Fred Hamilton Fred Hamilton - 1 year ago 116
Swift Question

How do I load and edit a bitmap file at the pixel level in Swift for iOS?

I want to manipulate an image at the pixel level in Swift. This question was answered for objective c: How do I access and manipulate JPEG image pixels?, but I would like to see the equivalent source for Swift.

Answer Source

This is how I am getting a color from an image at a touch location.

I translated this answer:

(This sample does no error checking for nil)

func createARGBBitmapContext(inImage: CGImage) -> CGContext {
  var bitmapByteCount = 0
  var bitmapBytesPerRow = 0

  //Get image width, height
  let pixelsWide = CGImageGetWidth(inImage)
  let pixelsHigh = CGImageGetHeight(inImage)

  // Declare the number of bytes per row. Each pixel in the bitmap in this
  // example is represented by 4 bytes; 8 bits each of red, green, blue, and
  // alpha.
  bitmapBytesPerRow = Int(pixelsWide) * 4
  bitmapByteCount = bitmapBytesPerRow * Int(pixelsHigh)

  // Use the generic RGB color space.
  let colorSpace = CGColorSpaceCreateDeviceRGB()

  // Allocate memory for image data. This is the destination in memory
  // where any drawing to the bitmap context will be rendered.
  let bitmapData = malloc(bitmapByteCount)

  // Create the bitmap context. We want pre-multiplied ARGB, 8-bits
  // per component. Regardless of what the source image format is
  // (CMYK, Grayscale, and so on) it will be converted over to the format
  // specified here by CGBitmapContextCreate.
  let context = CGBitmapContextCreate(bitmapData, pixelsWide, pixelsHigh, 8, bitmapBytesPerRow, colorSpace, CGImageAlphaInfo.PremultipliedFirst.rawValue)

  // Make sure and release colorspace before returning
  return context!

func getPixelColorAtLocation(point:CGPoint, inImage:CGImageRef) -> NSColor {
  // Create off screen bitmap context to draw the image into. Format ARGB is 4 bytes for each pixel: Alpa, Red, Green, Blue
  let context = self.createARGBBitmapContext(inImage)

  let pixelsWide = CGImageGetWidth(inImage)
  let pixelsHigh = CGImageGetHeight(inImage)
  let rect = CGRect(x:0, y:0, width:Int(pixelsWide), height:Int(pixelsHigh))

  //Clear the context
  CGContextClearRect(context, rect)

  // Draw the image to the bitmap context. Once we draw, the memory
  // allocated for the context for rendering will then contain the
  // raw image data in the specified color space.
  CGContextDrawImage(context, rect, inImage)

  // Now we can get a pointer to the image data associated with the bitmap
  // context.
  let data = CGBitmapContextGetData(context)
  let dataType = UnsafePointer<UInt8>(data)

  let offset = 4*((Int(pixelsWide) * Int(point.y)) + Int(point.x))
  let alpha = dataType[offset]
  let red = dataType[offset+1]
  let green = dataType[offset+2]
  let blue = dataType[offset+3]
  let color = NSColor(red: CGFloat(red)/255.0, green: CGFloat(green)/255.0, blue: CGFloat(blue)/255.0, alpha: CGFloat(alpha)/255.0)

  // Free image data memory for the context
  return color;
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