java-native-access/jna · error · RuntimeException

Area is not polygonal: " + area

Error message

Area is not polygonal: " + area

What it means

W32WindowUtils converts a java.awt.Area into a Win32 polygon region for setWindowMask by walking its PathIterator and accepting only SEG_LINETO/SEG_MOVETO/SEG_CLOSE segments. If the Area contains curves (SEG_QUADTO, SEG_CUBICTO, arcs, ovals), the code throws RuntimeException('Area is not polygonal: <area>').

Source

Thrown at contrib/platform/src/com/sun/jna/platform/WindowUtils.java:1068

            float[] coords = new float[6];
            List<POINT> points = new ArrayList<>();
            int size = 0;
            List<Integer> sizes = new ArrayList<>();
            while (!pi.isDone()) {
                int type = pi.currentSegment(coords);
                if (type == PathIterator.SEG_MOVETO) {
                    size = 1;
                    points.add(new POINT((int)coords[0], (int)coords[1]));
                }
                else if (type == PathIterator.SEG_LINETO) {
                    ++size;
                    points.add(new POINT((int)coords[0], (int)coords[1]));
                }
                else if (type == PathIterator.SEG_CLOSE) {
                    sizes.add(Integer.valueOf(size));
                }
                else {
                    throw new RuntimeException("Area is not polygonal: " + area);
                }
                pi.next();
            }
            POINT[] lppt = (POINT[])new POINT().toArray(points.size());
            POINT[] pts = points.toArray(new POINT[points.size()]);
            for (int i=0;i < lppt.length;i++) {
                lppt[i].x = pts[i].x;
                lppt[i].y = pts[i].y;
            }
            int[] counts = new int[sizes.size()];
            for (int i=0;i < counts.length;i++) {
                counts[i] = sizes.get(i).intValue();
            }
            HRGN hrgn = gdi.CreatePolyPolygonRgn(lppt, counts, counts.length, mode);
            setWindowRegion(w, hrgn);
        }

        @Override

View on GitHub (pinned to d036ad9781)

Solutions

1. Flatten the shape before building the Area: new Area(shape.getPathIterator(null, flatness)) — flattening converts curves to line segments.
2. Build the mask from Polygon/rectangular shapes only, or approximate curves with many line segments.
3. Use a newer JNA version if available (later releases handle non-polygonal areas by flattening).
4. Catch RuntimeException and fall back to a rectangular mask.

Example fix

// before
Area area = new Area(new Ellipse2D.Float(0, 0, 200, 200));
WindowUtils.setWindowMask(frame, area); // RuntimeException: not polygonal
// after
Area area = new Area(
    new Ellipse2D.Float(0, 0, 200, 200).getPathIterator(null, 1.0)); // flattened to line segments
WindowUtils.setWindowMask(frame, area);

PathIterator pi = area.getPathIterator(null);
boolean polygonal = true;
while (!pi.isDone()) { int t = pi.currentSegment(new double[6]); if (t == PathIterator.SEG_QUADTO || t == PathIterator.SEG_CUBICTO) { polygonal = false; break; } pi.next(); }
if (!polygonal) { area = new Area(shape.getPathIterator(null, 1.0)); }

try { WindowUtils.setWindowMask(frame, area); } catch (RuntimeException e) { WindowUtils.setWindowMask(frame, new Area(frame.getBounds())); }

When it happens

Trigger: Calling WindowUtils.setWindowMask(window, area) where the Area was built from curves: new Area(new Ellipse2D.Float(...)), new Area(new RoundRectangle2D...), or any shape whose outline flattens to quadratic/cubic segments without prior flattening.

Common situations: Making rounded or elliptical window shapes with setWindowMask(Area) using oval/round-rect shapes directly; building Areas from Arc2D; forgetting that Area retains curve segments unless flattened.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

• Set sun.java2d.noddraw=true to enable transparent windows
• Window must be a RootPaneContainer
• Component must be heavyweight: " + w
• No support for " + os
• Property Accessor name must start with 'get', or set the ano

AI-assisted analysis of java-native-access/jna@d036ad9781 (2026-09-12). Data as JSON: /api/errors/c0ca65fe022dbe1e. Report an issue: GitHub.