EDIT: Resolved, I answered the question below.
I am using the following to get metadata for PHAssets:
let data = NSData.init(contentsOf: url!)!
if let imageSource = CGImageSourceCreateWithData(data, nil) {
let metadata = CGImageSourceCopyPropertiesAtIndex(imageSource, 0, nil)! as NSDictionary
}
The metadata dictionary has all the values I am looking for. However a few fields like ShutterSpeedValue
, ExposureTime
which have fractions get printed as decimals:
ExposureTime = "0.05"
ShutterSpeedValue = "4.321956769055745"
When I look at this data on my Mac's preview app and exiftool, it shows:
ExposureTime = 1/20
ShutterSpeedValue = 1/20
How can I get the correct fraction string instead of the decimal string?
EDIT: I tried simply converting the decimal to a fraction string using this from SO code but this isn't correct:
func rationalApproximation(of x0 : Double, withPrecision eps : Double = 1.0E-6) -> String {
var x = x0
var a = x.rounded(.down)
var (h1, k1, h, k) = (1, 0, Int(a), 1)
while x - a > eps * Double(k) * Double(k) {
x = 1.0/(x - a)
a = x.rounded(.down)
(h1, k1, h, k) = (h, k, h1 + Int(a) * h, k1 + Int(a) * k)
}
return "\(h)/\(k)"
}
As you notice, the decimal value of ShutterSpeedValue
printed as 4.321956769055745
isn't even equal to 1/20.
Resolved.
As per
https://www.dpreview.com/forums/post/54376235
ShutterSpeedValue is defined as APEX value, where:
ShutterSpeed = -log2(ExposureTime)
So -log2(1/20) is 4.3219, just as what I observed.
So to get the ShutterSpeedValue, I use the following:
"1/\\(ceil(pow(2, Double(4.321956769055745))))"
I tested 3 different photos and 1/20, 1/15 and 1/1919 were all correctly calculated using your formula.
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