Saving NSImage in Different Formats Locally

Question

I need to allow the user to save an NSImage to a local file.

So I am using those extensions from this SO answer to can save the image in PNG Format

Answer 1

You can get a BMP, GIF, JPEG, JPEG2000, PNG, or a TIFF by converting your NSImage to a NSBitmapImageRep and then using its representation(using:) method, but it's a bit of a pain; you have to create an empty NSBitmapImageRep, set it as the current graphics context, and draw your NSImage into it (see this answer for details).

If you can require macOS 10.13, there are some handy methods on CIContext that are, in my opinion, easier to use, and which can convert an image to TIFF, JPEG, or PNG (no BMP, GIF, or JPEG2000 though; also, there's a method to convert to HEIF that appears to be iOS only as of the time of this writing):

func getPNGData(image: NSImage) -> NSData? {
    if let ci = image.cgImage(forProposedRect: nil, context: nil, hints: nil).map({ CIImage(cgImage: $0) }),
        let png = CIContext().pngRepresentation(of: ci, format: kCIFormatRGBAf, colorSpace: CGColorSpace(name: CGColorSpace.sRGB)!) {
        // I'm colorblind, so it's very possible that the constants above
        // were poorly chosen. Choose whatever makes your colors look right
        return png
    } else {
        return nil
    }
}

Answer 2

You can create a custom method to allow you to specify any image type and also the directory where you would like to save your NSImage. You can also set a default value to the destination directory as the current directory, so if you don't pass the directory url it will save to the current one:

extension NSImage {
    func save(as fileName: String, fileType: NSBitmapImageRep.FileType = .jpeg, at directory: URL = URL(fileURLWithPath: FileManager.default.currentDirectoryPath)) -> Bool {
        guard let tiffRepresentation = tiffRepresentation, directory.isDirectory, !fileName.isEmpty else { return false }
        do {
            try NSBitmapImageRep(data: tiffRepresentation)?
                .representation(using: fileType, properties: [:])?
                .write(to: directory.appendingPathComponent(fileName).appendingPathExtension(fileType.pathExtension))
            return true
        } catch {
            print(error)
            return false
        }
    }
}

---

You will need also to make sure the url passed to your method is a directory url. You can use URL resourceValues method to get the url isDirectoryKey value and check if it is true:

extension URL {
    var isDirectory: Bool {
       return (try? resourceValues(forKeys: [.isDirectoryKey]))?.isDirectory == true
    }
}

---

You can also extend NSBitmapImageRep.FileType to provide the associated file path extension:

extension NSBitmapImageRep.FileType {
    var pathExtension: String {
        switch self {
        case .bmp:
            return "bmp"
        case .gif:
            return "gif"
        case .jpeg:
            return "jpg"
        case .jpeg2000:
            return "jp2"
        case .png:
            return "png"
        case .tiff:
            return "tif"
        }
    }
}

---

Playground Testing:

let desktopDirectory = FileManager.default.urls(for: .desktopDirectory, in: .userDomainMask).first!
// lets change the current directory to the desktop directory
FileManager.default.changeCurrentDirectoryPath(desktopDirectory.path)

// get your nsimage
let picture  = NSImage(contentsOf: URL(string: "https://i.stack.imgur.com/Xs4RX.jpg")!)!

// this will save to the current directory
if picture.save(as: "profile") {
    print("file saved as profile.jpg which is the default type")
}
if picture.save(as: "profile", fileType: .png) {
    print("file saved as profile.png")
}
if picture.save(as: "profile", fileType: .tiff) {
    print("file saved as profile.tif")
}
if picture.save(as: "profile", fileType: .gif) {
    print("file saved as profile.gif")
}
if picture.save(as: "profile", fileType: .jpeg2000) {
    print("file saved as profile.jp2")
}

// you can also chose a choose another directory without the need to change the current directory
let url = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first!
if picture.save(as: "profile", at: url) {
    print("file saved as profile.jpg at documentDirectory")
}