Swift UIImage .jpegData() and .pngData() changes image size

Question

I am using Swift's Vision Framework for Deep Learning and want to upload the input image to backend using REST API - for which I am converting my UIImage to MultipartFormData using jpegData() and pngData() function that swift natively offers.

I use session.sessionPreset =.vga640x480 to specify the image size in my app for processing.

I was seeing different size of image in backend - which I was able to confirm in the app because UIImage(imageData) converted from the image is of different size.

This is how I convert image to multipartData -

let multipartData = MultipartFormData()
if let imageData = self.image?.jpegData(compressionQuality: 1.0) {
    multipartData.append(imageData, withName: "image", fileName: "image.jpeg", mimeType: "image/jpeg")
}

This is what I see in Xcode debugger -

Answer 1

This does not work, ending up with a Data representation of the image with an incorrect scale :

let ciImage = CIImage(cvImageBuffer: pixelBuffer) // 640×480
let image = UIImage(ciImage: ciImage)             // says it is 640×480 with scale of 1
guard let data = image.pngData() else { ... }     // but if you extract `Data` and then recreate image from that, the size will be off by a multiple of your device’s scale

However, if you create it via a CGImage , you will get the right result:

let ciImage = CIImage(cvImageBuffer: pixelBuffer)
let ciContext = CIContext()
guard let cgImage = ciContext.createCGImage(ciImage, from: ciImage.extent) else { return }
let image = UIImage(cgImage: cgImage)