What is the difference between NSRectFill and NSBezierPath(rect).fill - swift

Question

I know I can fill a rect using NSRectFill(bounds). However I wanted to preserve transparency for PDF output and I discovered that I can do that only with NSBezierPath(rect: bounds).fill()
What is the difference (behind the scenes) of those two?
func drawBackground() {
CGContextSaveGState(currentContext)
if (NSGraphicsContext.currentContextDrawingToScreen()) {
NSColor(patternImage: checkerboardImage).set()
NSRectFillUsingOperation(bounds, NSCompositingOperation.CompositeSourceOver)
}
NSColor.clearColor().setFill()
//NSRectFill(bounds) //option 1
NSBezierPath(rect: bounds).fill() // option 2
CGContextRestoreGState(currentContext)
}
extension NSImage {
static func checkerboardImageWithSize(size : CGFloat) -> NSImage {
let fullRect = NSRect(x: 0, y: 0, width: size, height: size)
let halfSize : CGFloat = size * 0.5;
let upperSquareRect = NSRect(x: 0, y: 0, width: halfSize, height: halfSize);
let bottomSquareRect = NSRect(x: halfSize, y: halfSize, width:halfSize, height: halfSize);
let image = NSImage(size: NSSize(width: size, height: size))
image.lockFocus()
NSColor.whiteColor()
NSRectFill(fullRect)
NSColor(deviceWhite: 0.0, alpha:0.1).set()
NSRectFill(upperSquareRect)
NSRectFill(bottomSquareRect)
image.unlockFocus()
return image
}
}

Answer 1

I'm mostly an iOS programmer and not very fluent these days over on the AppKit side of things, but my guess is that you're getting the wrong NSCompositingOperation. I see from the docs that NSRectFill uses NSCompositeCopy. Perhaps it would work better if you used NSRectFillUsingOperation, where you get to specify the compositing operation.