NSImage diff speed test

compare.swift

import Cocoa

func context(for cgImage: CGImage) -> CGContext? {
  guard
    let space = cgImage.colorSpace,
    let context = CGContext(
      data: nil,
      width: cgImage.width,
      height: cgImage.height,
      bitsPerComponent: cgImage.bitsPerComponent,
      bytesPerRow: cgImage.bytesPerRow,
      space: space,
      bitmapInfo: CGImageAlphaInfo.premultipliedLast.rawValue
    )
    else { return nil }

  context.draw(cgImage, in: CGRect(x: 0, y: 0, width: cgImage.width, height: cgImage.height))
  return context
}

private func NSImagePNGRepresentation(_ image: NSImage) -> Data? {
  guard let cgImage = image.cgImage(forProposedRect: nil, context: nil, hints: nil) else { return nil }
  let rep = NSBitmapImageRep(cgImage: cgImage)
  rep.size = image.size
  return rep.representation(using: .png, properties: [:])
}

func compare(_ old: NSImage, _ new: NSImage, precision: Float) -> Bool {
  guard let oldCgImage = old.cgImage(forProposedRect: nil, context: nil, hints: nil) else { return false }
  guard let newCgImage = new.cgImage(forProposedRect: nil, context: nil, hints: nil) else { return false }
  guard oldCgImage.width != 0 else { return false }
  guard newCgImage.width != 0 else { return false }
  guard oldCgImage.width == newCgImage.width else { return false }
  guard oldCgImage.height != 0 else { return false }
  guard newCgImage.height != 0 else { return false }
  guard oldCgImage.height == newCgImage.height else { return false }
  guard let oldContext = context(for: oldCgImage) else { return false }
  guard let newContext = context(for: newCgImage) else { return false }
  guard let oldData = oldContext.data else { return false }
  guard let newData = newContext.data else { return false }
  let byteCount = oldContext.height * oldContext.bytesPerRow
  if memcmp(oldData, newData, byteCount) == 0 { return true }
  let newer = NSImage(data: NSImagePNGRepresentation(new)!)!
  guard let newerCgImage = newer.cgImage(forProposedRect: nil, context: nil, hints: nil) else { return false }
  guard let newerContext = context(for: newerCgImage) else { return false }
  guard let newerData = newerContext.data else { return false }
  if memcmp(oldData, newerData, byteCount) == 0 { return true }
  if precision >= 1 { return false }
  let oldRep = NSBitmapImageRep(cgImage: oldCgImage)
  let newRep = NSBitmapImageRep(cgImage: newerCgImage)
  var differentPixelCount = 0
  let pixelCount = oldRep.pixelsWide * oldRep.pixelsHigh
  let threshold = 1 - precision
  for x in 0..<oldRep.pixelsWide {
    for y in 0..<oldRep.pixelsHigh {
      if oldRep.colorAt(x: x, y: y) != newRep.colorAt(x: x, y: y) { differentPixelCount += 1 }
      if Float(differentPixelCount) / Float(pixelCount) > threshold { return false}
    }
  }
  return true
}

func compare2(_ old: NSImage, _ new: NSImage, precision: Float) -> Bool {
  guard let oldCgImage = old.cgImage(forProposedRect: nil, context: nil, hints: nil) else { return false }
  guard let newCgImage = new.cgImage(forProposedRect: nil, context: nil, hints: nil) else { return false }
  guard oldCgImage.width != 0 else { return false }
  guard newCgImage.width != 0 else { return false }
  guard oldCgImage.width == newCgImage.width else { return false }
  guard oldCgImage.height != 0 else { return false }
  guard newCgImage.height != 0 else { return false }
  guard oldCgImage.height == newCgImage.height else { return false }
  guard let oldContext = context(for: oldCgImage) else { return false }
  guard let newContext = context(for: newCgImage) else { return false }
  guard let oldData = oldContext.data else { return false }
  guard let newData = newContext.data else { return false }
  let byteCount = oldContext.height * oldContext.bytesPerRow
  if memcmp(oldData, newData, byteCount) == 0 { return true }
  let newer = NSImage(data: NSImagePNGRepresentation(new)!)!
  guard let newerCgImage = newer.cgImage(forProposedRect: nil, context: nil, hints: nil) else { return false }
  guard let newerContext = context(for: newerCgImage) else { return false }
  guard let newerData = newerContext.data else { return false }
  if memcmp(oldData, newerData, byteCount) == 0 { return true }
  if precision >= 1 { return false }
  let oldRep = NSBitmapImageRep(cgImage: oldCgImage)
  let newRep = NSBitmapImageRep(cgImage: newerCgImage)
  var differentPixelCount = 0
  let pixelCount = oldRep.pixelsWide * oldRep.pixelsHigh
  let threshold = (1 - precision) * Float(pixelCount)

  let p1: UnsafeMutablePointer<UInt8> = oldRep.bitmapData!
  let p2: UnsafeMutablePointer<UInt8> = newRep.bitmapData!

  for offset in 0 ..< pixelCount {
    if p1[offset] != p2[offset]
        || p1[offset + 1] != p2[offset + 1]
        || p1[offset + 2] != p2[offset + 2]
        || p1[offset + 3] != p2[offset + 3] {
        differentPixelCount += 1
    }
    if Float(differentPixelCount) > threshold { return false }
  }
  return true
}

let orig = NSImage(contentsOfFile: "/Users/sas/Downloads/orig.png")!
let new = NSImage(contentsOfFile: "/Users/sas/Downloads/new.png")!

// compare

do {
    let start = Date()
    print(compare(orig, new, precision: 1))
    let elapsed = Date().timeIntervalSince(start)
    print("compare1, precision 1    : \(elapsed)")
}

do {
    let start = Date()
    print(compare(orig, new, precision: 0.999))
    let elapsed = Date().timeIntervalSince(start)
    print("compare1, precision 0.999: \(elapsed)")
}

// compare2

do {
    let start = Date()
    print(compare2(orig, new, precision: 1))
    let elapsed = Date().timeIntervalSince(start)
    print("compare2, precision 1    : \(elapsed)")
}

do {
    let start = Date()
    print(compare2(orig, new, precision: 0.999))
    let elapsed = Date().timeIntervalSince(start)
    print("compare2, precision 0.999: \(elapsed)")
}