F# lazy pixels reading

Question

I want to make a lazy loading of image pixels to the 3 dimensional array of integers. For example in simple way it looks like this:

   for i=0 to Width 
             


        

Answer 1

What would be slow is the call to GetPixel. If you want to call it only as needed, you could use something like this:

open System.Drawing

let lazyPixels (image:Bitmap) =
    let Width = image.Width
    let Height = image.Height
    let pixels : Lazy[,,] = Array3D.zeroCreate 3 Width Height
    for i = 0 to Width-1 do
        for j = 0 to Height-1 do
            let point = lazy image.GetPixel(i,j)
            pixels.[0,i,j] <- lazy point.Value.R
            pixels.[1,i,j] <- lazy point.Value.G
            pixels.[2,i,j] <- lazy point.Value.B
    pixels

GetPixel will be called at most once for every pixel, and then reused for the other components.

Another way of approaching this problem would be to do a bulk-load of the entire image. This will be a lot quicker than calling GetPixel over and over again.

open System.Drawing
open System.Drawing.Imaging

let pixels (image:Bitmap) =
    let Width = image.Width
    let Height = image.Height
    let rect = new Rectangle(0,0,Width,Height)

    // Lock the image for access
    let data = image.LockBits(rect, ImageLockMode.ReadOnly, image.PixelFormat)

    // Copy the data
    let ptr = data.Scan0
    let stride = data.Stride
    let bytes = stride * data.Height
    let values : byte[] = Array.zeroCreate bytes
    System.Runtime.InteropServices.Marshal.Copy(ptr,values,0,bytes)

    // Unlock the image
    image.UnlockBits(data)

    let pixelSize = 4 // <-- calculate this from the PixelFormat

    // Create and return a 3D-array with the copied data
    Array3D.init 3 Width Height (fun i x y ->
        values.[stride * y + x * pixelSize + i])

(adopted from the C# sample on Bitmap.LockBits)