Heightmap generation using midpoint displacement

I am writing a program to generate a height map following the midpoint displacement algorithm (somewhat similar to diamond-square).

I'm at the point where I have a recursive program that paints the whole map but still needs some modifications to provide the desired result.

Before I do, I would like a code review on what I have so far to make sure everything so far is ok.

height_map.fs

module HeightMap

// contains the height map types and common functions that can be re-used for 
// different generation algorithms

type HeightMap = {Size:int; Map:float array} with     
    member this.Get x y =
        this.Map.[x * this.Size + y]      

    member this.Set x y value =
        this.Map.[x * this.Size + y]  0.0
    | v when v > 1.0 -> 1.0
    | _ -> v

// converts a float point ranging from 0.0 to 1.0 to a rgb value
// 0.0 represents black and 1.0 white. The conversion is in greyscale 
let convertFloatToRgb (pct:float) : int * int * int =
    let greyscale = int (float 255 * pct)
    (greyscale, greyscale, greyscale)

// returns the average between two values    
let avgf (a:float) (b:float) =
    (a + b) / 2.0

// find the middle between two points in our map
let avgi (a:int) (b:int) =
    (a + b) / 2

// returns a floating number which is generated using bounds as a control of the range of possible values
let randomize (rnd:System.Random) (bound:int) : float = 
    float (rnd.Next(-bound, bound) / bound)

midpoint_displacement.fs

```
module MidpointDisplacement

open HeightMap

// set the four corners to random values
let initCorners (hm:HeightMap) =
let rnd = System.Random()
let size = hm.Size

hm.Set 0 0 (rnd.NextDouble())
hm.Set 0 (size - 1) (rnd.NextDouble())
hm.Set (size - 1) 0 (rnd.NextDouble())
hm.Set (size - 1) (size - 1) (rnd.NextDouble())

// set the middle values between each corner (c1 c2 c3 c4)
// variation is a function that is applied on each pixel to modify it's value
let middle (hm:HeightMap) (x1, y1) (x2

First thing to do when you see a boilerplate code is to represent code as data.
So your function middle is quite simple iterate all sides of a square(represented as a pair of points) use side points and a point between them and apply change. In short you could write something like that

let middle (hm:HeightMap) (x1, y1) (x2, y2) (x3, y3) (x4, y4) (variation) =
    let points = [|x1, y1; x2, y2; x4, y4; x3, y3; x1, y1|]//clockwise iterate sides
    for i in 0..3 do
        let x1, y1 = points.[i]
        let x2, y2 = points.[i + 1]
        let mx, my = avgi x1 x2, avgi y1 y2

        if hm.Get mx my = 0.0 then 
            hm.Set mx my (avgf (hm.Get x1 y1) (hm.Get x2 y2) |> variation |> normalizeValue)

One additional trick when you work with an array with some cursor inside it and have to do some recursive work then you could apply comonads.

StackExchange Code Review Q#122875, answer score: 3