Get the correct image width and height of an NSImage

Question

I use the code below to get the width and height of a NSImage:

NSImage *image = [[[NSImage alloc] initWithContentsOfFile:[NSString stringWithFormat:s]] autorelease];
imageWidth=[image size].width;
imageHeight=[image size].height;
NSLog(@"%f:%f",imageWidth,imageHeight);

But sometime imageWidth, imageHeight does not return the correct value. For example when I read an image, the EXIF info displays:

PixelXDimension = 2272;
PixelYDimension = 1704;

But imageWidth, imageHeight outputs

521:390

score 32 · Accepted Answer · answered Nov 05 '12 at 07:59

Dimensions of your image in pixels is stored in NSImageRep of your image. If your file contains only one image, it will be like this:

NSImageRep *rep = [[image representations] objectAtIndex:0];
NSSize imageSize = NSMakeSize(rep.pixelsWide, rep.pixelsHigh);

where image is your NSImage and imageSize is your image size in pixels.

score 10 · Answer 2 · edited May 23 '17 at 12:25

10

NSImage size method returns size information that is screen resolution dependent. To get the size represented in the actual file image you need to use an NSImageRep.

Refer nsimage-size-not-real-size-with-some-pictures link and get helped

edited May 23 '17 at 12:25

Community

1
1

answered Aug 09 '12 at 05:00

Paresh Navadiya

38,095
11
81
132

Hii Price,in iOS How to get size information that is screen resolution dependent? Have you any idea? then plz help me. – Nikunj Aug 24 '13 at 12:05

score 4 · Answer 3 · answered Aug 13 '15 at 08:11

4

the direct API gives also the correct results

CGImageRef cgImage = [oldImage CGImageForProposedRect:nil context:context hints:nil];
size_t width = CGImageGetWidth(cgImage);
size_t height = CGImageGetHeight(cgImage);

answered Aug 13 '15 at 08:11

Karsten

1,869
22
38

1

This appears to be the only correct answer when running on a Retina screen. NSImage lies (what it returns might depend on the DPI that the image data declares), NSImageRep lies (it will return 2x the true pixel size when on Retina); only CGImage gives the actual size of the image in pixels. I haven't been able to find any other way to get that information, on macOS 11. – bhaller Jul 11 '21 at 21:52

score 2 · Answer 4 · answered Aug 09 '12 at 05:01

Apple uses a point system based on DPI to map points to physical device pixels. It doesnt matter what the EXIF says, it matters how many logical screen points your canvas has to display the image.

iOS and OSX perform this mapping for you. The only size you should be concerned about is the size returned from UIImage.size

You cant (read shouldnt have to shouldnt care) do the mapping to device pixels yourself, thats why apple does it.

score 2 · Answer 5 · answered Jan 02 '19 at 22:32

SWIFT 4

You have to make a NSBitmapImageRep representation of the NSImage to get the correct pixel height and width. First a this extension to gather a CGImage from the NSImage:

extension NSImage {
    @objc var CGImage: CGImage? {
        get {
            guard let imageData = self.tiffRepresentation else { return nil }
            guard let sourceData = CGImageSourceCreateWithData(imageData as CFData, nil) else { return nil }
            return CGImageSourceCreateImageAtIndex(sourceData, 0, nil)
        }
    }
}

Then when you want to get the height and width:

let rep = NSBitmapImageRep(cgImage: (NSImage(named: "Your Image Name")?.CGImage)!)
let imageHeight = rep.size.height
let imageWidth = rep.size.width

score 1 · Answer 6 · answered Jul 11 '21 at 05:49

i make a extension like this:

extension NSImage{
    var pixelSize: NSSize?{
        if let rep = self.representations.first{
            let size = NSSize(width: rep.pixelsWide, height: rep.pixelsHigh)
            return size
        }
        return nil
    }
}

Get the correct image width and height of an NSImage

6 Answers6

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