How do I create a UIImage with the DPI set? - ios

Question

I have a UIImage, and I wish to save it to Photos with the DPI metadata set. I understand that a UIImage is immutable, so I have to create a new UIImage. I've used this answer as a reference to create an swift extension function on UIImage that produces a new UIImage with the DPI set. I'm successfully saving this to Photos with UIImageWriteToSavedPhotosAlbum(image, nil, nil, nil). I email this to myself (set as 'Actual Size') and open it in Preview but the DPI is always left at 72.
Here is the function that I converted:
func imageWithDPI(dpi :Int) -> UIImage? {
guard let sourceImageData = UIImageJPEGRepresentation(self, 0.8) else {
print("Couldn't make PNG Respresentation of UIImage")
return nil
}
guard let source = CGImageSourceCreateWithData(sourceImageData, nil) else {
print("Couldn't create source with sourceImageData")
return nil
}
var metadata = CGImageSourceCopyPropertiesAtIndex(source, 0, nil) as? [String:AnyObject] ?? [String:AnyObject]()
metadata[kCGImagePropertyDPIWidth as String] = dpi
metadata[kCGImagePropertyDPIHeight as String] = dpi
var exifDictionary = metadata[kCGImagePropertyTIFFDictionary as String] as? [String:AnyObject] ?? [String:AnyObject]()
exifDictionary[kCGImagePropertyTIFFXResolution as String] = dpi
exifDictionary[kCGImagePropertyTIFFYResolution as String] = dpi
metadata[kCGImagePropertyTIFFDictionary as String] = exifDictionary
var jfifDictionary = metadata[kCGImagePropertyJFIFDictionary as String] as? [String:AnyObject] ?? [String:AnyObject]()
jfifDictionary[kCGImagePropertyJFIFXDensity as String] = dpi
jfifDictionary[kCGImagePropertyJFIFYDensity as String] = dpi
jfifDictionary[kCGImagePropertyJFIFVersion as String] = 1
metadata[kCGImagePropertyJFIFDictionary as String] = jfifDictionary
guard let uti = CGImageSourceGetType(source) else {
print("Couldn't get type from source")
return nil
}
let destinationImageData = NSMutableData()
guard let destination = CGImageDestinationCreateWithData(destinationImageData, uti, 1, nil) else {
print("Couldn't create destination with destinationImageData")
return nil
}
CGImageDestinationAddImageFromSource(destination, source,0, metadata)
CGImageDestinationFinalize(destination)
return UIImage(data: destinationImageData)
}
I'm kind of out of my depth in Core Graphics, so any advice would be much appreciated.
Many thanks in advance.

Answer 1

UIImage may be stripping all of the metadata from the image when you create it from the destinationImageData, including the DPI. I recently ran into this issue when sharing images via iOS’s UIActivityItemProvider API. If I returned a UIImage * from the - (id)item method then all metadata was lost. The fix was to return either an NSData * or an NSURL *.
Try returning the destinationImageData directly, or saving it to a file & emailing that to yourself.
I’m curious as to why you want to set the DPI for an image? DPI is mostly meaningless for nearly all digital images these days (unless an image is meant to exactly represent the dimensions of a physical object). Actual pixel resolution is far more important than DPI.