diff -r 000781318c36 -r b54b350dff5d src/tools/drawtool.cpp
--- a/src/tools/drawtool.cpp	Fri Jul 30 01:28:39 2021 +0300
+++ b/src/tools/drawtool.cpp	Fri Aug 27 00:55:32 2021 +0300
@@ -10,6 +10,7 @@
 static const QPen polygonPen = {QBrush{Qt::black}, 2.0, Qt::DashLine};
 static const QPen pointPen = {QBrush{Qt::black}, 2.0};
 static const QBrush polygonBrush = {QColor{64, 255, 128, 192}};
+static const QBrush badPolygonBrush = {QColor{255, 96, 96, 192}};
 
 DrawTool::DrawTool(QObject* parent) :
 	BaseTool{parent} {}
@@ -83,6 +84,10 @@
 {
 	this->previewPolygon.resize(this->polygon.size() + 1);
 	this->previewPolygon.back() = this->previewPoint;
+	if (this->previewPolygon.size() > 2)
+	{
+		this->isconcave = not geom::convex(this->previewPolygon);
+	}
 }
 
 void DrawTool::reset()
@@ -92,7 +97,7 @@
 
 void DrawTool::overpaint(Canvas* canvas, QPainter* painter) const
 {
-	painter->setBrush(::polygonBrush);
+	painter->setBrush(this->isconcave ? ::badPolygonBrush : ::polygonBrush);
 	painter->setPen(::polygonPen);
 	canvas->drawWorldPolygon(painter, this->previewPolygon);
 	painter->setBrush(::pointBrush);