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C# Wrapper to the FreeImage DLL for graphical image format conversion

, 30 Sep 2003
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This article provides a simplified C# wrapper to the FreeImage project for graphical file format conversion.


FreeImage is an extremely useful Open Source project for reading, manipulating and converting a large amount of graphical formats. However at present the library exists as Win32 Dynamic Link Library. As my latest fascination is C#, I decided to create a simple Interop wrapper for it.

Using the code

The code is very straightforward to use, which is a reflection on the FreeImage implementation itself. For my needs I wanted to be able to take a picture file in Portable Bitmap (PBM) format and convert it to a standard Bitmap (BMP). For this I needed 3 methods exposed via the FreeImage DLL, FreeImage_Load, FreeImage_Save and FreeImage_Unload, the signature for which are:

       const char *filename, int flags FI_DEFAULT(0));
   FIBITMAP *dib, const char *filename, int flags FI_DEFAULT(0)); 
DLL_API void DLL_CALLCONV FreeImage_Unload(FIBITMAP* dib);

Now the C# Interop signatures for these are:

    public class FreeImage
         public static extern int FreeImage_Load(FIF format, 
                        string filename, int flags);
         public static extern void FreeImage_Unload(int handle);
         public static extern bool FreeImage_Save(FIF format, 
            int handle, string filename, int flags);

As an example, to convert a graphical image from PBM to BMP, all you need to do is the following (note the enum FIF is not documented here, but can be found in the source code).

namespace Test
    class MainClass
        public static void Main(string[] args)
            int handle = FreeImageAPI.FreeImage.FreeImage_Load(


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About the Author

David Boland
Web Developer
United States United States
No Biography provided

Comments and Discussions

GeneralResquest.OutputStream Pinmemberkernellius4-Apr-07 13:03 
QuestionWhat about large images? PinmemberGerrit Horeis6-Oct-06 4:17 
Questionhelp Pinmemberxenia gr3-Sep-06 23:48 
QuestionHow to convert double pointers (**)? Pinmembervor0nwe26-Apr-06 6:37 
I've noticed that none of the Metadata functions have been included. Frown | :(
I've been trying to add them, but I find it very hard to convert the double pointers correctly.
How can the following function call be translated to C#?
FIBITMAP *dib, FITAG **tag);

Thanks for any pointers! (no pun intended :->)
QuestionI can't make C# works with FreeImage Pinmembermesh20055-Mar-06 22:41 
Questioninserting images in word document? PinmemberHiral Patel27-Nov-05 0:51 
GeneralGrayscale PinmemberEmailSolidale21-Sep-05 21:55 
GeneralC:\projects\SVGTest\SVG.cs(145): Argument '2': cannot convert from 'int' to 'uint' Pinmembereric paul2-Aug-05 13:12 
GeneralRe: C:\projects\SVGTest\SVG.cs(145): Argument '2': cannot convert from 'int' to 'uint' PinmemberDavid Boland2-Aug-05 13:32 
GeneralRe: C:\projects\SVGTest\SVG.cs(145): Argument '2': cannot convert from 'int' to 'uint' Pinmemberminorello5-Aug-05 4:46 

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