Question

I am working with drawing polygons based on a given line. I have the logic working out well except in cases where it appears that the polygon intersects itself. However, it doesn’t seem to be 100% consistent, nor does it make sense based on what I’m reading. Below are two images created using the same code. The yellow polygons are the ones I’m concerned with.

Image: http://i31.tinypic.com/24cxxlf.png

I want every case to work like the first case (where the empty area “wrapped” by the polygon is not filled in).

These images are produced by this code:

BufferedImage drawingImage = new BufferedImage(width, height, BufferedImage.TYPE_INT_ARGB);
Graphics2D g = drawingImage.createGraphics();
Polygon polygon = new Polygon(parsedPoints[0], parsedPoints[1], parsedPoints[0].length);
g.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
g.setRenderingHint(RenderingHints.KEY_COLOR_RENDERING, RenderingHints.VALUE_COLOR_RENDER_QUALITY);
g.setColor(drawingColor);
g.fillPolygon(polygon);
float[] scales = {1f, 1f, 1f, 0.7f};
float[] offsets = new float[4];
RescaleOp rop = new RescaleOp(scales, offsets, null);
graphics.drawImage(drawingImage, rop, 0, 0);
graphics.setStroke(new BasicStroke(2));
graphics.setColor(drawingColor);
graphics.drawPolygon(polygon);

(I’m filling the polygon applying a rescale to get some transparency to the fill, and then drawing the border without transparency.)

According to the Java documentation for the Graphics.fillPolygon method:

The area inside the polygon is defined
using an even-odd fill rule, also
known as the alternating rule.

If I understand that correctly, then in both cases a pixel contained within the area “wrapped” by the thick polygon would cross exactly two paths, so it would be considered “outside” the polygon.

So my questions are: (a) am I understanding the even-odd fill rule and (b) is there a way in Java to make the second image work more like the first?

Any thoughts on this would be greatly appreciated.

Thanks.

Answer 1

1. The rule applies per polygon. Java doesn’t care about the polygon you drew even one statement ago.
2. You seem to be misunderstanding the even-odd rule a bit. The practical version of the rule goes a bit like this…for each ‘y’ coordinate the polygon crosses, there’s an ordered list of all the x coordinates where it crosses. The ‘inside’ of the polygon is the sections from each even-numbered index (0, 2, 4…) to the next odd-numbered index.