Downscale UIImage to fit a maximum Base64 length - ios

Question

In my app I have to upload a UIImage to a server in form of a Base64 string where the condition is that the string must not exceed 54999 characters.
The code I currently use generally works but it takes a lot of time, memory and the length of the uploaded image Base64 string is usually very off from 54999. Sometimes even by a factor of 10.
var imageData = UIImagePNGRepresentation(image)
var base64String = imageData.base64EncodedStringWithOptions(.allZeros)
var scaledImage: UIImage = image
var newSize: CGSize = image.size
while ((base64String as NSString).length > 54999)
{
newSize.width *= 0.5
newSize.height *= 0.5
scaledImage = image.imageScaledToFitSize(newSize)
imageData = UIImagePNGRepresentation(scaledImage)
base64String = imageData.base64EncodedStringWithOptions(.allZeros)
}
// proceed to upload Base64 string...
At the beginning I thought I could this the following way but this obviously didn't work at all because there is no linear correlation between the file size and its Base64 length:
let maxLength: NSInteger = 54999
if ((base64String as NSString).length > maxLength)
{
let downScaleFactor: CGFloat = CGFloat(maxLength) / CGFloat((base64String as NSString).length)
var size: CGSize = image.size
size.width *= downScaleFactor
size.height *= downScaleFactor
scaledImage = image.imageScaledToFitSize(size)
imageData = UIImagePNGRepresentation(scaledImage)
base64String = imageData.base64EncodedStringWithOptions(.allZeros)
}
There must be a better way to do this.

Answer 1

This is a tough problem. The base64 encoding creates a predictable decrease in image size (3 bytes becomes 4, or a 33% increase in size)
You can't be sure of the result size from PNG or JPEG compression since the decrease in byte size depends on the image being compressed. (A solid-colored rectangle compresses EXTREMELY well. A cartoon using solid colors compresses quite well, and a continuous tone photograph with lots of detail does not compress as well.)
You will probably have better luck using JPEG images, since you can adjust the compression level on those, and JPEG offers higher compression ratios.
I would suggest experimenting with the lowest image quality you can tolerate (use quality setting .5, look at the image, and adjust up/down from there until you get an image that looks "good enough".)
Then use that compression. You can skip the base64 encoding until the end since you can simply multiply your non base64 byte size by 4/3 to get the size after base 64 encoding.
I would suggest shrinking your image by 1/sqrt(2) (~0.7) instead of 0.5. A decrease of 50% will cause large jumps in image size.