Refactor ellipse algorithm to allow for dynamic midpoints

ifennna · Apr 19, 2019 · 6644937 · 6644937
1 parent 95bb623
commit 6644937
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Showing 2 changed files with 30 additions and 28 deletions.
diff --git a/ellipses/ellipses.go b/ellipses/ellipses.go
@@ -2,57 +2,59 @@ package ellipses
 
 import "go-graphics/pixel"
 
-func Draw(start pixel.Point, majorDistance float64) {
-	minorDistance := start.Y
+func Draw(midpoint pixel.Point, horizontalDistance float64, verticalDistance float64) {
 	var derivativeOfX float64 = 0
-	derivativeOfY := 2 * majorDistance * majorDistance * minorDistance
-	decisionVariable := (minorDistance * minorDistance) -
-		(majorDistance * majorDistance * minorDistance) +
-		(0.25 * majorDistance * majorDistance)
-	currentPixel := start
+	derivativeOfY := 2 * horizontalDistance * horizontalDistance * verticalDistance
+	decisionVariable := (verticalDistance * verticalDistance) -
+		(horizontalDistance * horizontalDistance * verticalDistance) +
+		(0.25 * horizontalDistance * horizontalDistance)
+	currentPixel := pixel.Point{X: 0, Y: verticalDistance}
 
 	for derivativeOfX < derivativeOfY {
-		drawMirroredPixels(currentPixel)
+		drawMirroredPixels(midpoint, currentPixel)
 		currentPixel.X++
 
-		derivativeOfX += minorDistance * minorDistance * 2
+		derivativeOfX += verticalDistance * verticalDistance * 2
 
 		if decisionVariable < 0 {
-			decisionVariable += derivativeOfX + (minorDistance * minorDistance)
+			decisionVariable += derivativeOfX + (verticalDistance * verticalDistance)
 		} else {
 			currentPixel.Y--
-			derivativeOfY -= majorDistance * majorDistance * 2
-			decisionVariable += derivativeOfX + (minorDistance * minorDistance) - derivativeOfY
+			derivativeOfY -= horizontalDistance * horizontalDistance * 2
+			decisionVariable += derivativeOfX + (verticalDistance * verticalDistance) - derivativeOfY
 		}
 	}
 
-	drawMirroredPixels(currentPixel)
+	drawMirroredPixels(midpoint, currentPixel)
 
-	decisionVariable = minorDistance*minorDistance*(currentPixel.X+0.5)*(currentPixel.X+0.5) +
-		majorDistance*majorDistance*(currentPixel.Y-1)*(currentPixel.Y-1) -
-		minorDistance*minorDistance*majorDistance*majorDistance
+	decisionVariable = verticalDistance*verticalDistance*(currentPixel.X+0.5)*(currentPixel.X+0.5) +
+		horizontalDistance*horizontalDistance*(currentPixel.Y-1)*(currentPixel.Y-1) -
+		verticalDistance*verticalDistance*horizontalDistance*horizontalDistance
 
 	for currentPixel.Y > 0 {
 		currentPixel.Y--
-		derivativeOfY -= majorDistance * majorDistance * 2
+		derivativeOfY -= horizontalDistance * horizontalDistance * 2
 
 		if decisionVariable >= 0 {
-			decisionVariable -= derivativeOfY + majorDistance*majorDistance
+			decisionVariable -= derivativeOfY + horizontalDistance*horizontalDistance
 		} else {
 			currentPixel.X++
-			derivativeOfX += minorDistance * minorDistance * 2
-			decisionVariable += derivativeOfX - derivativeOfY + majorDistance*majorDistance
+			derivativeOfX += verticalDistance * verticalDistance * 2
+			decisionVariable += derivativeOfX - derivativeOfY + horizontalDistance*horizontalDistance
 		}
 
-		drawMirroredPixels(currentPixel)
+		drawMirroredPixels(midpoint, currentPixel)
 	}
 }
 
-func drawMirroredPixels(point pixel.Point) {
-	sectorOne := point
-	sectorTwo := pixel.Point{X: -point.X, Y: point.Y}
-	sectorThree := pixel.Point{X: -point.X, Y: -point.Y}
-	sectorFour := pixel.Point{X: point.X, Y: -point.Y}
+func drawMirroredPixels(midpoint pixel.Point, point pixel.Point) {
+	x := midpoint.X + point.X
+	y := midpoint.Y + point.Y
+
+	sectorOne := pixel.Point{X: x, Y: y}
+	sectorTwo := pixel.Point{X: -x, Y: y}
+	sectorThree := pixel.Point{X: -x, Y: -y}
+	sectorFour := pixel.Point{X: x, Y: -y}
 
 	pixel.DrawPixel(sectorOne)
 	pixel.DrawPixel(sectorTwo)

diff --git a/main.go b/main.go
@@ -47,8 +47,8 @@ func run() {
 	circles.Draw(pixel.Point{0, 0}, 19, "midpoint")
 	circles.Draw(pixel.Point{0, 0}, 29, "bresenhams")
 
-	ellipse := pixel.Point{0, 15}
-	ellipses.Draw(ellipse, 31)
+	ellipse := pixel.Point{0, 0}
+	ellipses.Draw(ellipse, 31, 15)
 
 	arcStart := pixel.Point{0, 70}
 	arcEnd := pixel.Point{50, 20}