Are algorithm questions allowed here? Well here goes anyway.

I am loading a large dataset into an array, doing some data processing to generate an array of pixel data which I then save as a PNG file.

For the original data, the reference point for the data is the bottom-left corner, but when saving to a PNG file the reference point is the top-left corner - so I have to do a vertical flip.

To add a little complication, I am not able to use CoreGraphics image handling as my code will run on a RaspberryPi, so I am using PureSwift/PNG (https://github.com/PureSwift/png) instead.

The code I have works but my method could be a bit slow - the typical images that I am generating being 7200x7200px (~52M pixels). Redesigning my implementation such that the pixelData is [UInt8][UInt8] might allow me to process the pixels a line at a time, but that would require a big rewrite of other code - which I am trying to avoid.!

import Foundation
import PNG

func writePixelstoFilePNG(_ pixelData: [UInt8], height: Int, width: Int, to destinationURL: URL) -> Bool {
  let size: (x: Int, y: Int) = (width, height)
  let layout: (rgb: PNG.Layout, v: PNG.Layout) = (
    rgb:  .init(format: .rgb8(palette: [], fill: nil, key: nil)),
    v:    .init(format:   .v8(             fill: nil, key: nil))
  )

  var px = pixelData
  vFlipImagePixels(&px, height: height, width: width)

  let image: PNG.Data.Rectangular = .init(packing: px, size: size, layout: layout.v)
  do {
    try image.compress(path: destinationURL.path, level: 0)
  } catch {
    return false
  }
  return true
}

func vFlipImagePixels(_ px: inout [UInt8], height: Int, width: Int) {
  for r in 0 ..< ((height >> 1)) {
    for c in 0 ..< width {
      px.swapAt((height * r) + c, (height - 1 - r) * width + c)
    }
  }
}

Figured out an alternative way to vertically flip the array, by rows, using Array.replaceSubrange

Not sure if it is any more efficient than flipping pixel-by-pixel, but for me this seems a little more intuituive.

func vFlipImagePixelsByRow(_ px: inout [UInt8], height: Int, width: Int) {
  for r in 0 ..< (height >> 1) {
    let rangeA = (r*width) ..< (r*width + width)
    let rangeB = ((height - r - 1) * width) ..< (((height - r - 1) * width) + width)

    let tmpRow = px[rangeA]

    px.replaceSubrange(rangeA, with: px[rangeB])
    px.replaceSubrange(rangeB, with: tmpRow)
  }
}

@KieranConlon -sorry this is not about the question, but can you suggest a good resource for algorithm, paid/free ...thanks

Hi Amit,

Apple have a Swift algorithm's package which, I suppose is a general purpose 'toolbox' for anyone to use, swift-algorithms.

The Ray Wenderlich site (is it OK to mention then on here?) is a good resource, in fact they even have a tutorial for the swift-algorithms mentioned above, RW-Swift-Algorithms.

You could try searching StackOverflow, chances are that someone has asked a similar question in the past.

The application I am writing has very specific algorithms and mathematical methods that I had to implement. I found Mathematics and GIS communities on StackExchange to be really helpful.

Also, for mathematics algorithms and methods I found WolframAlpha's Mathworld to be excellent.

Otherwise, wikipedia is also a good place to look.

Kieran

Thanks a lot never saw MathWorld before, seems great