Half-working StrokeEllipse

artist/ellipse.go

@@ -2,6 +2,7 @@ package artist
 
 import "math"
 import "image"
+import "image/color"
 import "git.tebibyte.media/sashakoshka/tomo"
 
 // FillEllipse draws a filled ellipse with the specified pattern.
@@ -12,17 +13,18 @@ func FillEllipse (
 ) (
 	updatedRegion image.Rectangle,
 ) {
+	bounds = bounds.Canon()
 	data, stride := destination.Buffer()
 	realWidth, realHeight := bounds.Dx(), bounds.Dy()
-	bounds = bounds.Canon().Intersect(destination.Bounds()).Canon()
+	bounds = bounds.Intersect(destination.Bounds()).Canon()
 	if bounds.Empty() { return }
 	updatedRegion = bounds
 
-	width,  height := bounds.Dx(), bounds.Dy()
+	width, height := bounds.Dx(), bounds.Dy()
 	for y := 0; y < height; y ++ {
 	for x := 0; x < width;  x ++ {
-		xf := float64(x) / float64(width)  - 0.5
-		yf := float64(y) / float64(height) - 0.5
+		xf := float64(x) / float64(realWidth)  - 0.5
+		yf := float64(y) / float64(realHeight) - 0.5
 		if math.Sqrt(xf * xf + yf * yf) <= 0.5 {
 			data[x + bounds.Min.X + (y + bounds.Min.Y) * stride] =
 				source.AtWhen(x, y, realWidth, realHeight)
@@ -31,4 +33,109 @@ func FillEllipse (
 	return
 }
 
-// TODO: StrokeEllipse
+// StrokeEllipse draws the outline of an ellipse with the specified line weight
+// and pattern.
+func StrokeEllipse (
+	destination tomo.Canvas,
+	source Pattern,
+	weight int,
+	bounds image.Rectangle,
+) {
+	if weight < 1 { return }
+
+	data, stride := destination.Buffer()
+	bounds = bounds.Canon().Inset(weight)
+	radii := image.Pt (
+		bounds.Dx() / 2,
+		bounds.Dy() / 2)
+	center := bounds.Min.Add(radii)
+	width, height := bounds.Dx(), bounds.Dy()
+
+	context := ellipsePlottingContext {
+		data: data,
+		stride: stride,
+		source: source,
+		width: width,
+		height: height,
+		weight: weight,
+		bounds: bounds,
+	}
+
+	x := float64(0)
+	y := float64(radii.Y)
+
+	// region 1 decision parameter
+	decision1 :=
+		float64(radii.Y * radii.Y) -
+		float64(radii.X * radii.X * radii.Y) +
+		(0.25 * float64(radii.X) * float64(radii.X))
+	decisionX := float64(2 * radii.Y * radii.Y * int(x))
+	decisionY := float64(2 * radii.X * radii.X * int(y))
+
+	// draw region 1
+	for decisionX < decisionY {
+		context.plot( int(x) + center.X,  int(y) + center.Y)
+		context.plot(-int(x) + center.X,  int(y) + center.Y)
+		context.plot( int(x) + center.X, -int(y) + center.Y)
+		context.plot(-int(x) + center.X, -int(y) + center.Y)
+
+		if (decision1 < 0) {
+			x ++
+			decisionX += float64(2 * radii.Y * radii.Y)
+			decision1 += decisionX + float64(radii.Y * radii.Y)
+		} else {
+			x ++
+			y --
+			decisionX += float64(2 * radii.Y * radii.Y)
+			decisionY -= float64(2 * radii.X * radii.X)
+			decision1 +=
+				decisionX - decisionY +
+				float64(radii.Y * radii.Y)
+		}
+	}
+
+	// region 2 decision parameter
+	decision2 :=
+		float64(radii.Y * radii.Y) * (x + 0.5) * (x + 0.5) +
+		float64(radii.X * radii.X) * (y - 1)   * (y - 1) -
+		float64(radii.X * radii.X * radii.Y * radii.Y)
+
+	// draw region 2
+	for y >= 0 {
+		context.plot( int(x) + center.X,  int(y) + center.Y)
+		context.plot(-int(x) + center.X,  int(y) + center.Y)
+		context.plot( int(x) + center.X, -int(y) + center.Y)
+		context.plot(-int(x) + center.X, -int(y) + center.Y)
+
+		if decision2 > 0 {
+			y --
+			decisionY -= float64(2 * radii.X * radii.X)
+			decision2 += float64(radii.X * radii.X) - decisionY
+		} else {
+			y --
+			x ++
+			decisionX += float64(2 * radii.Y * radii.Y)
+			decisionY -= float64(2 * radii.X * radii.X)
+			decision2 +=
+				decisionX - decisionY +
+				float64(radii.X * radii.X)
+		}
+	}
+}
+
+type ellipsePlottingContext struct {
+	data []color.RGBA
+	stride int
+	source Pattern
+	width, height int
+	weight int
+	bounds image.Rectangle
+}
+
+func (context ellipsePlottingContext) plot (x, y int) {
+	if (image.Point { x, y }).In(context.bounds) {
+		squareAround (
+			context.data, context.stride, context.source, x, y,
+			context.width, context.height, context.weight)
+	}
+}

elements/testing/artist.go

@@ -89,6 +89,18 @@ func (element *Artist) Resize (width, height int) {
 	// 0, 3
 	artist.FillEllipse(element, uhex(0x00FF00FF), element.cellAt(0, 3))
 
+	// 1, 3
+	for x := 1; x < 4; x ++ {
+		artist.StrokeEllipse (
+			element,
+			[]artist.Pattern {
+				uhex(0xFF0000FF),
+				uhex(0x00FF00FF),
+				uhex(0xFF00FFFF),
+			} [x - 1],
+			x, element.cellAt(x, 3))
+	}
+
 	drawTime := time.Since(drawStart)
 	textDrawer := artist.TextDrawer { }
 	textDrawer.SetFace(defaultfont.FaceRegular)