Finding image type from NSData or UIImage

Question

I am loading an image from a URL provided by a third-party. There is no file extension (or filename for that matter) on the URL (as it is an obscured URL). I can take the data from this (in the form of NSData) and load it into a UIImage and display it fine.

I want to persist this data to a file. However, I don't know what format the data is in (PNG, JPG, BMP)? I assume it is JPG (since it's an image from the web) but is there a programmatic way of finding out for sure? I've looked around StackOverflow and at the documentation and haven't been able to find anything.

TIA.

---

Edit: Do I really need the file extension? I'm persisting it to an external storage (Amazon S3) but considering that it will always be used in the context of iOS or a browser (both of whom seem fine in interpreting the data without an extension) perhaps this is a non-issue.

Solution 1

If you have NSData for the image file, then you can guess at the content type by looking at the first byte:

+ (NSString *)contentTypeForImageData:(NSData *)data {
    uint8_t c;
    [data getBytes:&c length:1];

    switch (c) {
    case 0xFF:
        return @"image/jpeg";
    case 0x89:
        return @"image/png";
    case 0x47:
        return @"image/gif";
    case 0x49:
    case 0x4D:
        return @"image/tiff";
    }
    return nil;
}

Solution 2

Improving upon wl.'s answer, here's a much more extended and precise way to predict the image's MIME type based on the signature. The code was largely inspired by php's ext/standard/image.c.

- (NSString *)mimeTypeByGuessingFromData:(NSData *)data {

    char bytes[12] = {0};
    [data getBytes:&bytes length:12];

    const char bmp[2] = {'B', 'M'};
    const char gif[3] = {'G', 'I', 'F'};
    const char swf[3] = {'F', 'W', 'S'};
    const char swc[3] = {'C', 'W', 'S'};
    const char jpg[3] = {0xff, 0xd8, 0xff};
    const char psd[4] = {'8', 'B', 'P', 'S'};
    const char iff[4] = {'F', 'O', 'R', 'M'};
    const char webp[4] = {'R', 'I', 'F', 'F'};
    const char ico[4] = {0x00, 0x00, 0x01, 0x00};
    const char tif_ii[4] = {'I','I', 0x2A, 0x00};
    const char tif_mm[4] = {'M','M', 0x00, 0x2A};
    const char png[8] = {0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a};
    const char jp2[12] = {0x00, 0x00, 0x00, 0x0c, 0x6a, 0x50, 0x20, 0x20, 0x0d, 0x0a, 0x87, 0x0a};


    if (!memcmp(bytes, bmp, 2)) {
        return @"image/x-ms-bmp";
    } else if (!memcmp(bytes, gif, 3)) {
        return @"image/gif";
    } else if (!memcmp(bytes, jpg, 3)) {
        return @"image/jpeg";
    } else if (!memcmp(bytes, psd, 4)) {
        return @"image/psd";
    } else if (!memcmp(bytes, iff, 4)) {
        return @"image/iff";
    } else if (!memcmp(bytes, webp, 4)) {
        return @"image/webp";
    } else if (!memcmp(bytes, ico, 4)) {
        return @"image/vnd.microsoft.icon";
    } else if (!memcmp(bytes, tif_ii, 4) || !memcmp(bytes, tif_mm, 4)) {
        return @"image/tiff";
    } else if (!memcmp(bytes, png, 8)) {
        return @"image/png";
    } else if (!memcmp(bytes, jp2, 12)) {
        return @"image/jp2";
    }

    return @"application/octet-stream"; // default type

}

The above method recognizes the following image types:

• image/x-ms-bmp (bmp)
• image/gif (gif)
• image/jpeg (jpg, jpeg)
• image/psd (psd)
• image/iff (iff)
• image/webp (webp)
• image/vnd.microsoft.icon (ico)
• image/tiff (tif, tiff)
• image/png (png)
• image/jp2 (jp2)

Unfortunately, there is no simple way to get this kind of information from a UIImage instance, because its encapsulated bitmap data cannot be accessed.

Solution 3

@Tai Le's solution for Swift 3 is assigning whole data into byte array. If an image large, it can cause crash. This solution just assigns single byte:

import Foundation

public extension Data {
    var fileExtension: String {
        var values = [UInt8](repeating:0, count:1)
        self.copyBytes(to: &values, count: 1)

        let ext: String
        switch (values[0]) {
        case 0xFF:
            ext = ".jpg"
        case 0x89:
            ext = ".png"
        case 0x47:
            ext = ".gif"
        case 0x49, 0x4D :
            ext = ".tiff"
        default:
            ext = ".png"
        }
        return ext
    }
}

Solution 4

If you're retrieving the image from a URL, then presumably you can inspect the HTTP response headers. Does the Content-Type header contain anything useful? (I'd imagine it would since a browser would probably be able to display the image correctly, and it could only do that if the content type were appropriately set)

Solution 5

Swift3 version:

let data: Data = UIImagePNGRepresentation(yourImage)!

extension Data {
    var format: String {
        let array = [UInt8](self)
        let ext: String
        switch (array[0]) {
        case 0xFF:
            ext = "jpg"
        case 0x89:
            ext = "png"
        case 0x47:
            ext = "gif"
        case 0x49, 0x4D :
            ext = "tiff"
        default:
            ext = "unknown"
        }
        return ext
    }
}

View more solutions

Comments

[pschang]

Well I'm storing it on a third-party (Amazon S3) and eventually intend on using this image on a web site as well. Although, now that I have it working, I see that the browser knows how to render the image regardless of a file extension. And we know iOS's UIImage doesn't care. So maybe it doesn't matter?

[pschang]

Oh yes. I hadn't thought of this! Good idea.

[Steven Fisher]

My guess is that it doesn't matter, but if you care about IE doing the right thing to the files without extensions you should probably test it as well.

[Devarshi]

is there any equivalent solution to identify type of files of other kinds ... such as rtf, mov, mp3?

[Shyam Bhat]

This doesn't help me in case of identifying an UIImage object. To extract NSData from an UIImage, I need to use either UIImagePNGRepresentation() or UIImageJPEGRepresentation() which converts the imageData by itself.

[nzeltzer]

Unfortunately, UIImage doesn't provide any access to image context – you won't be able to determine the image type without reference to the original byte stream.

[nzeltzer]

An alternative approach: CGImageSource (part of the ImageIO framework) can provide you with the appropriate uniform type identifier for image data.

[rob5408]

I had a jpg with a first byte of 0xDB?

[Arun Gupta]

Did anyone know bytes for pdf, word(doc, docx), excel(xls, xlsx), powerpoint(ppt, pptx), text, rtf..

[alfonso]

Great list, thanks. I'm having trouble with the new .heic image type used in iOS11. Could you update your answer to include this new type?

[Leo Dabus]

if you use UIImagePNGRepresentation to get the UIImage data your data property will always return PNG. Btw in Swift 3 or later Data it is already a collection of bytes why not simply first?

[Leo Dabus]

@ArunGupta a text can start with any byte. Btw a text that starts with the letter "G" could be interpreted as a gif image using this method. try String(data: Data([0x47]), encoding: .utf8)!. // G