Creating CGImage from BGRA data (but A=0 pix are opaq white)

// this is producing an image with a white background, rather than clear. wtf.
// chromaKeyCFData is BGRA, with background pixels having A==0.
// cgImage is source image (from a PNG) that I'm punching green pixels out of.
// colorSpace is cgImage.colorSpace! (ie, force unwrapped)
guard let chromaKeyDataProvider = CGDataProvider(data: chromaKeyCFData),
    let chromaKeyCGImage = CGImage(width: cgImage.width,
                                   height: cgImage.height,
                                   bitsPerComponent: cgImage.bitsPerComponent,
                                   bitsPerPixel: cgImage.bitsPerPixel,
                                   bytesPerRow: cgImage.bytesPerRow,
                                   space: colorSpace,
                                   bitmapInfo: cgImage.bitmapInfo,
                                   provider: chromaKeyDataProvider,
                                   decode: nil,
                                   shouldInterpolate: false,
                                   intent: CGColorRenderingIntent.defaultIntent) else { return nil }
let chromaKeyImage = UIImage(cgImage: chromaKeyCGImage)
return chromaKeyImage