// Rosetta Code problem: http://rosettacode.org/wiki/Canny_edge_detector
// Adapted from Phix to Yabasic by Galileo, 01/2022

import ReadFromPPM2

MaxBrightness = 255

readPPM("Valve.ppm")
print "Be patient, please ..."

width = peek("winwidth")
height = peek("winheight")
dim pixels(width, height), C_E_D(3, 3)

data -1, -1, -1, -1,  8, -1, -1, -1, -1
for i = 0 to 2
    for j = 0 to 2
        read C_E_D(i, j)
    next
next

// convert image to gray scale
for y = 1 to height
    for x = 1 to width
        c$ = right$(getbit$(x, y, x, y), 6)
        r = dec(left$(c$, 2))
        g = dec(mid$(c$, 3, 2))
        b = dec(right$(c$, 2))
        lumin = floor(0.2126 * r + 0.7152 * g + 0.0722 * b)
        pixels(x, y) = lumin
    next
next

dim new_image(width, height)

divisor = 1

// apply an edge detection filter

for y = 2 to height-2
    for x = 2 to width-2
        newrgb = 0
        for i = -1 to 1
            for j = -1 to 1
                newrgb = newrgb + C_E_D(i+1, j+1) * pixels(x+i, y+j)
            next
            new_image(x, y) = max(min(newrgb / divisor,255),0)
        next
    next
next

// show result

for x = 1 to width
    for y = 1 to height
        c = new_image(x, y)
        color c, c, c
        dot x, y
    next
next