MediaFoundation decoder for LOVE & decoding in-memory.

Windows 7 has its own COM-based API to aid decoding variety of audio formats, and if you concerned about patents (AAC, why), don’t worry as it’s a licensed decoder. This blog post is about how I wrote LOVE decoder that uses MediaFoundation.

Basics

First thing to know is how LOVE decoder class looks like.

class Decoder : public Object
{
public:
    static love::Type type;
    Decoder(Data *data, int bufferSize);
    virtual ~Decoder();
    static const int DEFAULT_BUFFER_SIZE = 16384;
    static const int DEFAULT_SAMPLE_RATE = 44100;
    static const int DEFAULT_CHANNELS = 2;
    static const int DEFAULT_BIT_DEPTH = 16;
    virtual Decoder *clone() = 0;
    virtual int decode() = 0;
    virtual int getSize() const;
    virtual void *getBuffer() const;
    virtual bool seek(double s) = 0;
    virtual bool rewind() = 0;
    virtual bool isSeekable() = 0;
    virtual bool isFinished();
    virtual int getChannelCount() const = 0;
    virtual int getBitDepth() const = 0;
    virtual int getSampleRate() const;
    virtual double getDuration() = 0;

protected:
    StrongRef<Data> data;
    int bufferSize;
    int sampleRate;
    void *buffer;
    bool eof;
};

Anything that ends with = 0; means pure virtual method which we must implement in our derived class. Now let’s derive the Decoder class.

MFDecoder Class

class MFDecoder: public Decoder
{
public:
    MFDecoder(Data *data, int bufferSize);
    virtual ~MFDecoder();

    static bool accepts(const std::string &ext);
    static void quit();
    Decoder *clone();
    int decode();
    bool seek(double s);
    bool rewind();
    bool isSeekable();
    int getChannelCount() const;
    int getBitDepth() const;
    double getDuration();

private:
    static bool initialize();
    static void *initData;
    // non-exposed datatype to prevent cluttering LOVE
    // includes with Windows.h
    void *mfData;
    // channel count
    int channels;
    // temporary buffer
    std::vector<char> tempBuffer;
    // amount of temporary PCM buffer
    int tempBufferCount;
    // byte depth
    int byteDepth;
    // duration
    double duration;
};

There are few points that must be noted here.

• MFDecoder contstructor receives LOVE Data object, which is data located in block of memory, and the desired buffer size.
• MediaFoundation API can return any number of samples so we use temporary buffer to contain leftovers after decoding.
• You may notice that mfData is void*. The reason to do this is that there’s problem compiling LOVE if Windows.h is included BEFORE the keyboard module. We also don’t want to clutter the includes with Windows-specific includes and drag down the compilation time.
• The tempBuffer is std::vector. Yes this is intentional so we don’t have to manage the allocated memory and take advantage of RAII. It also helps in case MediaFoundation returns data bigger than provided buffer by simply reallocating bigger temporary buffer.
• Then there’s initData member. This is set at initialize function above it, which is called when new MediaFoundation decoder is created or if the compatible extensions are being checked.

Problems

Now there are some problems.

• MediaFoundation doesn’t officially support loading media from memory.
  MediaFoundation assume you have the media file reside somewhere in local filesystem or in network, probably due to their DRM nature. There’s this blog post, but it only works on Windows 8 and using C++/CLI which has its own 2 problems:
  
  1. We can’t use C++/CLI when compiling LOVE, that would reduce our compilation times and increase the bloat which we want to try to minimize as possible.
  2. My target is to make the decoder available for Windows 7 and later. IRandomAccessStream and the function it uses are Windows 8 and later.
  Then I found out there’s MFCreateMFByteStreamOnStream which accepts IStream interface. IStream interface is available since Windows 2000, which then SHCreateMemStream can be used to create one.
  
• The functions that I use requires linking to Mfplat.dll and Mfreadwrite.dll.
  The latter is only available in Windows 7. Since I want to make sure it runs in Windows Vista too (without the MediaFoundation decoding capabilities of course), I have to dynamically load it, hence the MFDecoder::initialize() static function.
  
• Once IMFByteStream is created, you have to set the MIME.
  It’s done by casting the IMFByteStream to IMFAttributes and set the MIME. Unfortunately, as of LOVE commit ccf9e63, the decoder constructor no longer receives the audio extensions, so we have to test for every possible supported MIME types. Fortunately, Microsoft gives us list of supported media formats in their documentation, so I just get the MIME string from IIS MIME Types.
  

After those problems is resolved, it’s only matter of setting the properties of the IMFSourceReader like creating decoder which outputs PCM.

Seeking

Yes, the IMFSourceReader supports seeking, but there’s no guarantee that it will be accurate. The function you’re looking for is IMFSourceReader::SetCurrentPosition which accepts time as 100-nanoseconds (second to 100-nanosecond is multiply by 1e+7).

Another Problem: GUID

I’m getting linker errors when compiling LOVE with the MediaFoundation decoder as it complains about unresolved GUID. I also don’t want to link to any of MediaFoundation DLLs so it’s binary compatible with Windows Vista (XP support is dropped as of LOVE 11.0). Temporary workaround to fix this is to copy the GUID declaration in header files to const GUID variables.

Aftermath

After I get everything running, now I have LOVE build which can loads AAC and WMA using MediaFoundation, how good is that, huh? You can check the full source code in here. The respective header lies in same directory as the C++ file.

Now you may ask, what about MinGW? Well, unfortunately LOVE doesn’t support being compiled under MinGW in the first place, so compiling LOVE under Windows is only supported using MSVC compiler.

And if anyone wants to decode audio from memory using MediaFoundation, then this blog post is what he/she’s looking for.

Post is written in Markdown first then converted to HTML lol.