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August 19, 2008

Image Format Error when Loading from a Stream

The Microsoft Windows Imaging Component (WIC) is “an extensible framework for encoding, decoding, and manipulating images”. It's also the core of WPF’s System.Windows.Media.Imaging classes; this meant that a curious exception I got when using BitmapSource eventually led me to discover a possible bug in IWICImagingFactory::CreateDecoderFromStream.

My code was loading a large number of images from disk. The files contained a header with some image metadata, followed immediately by a regular Windows bitmap (in the ubiquitous BMP file format). The code would read the header from the stream, then load the bitmap from the rest of the stream, as follows:

using (Stream stream = new FileStream(filename, FileMode.Open))

{

    // read header

    stream.Read(header, 0, header.Length);

    // etc.

 

    BitmapImage bitmap = new BitmapImage();

    bitmap.BeginInit();

    bitmap.CacheOption = BitmapCacheOption.OnLoad;

    bitmap.StreamSource = stream;

    bitmap.EndInit();

    return bitmap;

}

Some images would fail to load, with EndInit throwing a mysterious System.IO.FileFormatException: “The image format is unrecognized”. The InnerException was System.Runtime.InteropServices.COMException (0x88982F07), with an HRESULT of a WIC error code: WINCODEC_ERR_UNKNOWNIMAGEFORMAT.

My first thought was that the images were somehow corrupted, but further investigation showed that the files loaded without errors if the header preceding the bitmap was removed from the file, or if the bitmap data following the header was first copied to a new MemoryStream before being loaded. I observed the same behaviour with IWICImagingFactory::CreateDecoderFromStream when I rewrote the test harness as a C++ COM application: if the IStream containing the image contained any data preceding the bitmap data, an error HRESULT would sometimes be returned.

It appears that, in certain circumstances, CreateDecoderFromStream assumes that the bitmap data begins at the stream's origin, and absolute offsets within the stream are used when seeking; thus, the image data must begin at offset 0 within the stream. As a workaround, you can copy the image data to a new MemoryStream (but note that this may increase memory usage). The solution I chose was to write a thin Stream wrapper class that handles calls to Position, Seek, Length, etc. and adjusts the offsets so that the image now appears to start at offset 0; all other calls are passed straight through to the underlying FileStream. This allows WIC and WPF to load all the images without having to make an unnecessary copy of the bitmap, or having to change the legacy file format.

Posted by Bradley Grainger at August 19, 2008 6:33 PM

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