Blend two NSImages using Swift on OS X - macos

Question

I have two instances of NSImage. I want image2 to be placed on top of image1, with a certain level of opacity. They are of matching dimensions. Neither of the images need to be visible in the UI.
How can I correctly set up a graphics context and draw images to it, one with full opacity and another semi-transparent?
I have been reading several answers here but I find it complicated, especially since most seem to be for either Objective-C or only apply to iOS. Any pointers are appriciated. If this can be accomplished without needing a CGContext at all that would be even better.
func blendImages(image1: NSImage, image2: NSImage, alpha: CGFloat) -> CGImage {
// Create context
var ctx: CGContextRef = CGBitmapContextCreate(0, inputImage.size.width, inputImage.size.height, 8, inputImage.size.width*4, NSColorSpace.genericRGBColorSpace(), PremultipliedLast)
let area = CGRectMake(0, 0, inputImage.size.width, inputImage.size.height)
CGContextScaleCTM(ctx, 1, -1)
// Draw image1 in context
// Draw image2 with alpha opacity
CGContextSetAlpha(ctx, CGFloat(0.5))
// Create CGImage from context
let outputImage = CGBitmapContextCreateImage(ctx)
return outputImage
}
Elsewhere I have this extension to get CGImages from my NSImages:
extension NSImage {
var CGImage: CGImageRef {
get {
let imageData = self.TIFFRepresentation
let source = CGImageSourceCreateWithData(imageData as! CFDataRef, nil)
let maskRef = CGImageSourceCreateImageAtIndex(source, 0, nil)
return maskRef
}
}
}

Answer 1

I managed to solve this with NSGraphicsContext.
func mergeImagesAB(pathA: String, pathB: String, fraction: CGFloat) -> CGImage {
guard let imgA = NSImage(byReferencingFile: pathA),
let imgB = NSImage(byReferencingFile: pathB),
let bitmap = imgA.representations[0] as? NSBitmapImageRep,
let ctx = NSGraphicsContext(bitmapImageRep: bitmap)
else {fatalError("Failed to load images.")}
let rect = NSRect(origin: CGPoint(x: 0, y: 0), size: bitmap.size)
NSGraphicsContext.saveGraphicsState()
NSGraphicsContext.setCurrentContext(ctx)
imgB.drawInRect(rect, fromRect: rect, operation: .CompositeSourceOver, fraction: fraction)
NSGraphicsContext.restoreGraphicsState()
guard let result = bitmap.CGImage
else {fatalError("Failed to create image.")}
return result;
}

Answer 2

Thanks Henrik, I needed this for Obj-C. Here's that version in case someone also needs:
-(CGImageRef) mergeImage:(NSImage*)a andB:(NSImage*)b fraction:(float)fraction{
NSBitmapImageRep *bitmap = (NSBitmapImageRep*)[[a representations] objectAtIndex:0];
NSGraphicsContext *ctx = [NSGraphicsContext graphicsContextWithBitmapImageRep:bitmap];
[NSGraphicsContext saveGraphicsState];
[NSGraphicsContext setCurrentContext:ctx];
CGRect rect = CGRectMake(0, 0, bitmap.size.width, bitmap.size.height);
[NSGraphicsContext saveGraphicsState];
[NSGraphicsContext setCurrentContext:ctx];
[b drawInRect:rect fromRect:rect operation:NSCompositeSourceOver fraction:fraction];
[NSGraphicsContext restoreGraphicsState];
return [bitmap CGImage];
}