Add paused-state vignette border to game world view

src/ui/GameWorldView.cpp

@@ -19,6 +19,7 @@
 #include <QMessageBox>
 #include <QMouseEvent>
 #include <QPainter>
+#include <QPainterPath>
 #include <QPen>
 #include <QPolygonF>
 #include <QRadialGradient>
@@ -431,6 +432,7 @@ void GameWorldView::paintGL()
     drawBeams(painter);
     drawOverlays(painter);
     drawScreenSpace(painter);
+    drawPauseBorder(painter);
     drawReplayOverlay(painter);
 }
 
@@ -1695,6 +1697,67 @@ void GameWorldView::drawScreenSpace(QPainter& /*painter*/)
 {
 }
 
+void GameWorldView::drawPauseBorder(QPainter& painter)
+{
+    if (m_gameSpeedMultiplier > 0.0) { return; }
+
+    // Border thickness in pixels, capped so it never exceeds half the viewport on
+    // very small windows (which would leave no un-darkened center).
+    const qreal borderPx = std::min<qreal>(
+        100.0, std::min(width(), height()) / 2.0);
+    if (borderPx <= 0.0) { return; }
+
+    const QColor edge(0, 0, 0, 128);        // 50% black at the viewport edge
+    const QColor inner(0, 0, 0, 0);         // fully transparent toward the center
+
+    const qreal w = width();
+    const qreal h = height();
+
+    painter.save();
+
+    // Each side is a linear gradient fading from `edge` at the viewport border to
+    // `inner` toward the center. The four sides are clipped to trapezoids that meet
+    // along the corner diagonals (a mitred picture frame), so the strips never
+    // overlap and don't double-darken the corners. Because the border thickness is
+    // the same on every side, both adjoining gradients evaluate equally along each
+    // 45-degree diagonal and join seamlessly.
+    const std::function<void(const QLinearGradient&, const QPolygonF&)> drawSide =
+        [&](const QLinearGradient& gradient, const QPolygonF& trapezoid) {
+        QPainterPath clip;
+        clip.addPolygon(trapezoid);
+        painter.save();
+        painter.setClipPath(clip);
+        painter.fillRect(QRectF(0, 0, w, h), gradient);
+        painter.restore();
+    };
+
+    QLinearGradient left(0, 0, borderPx, 0);
+    left.setColorAt(0.0, edge);
+    left.setColorAt(1.0, inner);
+    drawSide(left, QPolygonF({ QPointF(0, 0), QPointF(borderPx, borderPx),
+                              QPointF(borderPx, h - borderPx), QPointF(0, h) }));
+
+    QLinearGradient right(w, 0, w - borderPx, 0);
+    right.setColorAt(0.0, edge);
+    right.setColorAt(1.0, inner);
+    drawSide(right, QPolygonF({ QPointF(w, 0), QPointF(w - borderPx, borderPx),
+                               QPointF(w - borderPx, h - borderPx), QPointF(w, h) }));
+
+    QLinearGradient top(0, 0, 0, borderPx);
+    top.setColorAt(0.0, edge);
+    top.setColorAt(1.0, inner);
+    drawSide(top, QPolygonF({ QPointF(0, 0), QPointF(w, 0),
+                             QPointF(w - borderPx, borderPx), QPointF(borderPx, borderPx) }));
+
+    QLinearGradient bottom(0, h, 0, h - borderPx);
+    bottom.setColorAt(0.0, edge);
+    bottom.setColorAt(1.0, inner);
+    drawSide(bottom, QPolygonF({ QPointF(0, h), QPointF(w, h),
+                                QPointF(w - borderPx, h - borderPx), QPointF(borderPx, h - borderPx) }));
+
+    painter.restore();
+}
+
 void GameWorldView::drawReplayOverlay(QPainter& painter)
 {
     if (!m_replayPlayer) { return; }