CoreGraphics resizing function from iOS (UIImage) to MacOS (NSImage)

[alelordelo]

Hi Matthias,

Were you able to run your helper functions on MacOS? I replaced iOS (UIImage) to MacOS (NSImage), and got almost everything to work... Only the cgImage.bitsPerPixel part I didn't find a MacOS equivalent.

Have you tried running it on MacOS?

Cheers from Sweden! Alex

https://stackoverflow.com/questions/67202976/coregraphics-resizing-function-from-ios-uiimage-to-macos-nsimage

What I did: Code bellow

Problem: Getting error on this line of code: let bytesPerPixel = cgImage.bitsPerPixel / bitsPerComponent

Error: Value of type '(UnsafeMutablePointer?, NSGraphicsContext?, [NSImageRep.HintKey : Any]?) -> CGImage?' (aka '(Optional<UnsafeMutablePointer>, Optional, Optional<Dictionary<NSImageRep.HintKey, Any>>) -> Optional') has no member 'bitsPerPixel'

iOS: UIImage

extension UIImage {
    
    // Resizeing using CoreGraphics
    func resize(to size:CGSize) -> UIImage? {
        
        let cgImage = self.cgImage!

        let destWidth = Int(size.width)
        let destHeight = Int(size.height)
        let bitsPerComponent = 8
        let bytesPerPixel = cgImage.bitsPerPixel / bitsPerComponent
        let destBytesPerRow = destWidth * bytesPerPixel
        
        let context = CGContext(data: nil,
                                width: destWidth,
                                height: destHeight,
                                bitsPerComponent: bitsPerComponent,
                                bytesPerRow: destBytesPerRow,
                                space: cgImage.colorSpace!,
                                bitmapInfo: cgImage.bitmapInfo.rawValue)!
        context.interpolationQuality = .high
        context.draw(cgImage, in: CGRect(origin: CGPoint.zero, size: size))
        return context.makeImage().flatMap { UIImage(cgImage: $0) }
    }
}

MacOS: NSImage

extension NSImage {
    
    // Resizeing using CoreGraphics
    func resize(to size:CGSize) -> NSImage? {
        
        let cgImage = self.cgImage

        let destWidth = Int(size.width)
        let destHeight = Int(size.height)
        let bitsPerComponent = 8
        let bytesPerPixel = cgImage.bitsPerPixel / bitsPerComponent
        let destBytesPerRow = destWidth * bytesPerPixel
        
        let context = CGContext(data: nil,
                                width: destWidth,
                                height: destHeight,
                                bitsPerComponent: bitsPerComponent,
                                bytesPerRow: destBytesPerRow,
                                space: cgImage.colorSpace!,
                                bitmapInfo: cgImage.bitmapInfo.rawValue)!
        context.interpolationQuality = .high
        context.draw(cgImage, in: CGRect(origin: CGPoint.zero, size: size))
        return context.makeImage().flatMap { NSImage(cgImage: $0) }
    }
}

[hollance]

CGImage should work both on iOS and macOS. It looks like you forgot to unwrap self.cgImage, since the error message is about using an optional, not about CGImage itself.

[alelordelo]

Thanks Matthias!

I unwrapped self.cgImage, but got same error.

Looks like bitsPerPixel is only available for iOS... : /

Any idea what would be MacOS equivalent to bitsPerPixel?

/* Return the number of bits/pixel of `image'. */

**@available(iOS 2.0, *)**
public var bitsPerPixel: Int { get }

Value of type '(UnsafeMutablePointer<NSRect>?, NSGraphicsContext?, [NSImageRep.HintKey : Any]?) -> CGImage?' (aka '(Optional<UnsafeMutablePointer<CGRect>>, Optional<NSGraphicsContext>, Optional<Dictionary<NSImageRep.HintKey, Any>>) -> Optional<CGImage>') has no member 'bitsPerPixel'

[hollance]

Ah I see what's going on, NSImage.cgImage is not a property but a function. Namely the function:

func cgImage(forProposedRect proposedDestRect: UnsafeMutablePointer<NSRect>?, context referenceContext: NSGraphicsContext?, hints: [NSImageRep.HintKey : Any]?) -> CGImage?

You're trying to call bitsPerPixel on the function itself, not on the CGImage that comes out of the function. ;-)

[alelordelo]

thanks for spotting that Matthias! : )

Weird that UIImage.cgImage is a property, and NSImage.cgImage is a function.

Im a bit lost here... Do you have any idea what the final resize function would look like for MacOS?

[hollance]

Check out this StackOverflow answer: https://stackoverflow.com/questions/11949250/how-to-resize-nsimage

[alelordelo]

Thanks again!

Finally made it work with some help from Apple, this is the final function in case you also need. : )

// Resizeing using CoreGraphics

  func resize(to size:CGSize) -> NSImage? {
       
       let cgImage: CGImage = self.cgImage( forProposedRect: nil,  context: nil, hints: nil)!

       let destWidth = Int(size.width)
       let destHeight = Int(size.height)
       let bitsPerComponent = 8
       let bytesPerPixel = cgImage.bitsPerPixel / bitsPerComponent
       let destBytesPerRow = destWidth * bytesPerPixel
       
       let context = CGContext(data: nil,
                               width: destWidth,
                               height: destHeight,
                               bitsPerComponent: bitsPerComponent,
                               bytesPerRow: destBytesPerRow,
                               space: cgImage.colorSpace!,
                               bitmapInfo: cgImage.bitmapInfo.rawValue)!
       context.interpolationQuality = .high
       context.draw(cgImage, in: CGRect(origin: CGPoint.zero, size: size))
       return context.makeImage().flatMap { NSImage(cgImage: $0, size: NSSize(width: 1024, height: 1024)) }
   }