{"id":402,"date":"2008-07-05T13:31:11","date_gmt":"2008-07-05T11:31:11","guid":{"rendered":"https:\/\/alax.info\/blog\/?p=402"},"modified":"2008-07-18T18:46:20","modified_gmt":"2008-07-18T16:46:20","slug":"how-to-implement-directshow-fitler-using-directx-media-object-part-1","status":"publish","type":"post","link":"https:\/\/alax.info\/blog\/402","title":{"rendered":"How To: Implement DirectShow Filter using DirectX Media Object DMO (Part 1: Starting the Project)"},"content":{"rendered":"

This post is starting a step by step tutorial on writing a simple DirectShow<\/a> filter using a simplified DirectX Media Objects (DMO)<\/a> API. From the very scratch, the goal is to make a DirectShow\/DMO video processing filter which implements video brightness and contrast correction.<\/p>\n

DirectX Media Objects are COM<\/a>-based components. To implement a COM object we will use Visual Studio .NET 2008<\/a> and Active Template Library (ATL)<\/a>.<\/p>\n

We are starting creating a no thrills ATL DLL project following by adding a no thrills ATL Simple Object Class<\/em>:<\/p>\n

\"ATL<\/a><\/p>\n

<\/p>\n

\"ATL<\/a><\/p>\n

The only thing to make sure is to choose “Both” Threading Model since DirectX Media Object must function correctly in a free-threaded environment:<\/p>\n

\"ATL<\/a><\/p>\n

Visual Studio wizard creates a class which implements custom IDispatch-derived dual interface. We will not need it until we get to persistence and property pages.<\/p>\n

First of all, we need to reference relevant SDK headers to be able to use DirectShow and DMO API<\/a>:<\/p>\n

#include < dshow.h >\r\n#include < dmo.h >\r\n#include < dmodshow.h >\r\n...\r\n#pragma comment(lib, \"strmiids.lib\")\r\n#pragma comment(lib, \"dmoguids.lib\")\r\n#pragma comment(lib, \"msdmo.lib\")<\/pre>\n
We reference header files and immediately leave a comment to linker to link library files (to avoid specifying them explicitly in project settings.<\/p>\n
From the minimum requirements for every DMO<\/a> we implemented aggregation (ATL<\/a> provides implementation by default) and threading model:<\/p>\n
Every DMO should meet the following minimum requirements:
\n– It must support aggregation.
\n– It must expose the IMediaObject<\/a> interface.
\n– The threading model must be ‘both’. DMOs must function correctly in a free-threaded environment.<\/p><\/blockquote>\n
We yet need to expose IMediaObject<\/a> interface and add registration code to expose the object\/filter to DirectShow-enabled applications and software components. While registration step is not necessary, it still allows visual graph construction with the filter being developed using GraphEdit<\/a> utility.<\/p>\n
To implement IMediaObject<\/a>, we inherit from it the media object class:<\/p>\n
class ATL_NO_VTABLE CBrightnessContrastObject :\r\n    public CComObjectRootEx< CComMultiThreadModel >,\r\n    public CComCoClass< CBrightnessContrastObject, &CLSID_BrightnessContrastObject >,\r\n    public IDispatchImpl< IBrightnessContrastObject >,\r\n    public IMediaObject<\/strong><\/pre>\n
then add the interface into COM MAP:<\/p>\n
BEGIN_COM_MAP(CBrightnessContrastObject)\r\n    COM_INTERFACE_ENTRY(IMediaObject)<\/strong><\/pre>\n
and add IMediaObject<\/a> methods to the class.<\/p>\n
Since IMediaObject<\/a> contains Lock method, it is confused with CComObjectRootEx<\/a>‘s Lock and breaks project from compilation. To resolve this ambiguity, we add another method to the media object class:<\/p>\n
\/\/ CComObjectRootEx\r\n    VOID Lock()\r\n    {\r\n        CComObjectRootEx::Lock();\r\n    }<\/pre>\n
To enable automatic registration of the media object with DirectShow\/DMO infrastructure, we are putting registration as a part of COM registration of the DLL module. This way the object will be automatically exposed to DirectShow software once the DLL is installed and registered with the system.<\/p>\n
First of all, we are adding GetObjectFriendlyName<\/a> method to provide a friendly name for the object, to be used by both ATL to register the class as COM class and the same name will be available to DirectShow software:<\/p>\n
static LPCTSTR GetObjectFriendlyName() throw()\r\n{\r\n    return _T(\"Alax.Info BrightnessContrast Sample\");\r\n}<\/pre>\n
To intercept COM registration and register the object as DMO we are overriding UpdateRegistry method. The implementation for this method is added to th class through DECLARE_REGISTRY_RESOURCEID<\/a> macro. We comment this out and add implementation for this method explicitly:<\/p>\n
Except for default handling through CAtlModule::UpdateRegistryFromResource<\/a>, we will add calls to our RegisterMediaObject<\/strong> and UnregisterMediaObject<\/strong> functions which in their turn will use DMORegister<\/a> and DMOUnregister<\/a> API.<\/p>\n
static HRESULT WINAPI UpdateRegistry(BOOL bRegister) throw()\r\n{\r\n    _ATLTRY\r\n    {\r\n        if(!bRegister)\r\n            UnregisterMediaObject();\r\n        ATLASSERT(_pAtlModule);\r\n        _pAtlModule->UpdateRegistryFromResource(IDR_BRIGHTNESSCONTRASTOBJECT, bRegister);\r\n        if(bRegister)\r\n            RegisterMediaObject();\r\n    }\r\n    _ATLCATCH(Exception)\r\n    {\r\n        return Exception;\r\n    }\r\n    return S_OK;\r\n}<\/pre>\n
Our media object will expose one input and one output stream\/pin both with media type MEDIATYPE_Video, MEDIASUBTYPE_YUY2<\/a>.<\/p>\n
Once the project is compiled, COM registration is performed as a part of build task and DMO registration is a subtask. So, the object is already exposed to DirectShow software and applications:<\/p>\n
\"\"<\/a><\/p>\n
The framework part is done, we are ready to add real implementation for IMediaObject<\/a> interface and video processing methods.<\/p>\n
Source code: DmoBrightnessContrastSample.01.zip<\/a><\/p>\n
Additional Notes:<\/p>\n