a.go

func main() {
 flag.Parse()
 log.Printf("coloring method: %s", *coloringMethod)
 log.Printf("concurrency level: %d", *concurrencyLevel)
 sem = make(chan bool, *concurrencyLevel)
...
...
}
func renderImage() {
for i := 0; i < imHeight; i++ {
  for j := 0; j < imWidth; j++ {
   plotPixel(complex(float64(i), float64(j)))
   currentPixel++
  }
 }
for i := 0; i < cap(sem); i++ {
  sem <- true
 }
}
// check if c belongs to set, assign color accordingly and plot
func plotPixel(c complex128) {
 sem <- true
switch *coloringMethod {
 case "ml":
  go func(c complex128) {
   res := getColorFromMLMandelbrot(c)
   <-sem
   plot(res.C, res.Color)
  }(c)
 case "classic":
  go func(c complex128) {
   res := getColorFromClassicMandelbrot(c)
   <-sem
   plot(res.C, res.Color)
  }(c)
 }
}