How can I get the resolution of an image? (JPEG, GIF, PNG, JPG)

Question

I found this method:

Graphics g = e.Graphics;
Bitmap bmp = new Bitmap("winter.jpg");
g.DrawImage(bmp, 0, 0);
Console.WriteLine("Screen resolution: " + g.DpiX + "DPI");
Console.WriteLine("Image resolution: " + bmp.HorizontalResolution + "DPI");
Console.WriteLine("Image Width: " + bmp.Width);
Console.WriteLine("Image Height: " + bmp.Height);

SizeF s = new SizeF(bmp.Width * (g.DpiX / bmp.HorizontalResolution),
                    bmp.Height * (g.DpiY / bmp.VerticalResolution));

Console.WriteLine("Display size of image: " + s);

But I don't really understand how to fetch what I'm looking for. I'm not interested in DPI, I just need the 1024x768, 1200x1024 etc numbers. Also, would I have to create a new image object every time I want to find the resolution of the image?

I'm making an application that lists current images in a given folder, so any help would be appreciated. :)

Iodism

Answer 1

"1024x768" is only called a "resolution" in the context of a computer's display settings, and even then it's an unfortunate misnomer. In imaging "1024x768" is simply the width & height of the image, so as others have mentioned, you've already shown the code that retrieves these numbers. Tweaked slightly:

var img = Image.FromFile(@"C:\image.jpg");
Console.WriteLine(img.Width + "x" + img.Height); // prints "1024x768" (etc)

The only built-in method to get the numbers you're after is by creating a new instance (and, in fact, decoding the entire image) - which is going to be highly inefficient if you need to retrieve just these numbers for several hundred images. Avoiding this is hard; here's a starting point: How do I reliably get an image dimensions in .NET without loading the image?

---

When you speak of "resolution" you normally refer to the number of dots, or pixels, per inch. The Bitmap class stores this in the HorizontalResolution / VerticalResolution properties, while the Graphics class in DpiX / DpiY.

This calculation:

bmp.Width * (g.DpiX / bmp.HorizontalResolution)

can be rearranged for clarity as follows:

(bmp.Width / bmp.HorizontalResolution) * g.DpiX

where the bracketed part computes the width of the image in inches. Multiplying by g.DpiX then gives you the number of pixels in the Graphics object that have the same width in inches as the image bmp.

Cotta

Can you provide a minimum working example/JS fiddle to get DPI of uploaded JPEG image using JS? – Tshirt

Fun stupid fact... if you call the rotate function MS decides it's 300DPI... no matter what the original image was set to. You have to catch if before rotate, rotate, reset it (with appropriate flip adjustments for each 90 degrees of rotation), and THEN save it. Because Microsoft.... – Lipid

Answer 2

To get the size:

string path = @"C:\winter.jpg";
Image img = Image.FromFile(path);
Console.WriteLine("Image Width: " + img.Width); //equals img.Size.Width
Console.WriteLine("Image Height: " + img.Height); //equals img.Size.Height

Here's a full list of System.Drawing.Image properties, if case want to display others.

I'm not sure if there's a quicker way for an entire directory, with Windows Vista/7 there may be a newer API that contains this info, haven't come across it though.

Concordat

Answer 3

Surely you've answered your own question, it's as simple as this:

Bitmap bmp = new Bitmap(@"winter.bmp");     
Console.WriteLine("Image Width: " + bmp.Width);
Console.WriteLine("Image Height: " + bmp.Height);

The DPI info is really only useful when you are displaying or printing the image. In the sample code it is converting the image to a Graphics object that can be used to draw it to the screen.

To answer the 2nd part of you question, you could parse the JPEG file header to find the width/height information without actually loading in the image. But to do this you'd need to know the header information and file format for JPEG files.

Equivalency