C# get width/height of image on web without downloading whole file?

Question

I believe with JPGs, the width and height information is stored within the first few bytes. What's the easiest way to get this information given an absolute URI?

Answer 1

First, you can request the first hundred bytes of an image using the Range header.

HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);
request.Headers.Set(HttpRequestHeader.UserAgent, "Range: bytes=0-100");

Next, you need to decode. The unix file command has a table of common formats, and the locations of key information. I'd suggest installing Cygwin and taking a look at /usr/share/file/magic.

For gif's and png's, you can easily get the image dimensions from the first 32 bytes. However, for JPEGs, @Andrew is correct in that you can't reliably get this information. You can figure out if it has a thumbnail, and the size of the thumbnail.

The get the actual jpeg size, you need to scan for the start of frame tag. Unfortunately, one can't reliably determine where this is going to be in advance, and a thumbnail could push it several thousand bytes into the file.

I'd recommend using the range request to get the first 32 bytes. This will let you determine the file type. After which, if it's a JPEG, then download the whole file, and use a library to get the size information.