Fooling Around

This does not seem to be documented anywhere, so it makes sense to mention. A video decoder, wrapped by DMO Wrapper Filter, will receive preroll media samples with AM_SAMPLE_PREROLL flag (alternatively available using IMediaSample::IsPreroll), but it won’t even forward these samples to the underlying DMO, instead they are just ignored.

MSDN says:

Preroll samples are processed but not displayed. They are located in the media stream before the displayable samples.

However this behavoir of DMO Wrapper Filter does not seem to be correct. A DMO might require (in my case, maybe there are other scenarios) to initialize its decoding context from a splice point and then be able to decode further samples. The way the wrapper is skipping samples it appears that DMO is not receiving splice point data and won’t be able to start decoding of non-preroll samples until it receives a non-preroll splice point media sample…

Recently it was the time to sort out an issue with a video DMO, which outputs YV12 video and in the same time is capable of supporting extended video strides in order to efficiently make a direct connection to Video Mixing Renderer Filters.

From past experience, I already knew that some bugs are definitely involved but their effect was yet unexplored. For a testbed application, I took good old FrameRateSample02 application, which generates multiple video feeds and routes it to video renderers:

With new source video choices the application is capable of constructing filter graphs that use a private DMO (that is hosted inside the executable) wrapped with DMO Wrapper Filter, with a graph topology shown below:

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Started as Is it possible to use local component in DLL? on microsoft.public.vc.atl newsgroup. The question is to embed a custom DirectX Media Object (DMO) into executable so that it is only available to proprietary application and not to entire system and could be reused in other applications. If in particular this DMO should be available to intelligent connect, e.g. used by embedded Windows Media Player control, the solution requires that private DMO appears fully registered as full featured regular DMO.

A while ago I proposed a solution on getting COM Class/DMO embedded into executable and temporarily exposed as availble without registry registration through CoRegisterClassObject/RegisterClassObject, however in addition to COM class registration, a DMO needs to also be specifically listed as a DMO, through DMORegister API.

In order to expose private DMO to DirectShow subsystem the following steps has to be performed:

• register COM class object as a COM class (CoRegisterClassObject) to enable its instantiation by its CLSID
• use DMORegister API to list DMO in global list (warning, seems to be requiring write permission on HKEY_CLASSES_ROOT key)
• have the filter graph of interest created
• use DMOUnregister API as early as possible to unlist private DMO from global list

ATLENSURE_SUCCEEDED(__objMap_CPrivateMediaObject.RegisterClassObject(CLSCTX_INPROC_SERVER, REGCLS_MULTI_SEPARATE));
DMO_PARTIAL_MEDIATYPE pInputMediaTypes[] = { { MEDIATYPE_Video, CPrivateMediaObject::GetXxxxSubtype() } };
ATLENSURE_SUCCEEDED(DMORegister(L"Private DMO", CLSID_PrivateMediaObject, DMOCATEGORY_VIDEO_DECODER, 0, _countof(pInputMediaTypes), pInputMediaTypes, 0, NULL));
...
ATLVERIFY(SUCCEEDED(DMOUnregister(CLSID_PrivateMediaObject, DMOCATEGORY_VIDEO_DECODER)));

The sample code/project takes an .AVI file which is a XVID video file with a FOURCC compression code changed from XVID to XXXX. The file is unplayable but there is a private DMO which accepts XXXX video and copies data as is into XVID format on its output.

A Visual C++ .NET 2008 source code is available from SVN.

Previously on the topic:

• Part 1: Starting the Project
• Part 2: Video Processing
• Part 3: Persistence, Automation and Property Pages
• Part 4: Merit

Due to the nature of the brightness and constract correction processing, it would make sense to combine and simplify processing to apply correction “in-place”, that is without copying data from input to output buffer, but instead processing the same buffer before it is passed further downstream.

DMO API offers additional optional IMediaObjectInPlace interface to be implemented on the DMO which the hosting object might prefer to regular IMediaObject.

The interface itself is simple with basically the only Process method to actually handle the processing:

// IMediaObjectInPlace
	STDMETHOD(Process)(ULONG nSize, BYTE* pnData, REFERENCE_TIME nStartTime, DWORD nFlags)
	STDMETHOD(Clone)(IMediaObjectInPlace** ppMediaObject)
	STDMETHOD(GetLatency)(REFERENCE_TIME* pnLatencyTime)

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Previously on the topic:

• Part 1: Starting the Project
• Part 2: Video Processing
• Part 3: Persistence, Automation and Property Pages

The implemented so far filter/DMO shown a problem related to its unexpectedly high “importance” in the system with the symptom of “auto-insertion” the filter when it is not necessary. For example, let us render an AVI file through Infinite Tee Pin Filter:

The problem is that DirectShow auto-inserts our Brightness/Contrast filter into the graph while it is obviously not expected, wanted or necessary:

The problem is high filter/DMO merit value and a popular YUY2 video format the filter is advertised to accept on input during DMO registration.

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See topic on MSDN Forums about coloring issue.

This is a demo of YUY2 to YV12 conversion as suggested copying Y values and averaging U and V.

Original image is on the right.

Code snippet to perform the transformation:

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Previously on the topic:

• Part 1: Starting the Project
• Part 2: Video Processing

The principal task of video processing is done but there are still things mandatory for the filter to be usable. First of all, a custom interface is required to be able to control the filter from higher level application and to adjust brightness and constract correction values on the run time. Additionally, persistence would not hurt at all to be able to store correction settings along with other graph settings in GraphEdit graph file or anywhere else. Additionally, it would also be convenient to have a property page for the filter to be able to adjust the correction settings through GUI, both on graph composition and while the graph is running.

All the mentioned tasks are interconnected and ATL has an answer in implementation of:

• IDispatch-derived automation interface through IDispatchImpl class to implement custom interface to be used for external control over the filter/DMO and also to be used to access persistent properties
• IPersistStream/IPersistStreamInit interfaces through IPersistStreamInitImpl class and PROP_MAP macro map to implement persistence capabilities
• ISpecifyPropertyPages interface through ISpecifyPropertyPagesImpl class to provide custom property page

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