Chapter 15. Using Multimedia in VR (4/10)

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400 15. Using Multimedia in VR

mentation. Because it is nicely self-contained, the facility can be added to any

Windows application with little difﬁculty. We won’t go into great detail, since

it takes us away from our main theme, but Listings 15.4 and 15.5 present a

C++ class and its methods to implement drag and drop. If you put this code

into a separate ﬁle, you can include it with all your programs and make use of

it by adding the code from Listing 15.6 in the appropriate places.

STDMETHODIMP CDropTarget::QueryInterface(REFIID riid, void

ppv){

if(ppv == NULL)return E_POINTER;

if(riid == IID_IDropTarget || riid == IID_IUnknown) {

AddRef();

ppv=this;

return S_OK;

}

ppv = NULL;

return E_NOINTERFACE;

}

STDMETHODIMP_(ULONG) CDropTarget::AddRef() {

return InterlockedIncrement(&m_cRef);

}

STDMETHODIMP_(ULONG) CDropTarget::Release(){

LONG lRef = InterlockedDecrement(&m_cRef);

if (lRef == 0)delete this;

return lRef;

}

STDMETHODIMP CDropTarget::DragEnter(

IDataObject

pDataObject,

//Pointer to the interface of the source data

// object

DWORD grfKeyState, //Current state of keyboard modifier keys

POINTL pt, //Current cursor coordinates

DWORD

pdwEffect //Pointer to the effect of the drag-and-drop

// operation

){ return S_OK; };

STDMETHODIMP CDropTarget::DragLeave(void){ return S_OK;}

STDMETHODIMP CDropTarget::DragOver(

DWORD grfKeyState, //Current state of keyboard modifier keys

POINTL pt, //Current cursor coordinates

DWORD

pdwEffect //Pointer to the effect of the drag-and-drop

// operation

){ return S_OK;}

Listing 15.5. Methods of the drag-and-drop class.

15.4. Video Sources 401

STDMETHODIMP CDropTarget::Drop(

IDataObject

pDataObject,

//Pointer to the interface for the source data

DWORD grfKeyState, //Current state of keyboard modifier keys

POINTL pt, //Current cursor coordinates

DWORD

pdwEffect //Pointer to the effect of the drag-and-drop

// operation

){

FORMATETC ff;

ff.cfFormat=CF_HDROP;

ff.ptd=NULL;

ff.dwAspect=DVASPECT_CONTENT;

ff.lindex= -1;

ff.tymed=TYMED_HGLOBAL;

STGMEDIUM pSM;

pDataObject->GetData(&ff,&pSM);

HDROP hDrop = (HDROP)(pSM.hGlobal);

TCHAR name[512];

int i=DragQueryFile(hDrop,0xFFFFFFFF,name,512);

DragQueryFile(hDrop,0,name,512);

// now that we’ve got the name as a text string use it ///////////

OpenDraggedClip(name); // this is the application defined function

//////////////////////////////////////////////////////////////////

ReleaseStgMedium(&pSM);

return S_OK;

}

Listing 15.5. (continued).

15.4 Video Sources

The other major use we will make of DirectShow concerns data acquisition.

Essentially, this means grabbing video or audio. In this section, we will set

up some example FilterGraphs to acquire data from live video sources. These

could be digital camcorders (DV) connected via the FireWire interface or

USB cameras ranging in complexity from a simple webcam using the USB 1

interface to a pair of miniature cameras on a head-mounted display device

multiplexed through a USB 2 hub. It could even be a digitized signal from an

analog video recorder or S-video output from a DVD player.

To DirectShow, all these devices appear basically the same. There are

subtle differences, but provided the camera/digitizer has a WDM (windows

driver model) device driver, DirectShow has a source ﬁlter which any appli-

cation program can load into its FilterGraph and use to generate a stream of

video samples. We shall see in this section that using a live video sour c e in a

402 15. Using Multimedia in VR

OleInitialize(NULL); // Drag and drop requires OLE not jsut COM so

// replace CoInitialize(NULL) with this.

SetUpDragAndDrop(hWnd) // make the application D&D compatible

Close DragAndDrop(); // destroy the D&D object

OleUninitialize(); //

void OpenDraggedClip(TCHAR

filename){ // this is called with file is dropped

if (g_psCurrent != Init)CloseClip();

OpenClip(T2A(filename)); // convert text string to basic "char" format

}

Listing 15.6. Modiﬁcations required in an application so that it can handle drag

and drop. When a ﬁle is dropped into the application window, the function

OpenDraggedClip() is called. Note also the change to OleInitialize() in-

stead of

CoInitialize().

DirectShow FilterGraph is as easy as using a ﬁle-based source. As a result, our

code listings will be relatively short and only complicated by the need to pro-

vide code to enable the application programs to select the desired input source

when more than one possible device is connected to the host computer.

There are, of course, some differences between the types of video signal

sources that we must be aware of as we plan our application design. We note

three of signiﬁcance:

1. Data rate. DV camcorders will typically provide 30 samples (or video

frames) per second (fps). A USB 1 webcam may only be able to deliver

15 fps.

2. Sample size or frame resolution. DV frame sizes in pixels are typically

720×576 for PAL regions and 640×480 in NTSC regions. A webcam

resolution might be 320 × 240.

