Tooling

Tools/Terrain/internal/template/despeckle.go

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+package template
+
+import "salty/terrain/internal/field"
+
+// Despeckle removes the hairline of a class nobody painted that appears along a boundary between two others.
+//
+// It exists because of a measured failure, and the arithmetic is worth keeping because it will happen to
+// anybody who exports a lossy image. Classification gives every pixel its nearest class in RGB, and a codec
+// blends across every boundary it finds. On the first template the antialiased edge between `surf`
+// (221,238,238) and `lowland` (153,204,102) comes out at about (186,219,174), whose distance to `desert`
+// (238,221,153) is **53.8** against **77.9** to either of the colours it was actually mixed from. So every
+// temperate coast on the map gained a one-pixel ribbon of desert - 1607 pixels of it nowhere near the real
+// desert - and it was invisible until the coast mask made those strays the nearest *land* to a stretch of
+// open water and handed their class to every cell it turned into shore. An eleven-pixel band of desert
+// appeared along a green continent, and the mask was blamed for it first.
+//
+// **The test is spatial, and it has to be.** The obvious fix is colorimetric - notice that the pixel lies on
+// the line between two class colours and give it to the nearer one - and it was built, measured and thrown
+// away, because it cannot work in general and this legend is the proof: `shelf` (153,204,221) sits 10 units
+// from the line between `ocean` and `surf`, so a real shelf pixel with a little codec noise on it and a
+// genuine ocean/surf blend are the same point in colour space. That rule reclassified 943 000 painted shelf
+// pixels. What actually distinguishes a stray is *where* it is: a class nobody painted here occupies a line
+// one pixel wide with two other classes on either side of it, and no painted feature at 12.9 m a pixel is
+// one pixel wide - the one thing that was, the decorative stroke, has a pass of its own.
+//
+// Hence a five by five window rather than three by three. A one-pixel ribbon running through the middle of a
+// 3x3 holds three of its nine cells and the two classes it divides hold three each, so nothing has a
+// majority and the rule cannot fire; over 5x5 the ribbon holds five of twenty-five against ten and ten, which
+// is the signature being looked for. A feature two pixels wide already holds ten and is left alone.
+func (r *Raster) Despeckle() int {
+	if r.W < despeckleWindow || r.H < despeckleWindow {
+		return 0
+	}
+	out := make([]uint8, len(r.Class))
+	copy(out, r.Class)
+
+	const rad = despeckleWindow / 2
+	counts := make([]int32, field.BandCount(r.H)*256)
+	changed := make([]int, field.BandCount(r.H))
+
+	field.RowsIndexed(r.H, func(band, y0, y1 int) {
+		c := counts[band*256 : band*256+256]
+		for y := y0; y < y1; y++ {
+			for x := 0; x < r.W; x++ {
+				self := r.Class[y*r.W+x]
+				for dy := -rad; dy <= rad; dy++ {
+					for dx := -rad; dx <= rad; dx++ {
+						c[r.At(x+dx, y+dy)]++
+					}
+				}
+				best, bestN := self, int32(0)
+				for i := range c {
+					if c[i] > bestN || (c[i] == bestN && uint8(i) < best) {
+						best, bestN = uint8(i), c[i]
+					}
+				}
+				selfN := c[self]
+				for i := range c {
+					c[i] = 0
+				}
+				if selfN <= despeckleThin && bestN >= despeckleMajority && best != self {
+					out[y*r.W+x] = best
+					changed[band]++
+				}
+			}
+		}
+	})
+
+	total := 0
+	for _, n := range changed {
+		total += n
+	}
+	r.Class = out
+	return total
+}
+
+const (
+	// despeckleWindow is five: see the note above on why three is too small to see a ribbon at all.
+	despeckleWindow = 5
+
+	// despeckleThin is how little of its own window a class may hold and still be called a stray. Five of
+	// twenty-five is a line one pixel wide straight through the middle; a feature two pixels wide holds ten.
+	despeckleThin = 5
+
+	// despeckleMajority is how much of the window the replacement has to hold. Eight of twenty-five means
+	// there is something clearly there to join, so a pixel in genuinely mixed country is left as it is.
+	despeckleMajority = 8
+)