How to calculate the area of a java.awt.geom.Area?

geometry, java, swing

Solution

One approach would be to `fill()` each scaled and transformed `Shape` with a different color using a suitable `AlphaComposite` and count the overlapping pixels in the underlying `Raster`.

Addendum 1: Using this calculator to see the effect of `AlphaComposite.Xor` shows that the intersetion of any two opaque colors is zero.

Addendum 2: Counting pixels may have performance problems; sampling may help. If each `Shape` is reasonably convex, it may be possible to estimate the overlap from the ratio of the `intersect()` area to the sum of the areas of the `Shape`s' `getBounds2D()`. For example,

Shape s1, s2 ...
Rectangle2D r1 = s1.getBounds2D();
Rectangle2D r2 = s2.getBounds2D();
Rectangle2D r3 = new Rectangle2D.Double();
Rectangle2D.intersect(r1, r2, r3);
double overlap = area(r3) / (area(r1) + area(r2));
...
private double area(Rectangle2D r) {
    return r.getWidth() * r.getHeight();
}

You may need to validate the results empirically.

Problem

I am looking for a way to calculate the area, in pixels, of an arbitrary instance of `java.awt.geom.Area`. The background: I have `Shape`s in my applications that may overlap. I want to know how much one `Shape` overlaps another. The `Shape`s may be skewed, rotated, etc. If I had a function `area(Shape)` (or `Area`), I could use the intersection of two `Shape`s like so: ``` double fractionObscured(Shape bottom, Shape top) { Area intersection = new Area(bottom); intersection.intersect(new Area(top)); return area(intersection) / area(bottom); } ```

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