Swift playground - cannot convert a filtered UIImage to a CGImage

Question

In a Swift playground, I am loading a JPEG, converting it to a UIImage and filtering it to monochrome. I then convert the resulting filtered image to a UIImage.

The input and the filtered images display correctly.

I then convert both images to a CGImage type. This works for the input image, but the filtered image returns nil from the conversion:

// Get an input image
let imageFilename = "yosemite.jpg"
let inputImage = UIImage(named: imageFilename )
let inputCIImage = CIImage(image:inputImage!)

// Filter the input image - make it monochrome
let filter = CIFilter(name: "CIPhotoEffectMono")
filter!.setDefaults()
filter!.setValue(inputCIImage, forKey: kCIInputImageKey)
let CIout = filter!.outputImage
let filteredImage = UIImage(CIImage: CIout!)        

// Convert the input image to a CGImage
let inputCGImageRef = inputImage!.CGImage           // Result: <CGImage 0x7fdd095023d0>
// THE LINE ABOVE WORKS

// Try to convert the filtered image to a CGImage
let filteredCGImageRef = filteredImage.CGImage      // Result: nil
// THE LINE ABOVE DOES NOT WORK
// Note that the compiler objects to 'filteredImage!.CGImage'

What's wrong?

Possible duplicate? stackoverflow.com/questions/10022534/… — Bryan Chen
@BryanChen - that's the same problem, but that is an Objective C solution. — Simon Youens
That's why I didn't close vote your question. But the link does answer you why CGImage is nil. — Bryan Chen

Simon Gladman Simon Gladman · Accepted Answer · 2016-05-04T07:27:29

A UIImage created from a CIImage as you've done isn't backed by a CGImage. You need to explicitly create one:

let context = CIContext()
let filteredCGImageRef = context.createCGImage(
    CIout!, 
    fromRect: CIout!.extent)

If you need a UIImage, create that from the CGImage rendered by the CIContext:

UIImage(CGImage: filteredCGImageRef)

Cheers,

Simon

Swift playground - cannot convert a filtered UIImage to a CGImage

1 Answers