Memory Management - CVPixelBuffer to UIImage, difference between using CIImage and VTCreateCGImageFromCVPixelBuffer - ios

Question

I got two solution converting CVPixelBuffer to UIImage.
Using CIImage and its context
First solution is using CIImage and CIContext.
func readAssetAndCache(completion: #escaping () -> Void) {
/** Setup AVAssetReader **/
reader.startReading()
let context = CIContext()
while let sampleBuffer = generator.trackoutput.copyNextSampleBuffer() {
autoreleasepool {
guard let imageBuffer: CVPixelBuffer = CMSampleBufferGetImageBuffer(sampleBuffer) else { return }
let ciImage = CIImage(cvImageBuffer: imageBuffer)
guard let cgImage = context.createCGImage(ciImage, from: ciImage.extent) else { return }
if let image = UIImage(cgImage: cgImage) {
CacheManager.default.cache(image: image, forKey: "CachedImages") {
completion()
}
}
}
}
}
Using VTCreateCGImageFromCVPixelBuffer
Code Looks like this.
func readAssetAndCache(completion: #escaping () -> Void) {
/** Setup AVAssetReader **/
reader.startReading()
while let sampleBuffer = generator.trackoutput.copyNextSampleBuffer() {
autoreleasepool {
guard let imageBuffer: CVPixelBuffer = CMSampleBufferGetImageBuffer(sampleBuffer) else { return }
if let image = UIImage(pixelBuffer: imageBuffer,
scale: 1.0,
orientation: self.imageOrientation) {
CacheManager.default.cache(image: image, forKey: "CachedImages") {
completion()
}
}
}
}
}
}
In this solution I use this extension method.
import UIKit
import VideoToolbox
extension UIImage {
public convenience init?(pixelBuffer: CVPixelBuffer, scale: CGFloat, orientation: UIImageOrientation) {
var image: CGImage?
VTCreateCGImageFromCVPixelBuffer(pixelBuffer, nil, &image)
guard let cgImage = image else { return nil }
self.init(cgImage: cgImage, scale: scale, orientation: orientation)
}
}
Whole process related to extracting images from video is as follows.
Whole Process
Extract All frames serially.
Cache each image in disk concurrently, using OperationQueue.
Question
The problem occurs when images are cached concurrently, using CIContext and its createCGImage(ciImage:rect:). Memory footprints gets higher until the end of all caching process in first scenario. But, Second way, it releases its memory when each workItem is finished.
I really don't know why.
Here is my Caching method related to this problem.
func cache(image: UIImage, forKey key: String, completion: #escaping () -> Void) {
let operation = BlockOperation(block: { [weak self] in
guard let `self` = self else { return }
self.diskCache(image: image)
})
operation.completionBlock = completion
operationQueue.addOperation(operation)
}
func diskCache(image: UIImage) {
let data = UIImageJPEGRepresentation(image, 1.0)
let key = "\(self.identifierPrefix).jpg"
FileManager.default.createFile(atPath: self.cachePath.appendingPathComponent(key).path,
contents: data, attributes: nil)
}