Sample NSImage and retain quality (swift 3) - swift

Question

I wrote an NSImage extension to allow me to take random samples of an image. I would like those samples to retain the same quality as the original image. However, they appear to be aliased or slightly blurry. Here's an example - the original drawn on the right and a random sample on the left:
I'm playing around with this in SpriteKit at the moment. Here's how I create the original image:
let bg = NSImage(imageLiteralResourceName: "ref")
let tex = SKTexture(image: bg)
let sprite = SKSpriteNode(texture: tex)
sprite.position = CGPoint(x: size.width/2, y:size.height/2)
addChild(sprite)
And here's how I create the sample:
let sample = bg.sample(size: NSSize(width: 100, height: 100))
let sampletex = SKTexture(image:sample!)
let samplesprite = SKSpriteNode(texture:sampletex)
samplesprite.position = CGPoint(x: 60, y:size.height/2)
addChild(samplesprite)
Here's the NSImage extension (and randomNumber func) that creates the sample:
extension NSImage {
/// Returns the height of the current image.
var height: CGFloat {
return self.size.height
}
/// Returns the width of the current image.
var width: CGFloat {
return self.size.width
}
func sample(size: NSSize) -> NSImage? {
// Resize the current image, while preserving the aspect ratio.
let source = self
// Make sure that we are within a suitable range
var checkedSize = size
checkedSize.width = floor(min(checkedSize.width,source.size.width * 0.9))
checkedSize.height = floor(min(checkedSize.height, source.size.height * 0.9))
// Get random points for the crop.
let x = randomNumber(range: 0...(Int(source.width) - Int(checkedSize.width)))
let y = randomNumber(range: 0...(Int(source.height) - Int(checkedSize.height)))
// Create the cropping frame.
var frame = NSRect(x: x, y: y, width: Int(checkedSize.width), height: Int(checkedSize.height))
// let ref = source.cgImage.cropping(to:frame)
let ref = source.cgImage(forProposedRect: &frame, context: nil, hints: nil)
let rep = NSBitmapImageRep(cgImage: ref!)
// Create a new image with the new size
let img = NSImage(size: checkedSize)
// Set a graphics context
img.lockFocus()
defer { img.unlockFocus() }
// Fill in the sample image
if rep.draw(in: NSMakeRect(0, 0, checkedSize.width, checkedSize.height),
from: frame,
operation: NSCompositingOperation.copy,
fraction: 1.0,
respectFlipped: false,
hints: [NSImageHintInterpolation:NSImageInterpolation.high.rawValue]) {
// Return the cropped image.
return img
}
// Return nil in case anything fails.
return nil
}
}
func randomNumber(range: ClosedRange<Int> = 0...100) -> Int {
let min = range.lowerBound
let max = range.upperBound
return Int(arc4random_uniform(UInt32(1 + max - min))) + min
}
I've tried this about 10 different ways and the results always seem to be a slightly blurry sample. I even checked for smudges on my screen. :)
How can I create a sample of an NSImage that retains the exact qualities of the section of the original source image?

Answer 1

Switching the interpolation mode to NSImageInterpolation.none was apparently sufficient in this case.
It's also important to handle the draw destination rect correctly. Since cgImage(forProposedRect:...) may change the proposed rect, you should use a destination rect that's based on it. You should basically use a copy of frame that's offset by (-x, -y) so it's relative to (0, 0) instead of (x, y).