Rebol3 Code Examplex

Rebol [
    title: "Rosetta code: Barnsley Fern"
    file:  %Barnsley_fern.r3
    url:   https://rosettacode.org/wiki/Barnsley_fern
    needs: 3.0.0
]
barnsley-fern: function [size [pair!]][
    img: make image! reduce [size 0.0.0]
    width:  size/x
    height: size/y

    p: 0x0 x: y: 0.0
    repeat i 100000 [
        xt: yt: 0.0
        r: random 100
        case [
            r <= 1  [xt: 0.0 yt: 0.16 * y]
            r <= 8  [xt: ( 0.20 * x) - (0.26 * y) yt: ( 0.23 * x) + (0.22 * y) + 1.60]
            r <= 15 [xt: (-0.15 * x) + (0.28 * y) yt: ( 0.26 * x) + (0.24 * y) + 0.44]
            'else   [xt: ( 0.85 * x) + (0.04 * y) yt: (-0.04 * x) + (0.85 * y) + 1.60]
        ]
        x: xt y: yt
        p/x: round (width / 2 + (60 * x))                    
        p/y: height - round (60 * y)             
        change at img p green
    ]
]

img: barnsley-fern 640x640
save %barnsley-fern.png img

if cv: attempt [import 'opencv][
    cv/imshow/name img "Barnsley"
    cv/waitKey 0
]