diff -r 48374309f3d1 -r b725b7fb63a5 src/widgets/patternviewer.cpp
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/widgets/patternviewer.cpp	Thu Mar 29 12:10:54 2018 +0300
@@ -0,0 +1,49 @@
+#include <QPainter>
+#include <QPaintEvent>
+#include "patterneditor.h"
+#include "patternviewer.h"
+
+/*
+ * Construct a pattern viewer for an existing pattern editor.
+ */
+PatternViewer::PatternViewer(PatternEditor* parent) :
+	QWidget {parent},
+	editor {parent}
+{
+	this->transformation.scale(4, 4);
+}
+
+/*
+ * Unfortunately, Qt doesn't provide an easy way to translate a floating point rectangle
+ * into a floating point polygon. So here's a manual implementation.
+ */
+static QPolygonF transformRect(const QRectF& rect, const QTransform& transform)
+{
+	QPolygonF transformed;
+	for (const QPointF& point : {
+		rect.topLeft(),
+		rect.topRight(),
+		rect.bottomRight(),
+		rect.bottomLeft()
+	}) {
+		transformed.append(transform.map(point));
+	}
+	return transformed;
+}
+
+/*
+ * Renders the pattern.
+ */
+void PatternViewer::paintEvent(QPaintEvent* event)
+{
+	const Pattern& pattern = this->editor->pattern;
+	static const QPixmap viewerBackground {":/data/pattern-background.png"};
+	static const QPixmap canvasBackground {":/data/transparent-background.png"};
+	QPainter painter {this};
+	painter.drawTiledPixmap(this->rect(), viewerBackground);
+	painter.setBrush(canvasBackground);
+	QRectF canvasRect = QRectF {{0.0f, 0.0f}, pattern.canvasSize};
+	painter.drawPolygon(::transformRect(canvasRect, this->transformation));
+	painter.setTransform(this->transformation);
+	event->accept();
+}