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Various methods for capturing the screen
VC截屏的一些实现方法 ：GDI, DirectShow, WM SDK( Windows Media SDK)
Some times, we want to capture the contents of the entire screen
programmatically. The following explains how it can be done. Typically,
the immediate options we have, among others, are using GDI and/or DirectX. Another option that is worth considering is Windows Media API.
Here, we would consider each of them and see how they can be used for
our purpose. In each of these approaches, once we get the screenshot
into our application defined memory or bitmap, we can use it in
generating a movie. Refer to the article
for more details about creating movies from bitmap sequences programmatically.
When performance is not an issue and when all that we want is just a
snapshot of the desktop, we can consider the GDI option. This mechanism
is based on the simple principle that the desktop is also a window -
that is it has a window Handle (HWND) and a device context (DC). If we can get the device context of the desktop to be captured, we can just blit
those contents to our application defined device context in the normal
way. And getting the device context of the desktop is pretty
straightforward if we know its window handle - which can be achieved
through the function GetDesktopWindow(). Thus, the steps involved are:
Acquire the Desktop window handle using the function GetDesktopWindow();
Get the DC of the desktop window using the function GetDC();
Create a compatible DC for the Desktop DC and a compatible bitmap to select into that compatible DC. These can be done using CreateCompatibleDC() and CreateCompatibleBitmap(); selecting the bitmap into our DC can be done with SelectObject();
Whenever you are ready to capture the screen, just blit
the contents of the Desktop DC into the created compatible DC - that's
all - you are done. The compatible bitmap we created now contains the
contents of the screen at the moment of the capture.
Do not forget to release the objects when you are done. Memory is precious (for the other applications).
Void&CaptureScreen()&{&&&&&int&nScreenWidth&=&GetSystemMetrics(SM_CXSCREEN);&&&&&int&nScreenHeight&=&GetSystemMetrics(SM_CYSCREEN);&&&&&HWND&hDesktopWnd&=&GetDesktopWindow();&&&&&HDC&hDesktopDC&=&GetDC(hDesktopWnd);&&&&&HDC&hCaptureDC&=&CreateCompatibleDC(hDesktopDC);&&&&&HBITMAP&hCaptureBitmap&=CreateCompatibleBitmap(hDesktopDC,&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&nScreenWidth,&nScreenHeight);&&&&&SelectObject(hCaptureDC,hCaptureBitmap);&&&&&&BitBlt(hCaptureDC,0,0,nScreenWidth,nScreenHeight,&&&&&&&&&&&&hDesktopDC,0,0,SRCCOPY|CAPTUREBLT);&&&&&&SaveCapturedBitmap(hCaptureBitmap);&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&ReleaseDC(hDesktopWnd,hDesktopDC);&&&&&DeleteDC(hCaptureDC);&&&&&DeleteObject(hCaptureBitmap);&}&
In the above code snippet, the function GetSystemMetrics() returns the screen width when used with SM_CXSCREEN, and returns the screen height when called with SM_CYSCREEN.
Refer to the accompanying source code for details of how to save the
captured bitmap to the disk and how to send it to the clipboard. Its
pretty straightforward. The source code implements the above technique
for capturing the screen contents at regular intervals, and creates a
movie out of the captured image sequences.
Capturing the screenshot with DirectX is a pretty easy task. DirectX offers a neat way of doing this.
Every DirectX application contains what we call a buffer, or a
surface to hold the contents of the video memory related to that
application. This is called the back buffer of the application. Some
applications might have more than one back buffer. And there is another
buffer that every application can access by default - the front buffer.
This one, the front buffer, holds the video memory related to the
desktop contents, and so essentially is the screen image.
By accessing the front buffer from our DirectX application, we can capture the contents of the screen at that moment.
