The pattern I use for chaining GPUImage filters on a single UIImage

UIImage+GPUImage.swift

import Foundation

extension UIImage {
  func blur() -> UIImage {
    let boxBlur = GPUImageBoxBlurFilter()
    boxBlur.blurRadiusInPixels = 4.0
    return boxBlur.imageByFilteringImage(self)
  }
    
  func canny() -> UIImage {
    let canny = GPUImageCannyEdgeDetectionFilter()
    return canny.imageByFilteringImage(self)
  }
}

// Example usage: 
//   let result = image.blur().canny()