Fastest way to compare two byte arrays in F#

Question

I am writing an application that uses the Kinect photo data and compares two frames against each other in F#. I am using Rob Miles Learn the Kinect Api page 74 as a guidepost but I am not using pointers and the performance is suffering. The byte from the Kinect frame is 1,228,800 bytes. I wrote the comparison like this:

member this.differenceCount(currentImageBytes) =
    if  previousImageBytes |> Seq.length = 0 then
        previousImageBytes <- currentImageBytes
        0
    else
        let bytes = Seq.zip previousImageBytes currentImageBytes
        let differenceCount = bytes |> Seq.mapi(fun i e -> i, e)
                                    |> Seq.filter(fun (i,e) -> i % 4 <> 0 )
                                    |> Seq.map snd
                                    |> Seq.filter(fun (p,c) -> p <> c)
                                    |> Seq.length
        previousImageBytes <- currentImageBytes
        differenceCount

When I run it, the screen lags because (I think) it is taking too long to process the arrays. In addition, the error rate is approaching 50%.

1) Am I approaching the problem wrong? 2) Is there a way to optimize my code to speed things up?

Answer 1

Your sequence mapping/filtering via tuples causes a lot of boxing overhead. The following example avoids the boxing and works in parallel, which (on my machine) is 50 times faster.

let parallelRanges =
    let kinectSize = 1228800
    let subSize = kinectSize / Environment.ProcessorCount
    let iMax = kinectSize - 1
    let steps = [| -1 .. subSize .. iMax |]
    steps.[steps.Length - 1] <- iMax
    steps |> Seq.pairwise |> Seq.toArray |> Array.map (fun (x, y) -> x + 1, y)

let countDiffs (prevBytes:byte[]) (curBytes:_[]) =
    let count (fromI, toI) =
        let rec aux i acc =
            if i > toI then acc
            elif i % 4 <> 0 && prevBytes.[i] <> curBytes.[i] then 
                aux (i + 1) (acc + 1)
            else aux (i + 1) acc
        aux fromI 0

    parallelRanges
    |> Array.Parallel.map count
    |> Array.sum