How to convert NSImage to PNG Data

[onmyway133]

Create NSBitmapImageRep with preferred size and draw NSImage onto that. Need to construct NSBitmapImageRep specifically instead of using convenient initializers like NSBitmapImageRep(data:), NSBitmapImageRep(cgImage:) to avoid device dependant resolution issue.

extension NSImage {
    func pngData(
        size: CGSize,
        imageInterpolation: NSImageInterpolation = .high
    ) -> Data? {
        guard let bitmap = NSBitmapImageRep(
            bitmapDataPlanes: nil,
            pixelsWide: Int(size.width),
            pixelsHigh: Int(size.height),
            bitsPerSample: 8,
            samplesPerPixel: 4,
            hasAlpha: true,
            isPlanar: false,
            colorSpaceName: .deviceRGB,
            bitmapFormat: [],
            bytesPerRow: 0,
            bitsPerPixel: 0
        ) else {
            return nil
        }

        bitmap.size = size
        NSGraphicsContext.saveGraphicsState()
        NSGraphicsContext.current = NSGraphicsContext(bitmapImageRep: bitmap)
        NSGraphicsContext.current?.imageInterpolation = imageInterpolation
        draw(
            in: NSRect(origin: .zero, size: size),
            from: .zero,
            operation: .copy,
            fraction: 1.0
        )
        NSGraphicsContext.restoreGraphicsState()

        return bitmap.representation(using: .png, properties: [:])
    }
}

Read more

• https://stackoverflow.com/questions/17507170/how-to-save-png-file-from-nsimage-retina-issues