package main

import (
    "fmt"
    "image"
    "image/color"
    "image/draw"
    "image/png"
    "math"
    "os"
)

func hough(im image.Image, ntx, mry int) draw.Image {
    nimx := im.Bounds().Max.X
    mimy := im.Bounds().Max.Y

    him := image.NewGray(image.Rect(0, 0, ntx, mry))
    draw.Draw(him, him.Bounds(), image.NewUniform(color.White),
        image.Point{}, draw.Src)

    rmax := math.Hypot(float64(nimx), float64(mimy))
    dr := rmax / float64(mry/2)
    dth := math.Pi / float64(ntx)

    for jx := 0; jx < nimx; jx++ {
        for iy := 0; iy < mimy; iy++ {
            col := color.GrayModel.Convert(im.At(jx, iy)).(color.Gray)
            if col.Y == 255 {
                continue
            }
            for jtx := 0; jtx < ntx; jtx++ {
                th := dth * float64(jtx)
                r := float64(jx)*math.Cos(th) + float64(iy)*math.Sin(th)
                iry := mry/2 - int(math.Floor(r/dr+.5))
                col = him.At(jtx, iry).(color.Gray)
                if col.Y > 0 {
                    col.Y--
                    him.SetGray(jtx, iry, col)
                }
            }
        }
    }
    return him
}

func main() {
    f, err := os.Open("Pentagon.png")
    if err != nil {
        fmt.Println(err)
        return
    }
    pent, err := png.Decode(f)
    if err != nil {
        fmt.Println(err)
        return
    }
    if err = f.Close(); err != nil {
        fmt.Println(err)
    }
    h := hough(pent, 460, 360)
    if f, err = os.Create("hough.png"); err != nil {
        fmt.Println(err)
        return
    }
    if err = png.Encode(f, h); err != nil {
        fmt.Println(err)
    }
    if cErr := f.Close(); cErr != nil && err == nil {
        fmt.Println(err)
    }
}