Take snapshot from UIView with lower resolution - swift

Question

I'm taking snapshot from a PDFView in PDFKit for streaming (20 times per sec), and I use this extesnsion
extension UIView {
func asImageBackground(viewLayer: CALayer, viewBounds: CGRect) -> UIImage {
let renderer = UIGraphicsImageRenderer(bounds: viewBounds)
return renderer.image { rendererContext in
viewLayer.render(in: rendererContext.cgContext)
}
}
}
But the output UIImage from this extension has a high resolution which make it difficult to stream. I can reduce it by this extension
extension UIImage {
func resize(_ max_size: CGFloat) -> UIImage {
// adjust for device pixel density
let max_size_pixels = max_size / UIScreen.main.scale
// work out aspect ratio
let aspectRatio = size.width/size.height
// variables for storing calculated data
var width: CGFloat
var height: CGFloat
var newImage: UIImage
if aspectRatio > 1 {
// landscape
width = max_size_pixels
height = max_size_pixels / aspectRatio
} else {
// portrait
height = max_size_pixels
width = max_size_pixels * aspectRatio
}
// create an image renderer of the correct size
let renderer = UIGraphicsImageRenderer(size: CGSize(width: width, height: height), format: UIGraphicsImageRendererFormat.default())
// render the image
newImage = renderer.image {
(context) in
self.draw(in: CGRect(x: 0, y: 0, width: width, height: height))
}
// return the image
return newImage
}
}
but it add an additional workload which make the process even worse. Is there any better way?
Thanks

Answer 1

You can downsample it using ImageIO which is recommended by Apple:
extension UIImage {
func downsample(to resolution: CGSize) -> UIImage? {
let imageSourceOptions = [kCGImageSourceShouldCache: false] as CFDictionary
guard let data = self.jpegData(compressionQuality: 0.75) as? CFData, let imageSource = CGImageSourceCreateWithData(data, imageSourceOptions) else {
return nil
}
let maxDimensionInPixels = Swift.max(resolution.width, resolution.height) * 3
let downsampleOptions = [
kCGImageSourceCreateThumbnailFromImageAlways: true,
kCGImageSourceShouldCacheImmediately: true,
kCGImageSourceCreateThumbnailWithTransform: true,
kCGImageSourceThumbnailMaxPixelSize: maxDimensionInPixels
] as CFDictionary
guard let downsampledImage = CGImageSourceCreateThumbnailAtIndex(imageSource, 0, downsampleOptions) else {
return nil
}
return UIImage(cgImage: downsampledImage)
}
}