Get the image size in the web page by downloading only a small amount of data in the image header

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A web page pointed to by a url contains many images(gif,png,jpg...), I just want to find the one with the largest size(eg: choose 1024x600 instead of 800x300) by Swift code.

Currently, I have to download all images locally and then get their sizes

I know that the size data of the image is actually only stored in its header, does this mean that I only need to download the small header data of the image to get its size?

If possible, how can I parse to the size of the image through its header data?

preferably Swift or C code, thanks! ;)

Or is there a better way to get the size of the image from web without downloading it locally? Please enlighten me, thank you!

Update:

I find some ruby code to do this:

PNG:

IO.read('image.png')[0x10..0x18].unpack('NN')
=> [713, 54]

GIF：

IO.read('image.gif')[6..10].unpack('SS')
=> [130, 50]

BMP：

d = IO.read('image.bmp')[14..28]
d[0] == 40 ? d[4..-1].unpack('LL') : d[4..8].unpack('SS')

Do these codes work? Thanks.

web swift image-processing objective-c image
1 Answers

You can just create an URLRequest and set the httpMethod to "HEAD". Then you can call an URLSession dataTask with your request and check the expectedContentLength property of the response:

import UIKit
import PlaygroundSupport
PlaygroundPage.current.needsIndefiniteExecution = true

let link = "https://i.stack.imgur.com/varL9.jpg"
let url = URL(string: link)!
var request = URLRequest(url: url)
request.httpMethod = "HEAD"
URLSession.shared.dataTask(with: request) { data, response , _ in
    guard let response = response, (response as? HTTPURLResponse)?.statusCode == 200 else { return }
    DispatchQueue.main.async() {
        print("data", data)                                                                // "data 0 bytes\n"
        print("suggestedFilename:", response.suggestedFilename ?? "no suggestedFilename")  // "suggestedFilename: varL9.jpg\n"
        print("expectedContentLength:", response.expectedContentLength)                    // "expectedContentLength: 60807\n"
    }
}.resume()
user_23038650
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