Extracting bitmaps from movies using DirectShow

• Download demo project - 132 Kb

Introduction

This article explains how to use the ISampleGrabber interface to grab a frame from a movie. We'll add the SampleGrabber filter to the graphbuilders filter list and use it to extract the bitmap. It'll show the necessary steps how to create the filter, create the graph, start it upp and grab the frame.

Setup the enviroment, we are using ATL smartpointers and DirectShow

#include "AtlBase.h"	// For atl smart pointers
#include "dShow.h"	// DirectShow header
#include "Qedit.h"	// SampleGrabber filter

The project has to be linked with Strmbase.lib

Since we're using COM we have to call CoInitialize() and CoUninitialize() in InitInstance, make sure the dialog destructor is called before CoUninitialize is called.

BOOL CFrameGrabberApp::InitInstance()
{
...
...
	CoInitialize(NULL);
	{
		CFrameGrabberDemoDlg dlg;
		m_pMainWnd = &dlg;
		int nResponse = dlg.DoModal();
	}
	CoUninitialize();
...
...
}

Step 1: Create the GraphBuilder

CComPtr<IGraphBuilder> pGraphBuilder;
HRESULT hr = ::CoCreateInstance(CLSID_FilterGraph, NULL, 
              CLSCTX_INPROC_SERVER,IID_IGraphBuilder,(void**)&pGraphBuilder);

Step 2: Create the Grabber filter and add it to the graph builder

CComPtr<IBaseFilter> pGrabberBaseFilter;
CComPtr<ISampleGrabber> pSampleGrabber;
AM_MEDIA_TYPE mt;
hr = ::CoCreateInstance(CLSID_SampleGrabber, NULL, CLSCTX_INPROC_SERVER,
                        IID_IBaseFilter, (LPVOID *)&pGrabberBaseFilter);
if (FAILED(hr))
	return hr;
pGrabberBaseFilter->QueryInterface(IID_ISampleGrabber, (void**)&pSampleGrabber);
if (pSampleGrabber == NULL)
	return E_NOINTERFACE;
hr = pGraphBuilder->AddFilter(pGrabberBaseFilter,L"Grabber");
if (FAILED(hr))
	return hr;

Step 3: Setup the media type we're interrested in and render the file. The graph builder will now setup all the filters it needs to render the movie including the sample grabber we added.

ZeroMemory(&mt,sizeof(AM_MEDIA_TYPE));
mt.majortype = MEDIATYPE_Video;
mt.subtype = MEDIASUBTYPE_RGB24;
mt.formattype = FORMAT_VideoInfo; 
hr = pSampleGrabber->SetMediaType(&mt);
if (FAILED(hr)) 
	return hr;
hr = pGraphBuilder->RenderFile(wFile,NULL); 
if (FAILED(hr)) 
	return hr;

Now when the graph is created we need to tell the sample grabber to stop the graph after receiving one sample, we also tell it to copy the sample data into it's internal buffer.

hr = pSampleGrabber->SetBufferSamples(TRUE);
if (FAILED(hr)) 
	return hr; 
hr = pSampleGrabber->SetOneShot(TRUE); 
if (FAILED(hr)) 
return hr;

Step 4: Now we run the graph and collects the data from the sample grabber.

hr = pMediaControl->Run();
if (FAILED(hr)) 
	return hr; 
long evCode;
hr = pMediaEventEx->WaitForCompletion(INFINITE, &evCode); 
if (FAILED(hr)) 
	return hr; 
AM_MEDIA_TYPE MediaType; 
ZeroMemory(&MediaType,sizeof(MediaType)); 
hr = pSampleGrabber->GetConnectedMediaType(&MediaType); 
if (FAILED(hr)) 
	return hr; 
// Get a pointer to the video header. 
VIDEOINFOHEADER *pVideoHeader = (VIDEOINFOHEADER*)MediaType.pbFormat; 
if (pVideoHeader == NULL) 
	return E_FAIL; 
// The video header contains the bitmap information. 
// Copy it into a BITMAPINFO structure. 
BITMAPINFO BitmapInfo; 
ZeroMemory(&BitmapInfo, sizeof(BitmapInfo)); 
CopyMemory(&BitmapInfo.bmiHeader, &(pVideoHeader->bmiHeader), 
           sizeof(BITMAPINFOHEADER)); 

// Create a DIB from the bitmap header, and get a pointer to the buffer. 
void *buffer = NULL; 
HBITMAP hBitmap = ::CreateDIBSection(0, &BitmapInfo, DIB_RGB_COLORS, &buffer, 
                                     NULL, 0); 
GdiFlush(); 
// Copy the image into the buffer. 
long size = 0; 
hr = pSampleGrabber->GetCurrentBuffer(&size,(long *)buffer);   
if (FAILED(hr)) 
	return  hr;

Now we have the bitmap handle, the demo program takes the sample one second in the movie and displays it to the user using an picture box.

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