNvy * m_nvx;
+        float px, py, qx, qy;
+
+        // If the segments are parallel, use bevel join.
+        if (qFuzzyIsNull(xprod))
+            break;
+
+        // Find the corners of the previous and next segment to join.
+        if (xprod < 0) {
+            px = m_vertices.at(count - 2);
+            py = m_vertices.at(count - 1);
+            qx = m_cx - m_nvx;
+            qy = m_cy - m_nvy;
+        } else {
+            px = m_vertices.at(count - 4);
+            py = m_vertices.at(count - 3);
+            qx = m_cx + m_nvx;
+            qy = m_cy + m_nvy;
+        }
+
+        // Find intersection point.
+        float pu = px * prevNvx + py * prevNvy;
+        float qv = qx * m_nvx + qy * m_nvy;
+        float ix = (m_nvy * pu - prevNvy * qv) / xprod;
+        float iy = (prevNvx * qv - m_nvx * pu) / xprod;
+
+        // Check that the distance to the intersection point is less than the miter limit.
+        if ((ix - px) * (ix - px) + (iy - py) * (iy - py) <= m_miter_limit * m_miter_limit) {
+            m_vertices.add(ix);
+            m_vertices.add(iy);
+            m_vertices.add(ix);
+            m_vertices.add(iy);
         }
         // else
         // Do a plain bevel join if the miter limit is exceeded or if