ExifLib - A Fast Exif Data Extractor for .NET 2.0

• Download source and demo project - 18.38 KB

Introduction

ExifLib simply reads Exif tags (i.e., camera model, GPS data, date picture taken, shutter speed etc.) from JPEG files, without the overhead introduced by using the GDI+ classes located in System.Drawing.Imaging, and with less lines of code for the developer.

Background

I've been using a simple command line application to move my photos into subdirectories based on the date on which they were created. As with all other .NET Exif implementations I've seen, I was using the PropertyItem class located in System.Drawing.Imaging. While this does the job, I often found myself processing thousands of images at a time, and the .NET classes were just too slow for the job. ExifLib goes back to the JPEG/TIFF standard itself, and only reads the essentials, using little more than the file input classes in System.IO.

Using the Code

ExifLib is very simple, with only one class and one enum in the namespace. Just add a reference to ExifLib.dll, and you're good to go! An example follows:

using ExifLib;
...
...
...
// Instantiate the reader
ExifReader reader = new ExifReader(@"C:\temp\testImage.jpg");

// Extract the tag data using the ExifTags enumeration
DateTime datePictureTaken;
if (reader.GetTagValue<DateTime>(ExifTags.DateTimeDigitized, 
                                    out datePictureTaken))
{
    // Do whatever is required with the extracted information
    MessageBox.Show(this, string.Format("The picture was taken on {0}", 
       datePictureTaken), "Image information", MessageBoxButtons.OK);
}

Points of Interest

Something strange that I learned while writing this library is that while JPEG stipulates "Big Endian" encoding (i.e., numbers read from left to right), the TIFF standard allows Big or Little Endian encoding. Since the Exif tags are encoded using TIFF encoding, often the JPEG will be read using "Big Endian" encoding until the TIFF section is reached, at which point the encoding reverses and the rest of the document is read using "Little Endian" encoding.

During coding, I realised from a comment on the ExifWorks CodeProject article that it's possible to increase performance when using System.Drawing.Image by setting the constructor's validateImageData parameter to false. However, even when using this enhancement, ExifLib still performs 50% faster, possibly because it does not read the tag values until they're requested. I have also noticed that ExifLib performs similarly with small (<1MP) images, but scales better when loading larger images. The screenshot at the top of this page was produced using a 5.5MP image.

History

Version 1.1

• Array extraction has been added, thanks to a comment from Justin Carasick. This is used in various fields, including GPS coordinates and Exif versioning. The previous version of ExifLib would only return the first element from an array.

Version 1.2

• Fixed bug when retrieving data for fields shorter than 4 bytes, thanks to a comment from bartsy. The previous version of ExifLib would lose important data from these fields when processing big-endian encoded files.
• Updated the project to Visual Studio 2010, refactored a little of the code. The project is still .NET 2.0+.

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)