Added draw-image.go which draws circle using Midpoint circle algorithm. To output full circle I used to draw smaller circles inside up to center but eventually there are not filled pixels left.

draw-image.go

@@ -0,0 +1,76 @@
+// Copyright 2018 Gytis Repečka. All rights reserved.
+// Use of this source code is governed by a GNU GPL
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+  "fmt"
+  "os"
+  "image"
+  "image/color"
+  "image/png"
+  "image/draw"
+)
+
+type Circle struct {
+  imageToDraw draw.Image
+  x, y, r int
+  c color.Color
+}
+
+// https://en.wikipedia.org/wiki/Midpoint_circle_algorithm
+func (circle *Circle) Draw() {
+  x := circle.r - 1
+  y := 0
+  dx := 1
+  dy := 1
+  err := dx - (circle.r * 2)
+
+  for x > y {
+    circle.imageToDraw.Set(circle.x + x, circle.y + y, circle.c)
+    circle.imageToDraw.Set(circle.x + y, circle.y + x, circle.c)
+    circle.imageToDraw.Set(circle.x - y, circle.y + x, circle.c)
+    circle.imageToDraw.Set(circle.x - x, circle.y + y, circle.c)
+    circle.imageToDraw.Set(circle.x - x, circle.y - y, circle.c)
+    circle.imageToDraw.Set(circle.x - y, circle.y - x, circle.c)
+    circle.imageToDraw.Set(circle.x + y, circle.y - x, circle.c)
+    circle.imageToDraw.Set(circle.x + x, circle.y - y, circle.c)
+
+    if err <= 0 {
+      y++
+      err += dy
+      dy += 2
+    }
+    if err > 0 {
+      x--
+      dx += 2
+      err += dx - (circle.r * 2)
+    }
+
+  }
+}
+
+func main() {
+  outFileName := "draw-image.png"
+
+  // Create 400x300 image
+  imageToDraw := image.NewRGBA(image.Rect(0, 0, 400, 300))
+
+  var myCircle Circle
+  
+
+  for r := 130; 0 < r; r-- {
+    myCircle = Circle{imageToDraw, 200, 150, r, color.RGBA{0, 200, 0, 255}}
+    myCircle.Draw()
+  }
+
+  f, errf := os.OpenFile(outFileName, os.O_WRONLY|os.O_CREATE, 0600)
+  if errf != nil {
+    fmt.Println(errf)
+    return
+  }
+  defer f.Close()
+  png.Encode(f, imageToDraw)
+  fmt.Printf("Image written: %s\n", outFileName)
+}