Edge Filter using F# and Accord.NET

Read Time: 2 minutes

This is a sample of how to apply edge filters to a set of images using F# and Accord.NET. The framework provides several Filters for image manipulating. Since I’m interested in edge enhancement I’ll limit my scope to those filters. In particular I’ve selected the DifferenceEdgeDetector().

Using Paket, here is a sample paket.dependencies file.

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source https://nuget.org/api/v2
nuget Accord
nuget Accord.Imaging

This is the boring setup stuff. 

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// Ref: http://accord-framework.net/docs/html/N_Accord_Imaging_Filters.htm

#I "../packages"
#r "../packages/Accord/lib/net45/accord.dll"
#r "../packages/Accord.Imaging/lib/net45/accord.imaging.dll"

open System
open System.Drawing
open System.Drawing.Imaging
open System.IO
open Accord
open Accord.Imaging
open Accord.Imaging.Filters

let imageRoot = Path.GetFullPath(Path.Combine(__SOURCE_DIRECTORY__, "../data/"))
let resultsRoot = Path.GetFullPath(Path.Combine(__SOURCE_DIRECTORY__, "../data/results/"))

Loading and applying the filter is straight-forward. The only additional point worthy of mention is that most of the edge filters require the image to be in grayscale. That conversion is included in this function.

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// Process image and save a result file with edge filter applied
let filterImage (resultsDir:string) (imageName:string) =
    let resultImageName = Path.Combine(resultsDir, Path.GetFileName(imageName))        
    let bitmap = new Bitmap(imageName)

    // Need to reduce to grayscale, filter needs a reduced color bitmap to process
    let bitmapGray = Grayscale.CommonAlgorithms.BT709.Apply(bitmap)
    let filter = new DifferenceEdgeDetector()

    filter.ApplyInPlace(bitmapGray)

    File.Delete resultImageName
    bitmapGray.Save(resultImageName)

The below code gets a list of qualifying images, then sends them through the filtering function.

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// Is file an image file?
let isImageFile (fileName:string) = 
    fileName.EndsWith(".jpg", StringComparison.OrdinalIgnoreCase)
    || fileName.EndsWith(".png", StringComparison.OrdinalIgnoreCase)

// Get Image list to process
let imageNames = 
    Directory.GetFiles imageRoot 
    |> Array.filter isImageFile

// Process images
imageNames 
|> Array.iter (filterImage resultsRoot)

Below is an example of the edge filter applied.

Before

After

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