3. Pixel data format. In most applications, we would like the source to de-

liver a 3-byte-per-pix el RGB24 data stream, but some video source de-

vices provide a YUV2 format. DirectShow’s intelligent connect mech-

anism inserts appropriate conversion ﬁlters so that if we want to use

RGB24 (or RGB32 for that matter), a YUV2-to-RGB conversion ﬁlter

will be loaded and connected into the graph.

A special DirectShow ﬁlter, called the grabber ﬁlter, is often put

into the ﬁlter chain by an application program. This not only

15.4. Video Sources 403

lets an application program peek into the data stream and grab

samples, but it is also used to tell the upstream source ﬁlters that

they must deliver samples in a particular format.

As w e build the code for a couple of examples using live video source ﬁl-

ters, you will see that the differences from the previous movie player program

are quite small. The longest pieces of code associated with video sources relate

to ﬁnding and selecting the source we want. In the context of F ilterGraphs

using video sources, the

ICaptureGraphBuilder2 interface comes into its

own by helping to put in place any necessary intermediate ﬁlters using intel-

ligent connect. Without the help of the

ICaptureGraphBuilder2 interface,

our program would have to search the source and sink ﬁlters for suitable in-

put and output pins and link them. Some of the project codes on the CD

and many examples from the DXSDK use this strategy, and this is one of the

main reasons why two DirectShow programs that do the same thing can look

so different.

15.4.1 Viewing Video from a DV or USB Camera

For the most part (the WinMain entry, message handler function etc.), the

code for this short program is very similar to the movie player code, so we

will not list it here. Instead, we go straight to look at the differences in the

structure of the FilterGraph given in Listing 15.7. A couple of functions merit

a comment:

g pCapture->RenderStream().ThisICaptureGraphBuilder2

method carries out a multitude of duties. Using its ﬁrst argument,

we select either a preview output or a rendered output. This gives an

application program the possibility of previewing the video while cap-

turing to a ﬁle at the same time. The second argument tells the method

that we need video samples. The last three arguments—

pSrcFilter,

NULL, NULL—put the graph together. They specify:

(a) The source of the video data stream.

(b) An optional intermediate ﬁlter between source and destination; a

compression ﬁlter for example.

NULL, the built-in renderer will be used.

If we are recording the video data into a ﬁle using a ﬁle writing ﬁlter (say

called

pDestFilter), the last three arguments would be (pSrcFilter,

404 15. Using Multimedia in VR

ICaptureGraphBuilder2

g_pCapture=NULL;// declare global pointer

// this is main function to build/run the capture graph

HRESULT CaptureVideo(){

HRESULT hr;

// this will point to the Video source filter

IBaseFilter

pSrcFilter=NULL;

hr = GetInterfaces(); // Get DirectShow interfaces

// Tell the capture graph builter to do its work on the graph.

hr = g_pCapture->SetFiltergraph(g_pGraph);

// Use the system device enumerator and class enumerator to find

// a video capture/preview device, such as a desktop USB video camera.

hr = FindCaptureDevice(&pSrcFilter);// see next listing

// Add Capture filter to our graph.

hr = g_pGraph->AddFilter(pSrcFilter, L"Video Capture");

// Render the preview pin on the video capture filter

// Use this instead of g_pGraph->RenderFile

hr = g_pCapture->RenderStream(&PIN_CATEGORY_PREVIEW,&MEDIATYPE_Video,

pSrcFilter, NULL, NULL);

// Now that the filter has been added to the graph and we have

// rendered its stream, we can release this reference to the filter.

pSrcFilter->Release();

hr = SetupVideoWindow(); // Setup video window

hr = g_pMC->Run(); // Start previewing video data

return S_OK;

}

// very similar to movie player - (Do check for errors in hr!!!

HRESULT GetInterfaces(void){

HRESULT hr;

// Create the filter graph

hr = CoCreateInstance (CLSID_FilterGraph, NULL, CLSCTX_INPROC,

IID_IGraphBuilder, (void

) &g_pGraph);

// Create the capture graph builder

hr = CoCreateInstance (CLSID_CaptureGraphBuilder2 , NULL, CLSCTX_INPROC,

IID_ICaptureGraphBuilder2, (void

) &g_pCapture);

// Obtain interfaces for media control and Video Window

hr = g_pGraph->QueryInterface(IID_IMediaControl,(LPVOID

) &g_pMC);

hr = g_pGraph->QueryInterface(IID_IVideoWindow, (LPVOID

) &g_pVW);

hr = g_pGraph->QueryInterface(IID_IMediaEvent, (LPVOID

) &g_pME);

// Set the window handle used to process graph events

hr = g_pME->SetNotifyWindow((OAHWND)ghApp, WM_GRAPHNOTIFY, 0);

return hr;

}

Listing 15.7. Build FilterGraph thatpreviews live video from a DV/USB camera. Note

the call to

FindCaptureDevice(), which ﬁnds the video source ﬁlter. Its code is

given in Listing 15.8.

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Table of Contents for Chapter 15. Using Multimedia in VR (4/10)

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Chapter 15. Using Multimedia in VR (4/10)