Accessing the front buffer from the DirectX application is pretty easy and straightforward. The interface IDirect3DDevice9 provides the GetFrontBufferData() method that takes a IDirect3DSurface9 object pointer and copies the contents of the front buffer onto that surface. The IDirect3DSurfce9 object can be generated by using the method IDirect3DDevice8::CreateOffscreenPlainSurface(). Once the screen is captured onto the surface, we can use the function D3DXSaveSurfaceToFile() to save the surface directly to the disk in bitmap format. Thus, the code to capture the screen looks as follows:
extern&IDirect3DDevice9*&g_pd3dD&Void&CaptureScreen()&{&&&&&IDirect3DSurface9*&pS&&&&&g_pd3dDevice-&CreateOffscreenPlainSurface(ScreenWidth,&ScreenHeight,&&&&&&&&&D3DFMT_A8R8G8B8,&D3DPOOL_SCRATCH,&&pSurface,&NULL);&&&&&g_pd3dDevice-&GetFrontBufferData(0,&pSurface);&&&&&D3DXSaveSurfaceToFile(&Desktop.bmp&,D3DXIFF_BMP,pSurface,NULL,NULL);&&&&&pSurface-&Release();&&}&
In the above, g_pd3dDevice is an IDirect3DDevice9
object, and has been assumed to be properly initialized. This code
snippet saves the captured image onto the disk directly. However,
instead of saving to disk, if we just want to operate on the image bits
directly - we can do so by using the method IDirect3DSurface9::LockRect().
This gives a pointer to the surface memory - which is essentially a
pointer to the bits of the captured image. We can copy the bits to our
application defined memory and can operate on them. The following code
snippet presents how the surface contents can be copied into our
application defined memory:
extern&void*&pB&extern&IDirect3DDevice9*&g_pd3dD&IDirect3DSurface9*&pS&g_pd3dDevice-&CreateOffscreenPlainSurface(ScreenWidth,&ScreenHeight,&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&D3DFMT_A8R8G8B8,&D3DPOOL_SCRATCH,&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&pSurface,&NULL);&g_pd3dDevice-&GetFrontBufferData(0,&pSurface);&D3DLOCKED_RECT&lockedR&pSurface-&LockRect(&lockedRect,NULL,&&&&&&&&&&&&&&&&&&&&D3DLOCK_NO_DIRTY_UPDATE|&&&&&&&&&&&&&&&&&&&&D3DLOCK_NOSYSLOCK|D3DLOCK_READONLY)));&for(&int&i=0&;&i&&&ScreenHeight&;&i++)&{&&&&&memcpy(&(BYTE*)&pBits&+&i&*&ScreenWidth&*&BITSPERPIXEL&/&8&,&&&&&&&&&&(BYTE*)&lockedRect.pBits&+&i*&lockedRect.Pitch&,&&&&&&&&&&ScreenWidth&*&BITSPERPIXEL&/&8);&}&g_pSurface-&UnlockRect();&pSurface-&Release();&
In the above, pBits is a void*. Make sure that we have allocated enough memory before copying into pBits. A typical value for BITSPERPIXEL
is 32 bits per pixel. However, it may vary depending on your current
monitor settings. The important point to note here is that the width of
the surface is not same as the captured screen image width. Because of
the issues involved in the memory alignment (memory aligned to word
boundaries are assumed to be accessed faster compared to non aligned
memory), the surface might have added additional stuff at the end of
each row to make them perfectly aligned to the word boundaries. The lockedRect.Pitch
gives us the number of bytes between the starting points of two
successive rows. That is, to advance to the correct point on the next
row, we should advance by Pitch, not by Width. You can copy the surface bits in reverse, using the following:
for(&int&i=0&;&i&&&ScreenHeight&;&i++)&{&&&&&memcpy((BYTE*)&pBits&+(&ScreenHeight&-&i&-&1)&*&&&&&&&&&&ScreenWidth&*&BITSPERPIXEL/8&,&&&&&&&&&&(BYTE*)&lockedRect.pBits&+&i*&lockedRect.Pitch&,&&&&&&&&&&ScreenWidth*&BITSPERPIXEL/8);&}&
This may come handy when you are converting between top-down and bottom-up bitmaps.
While the above technique of LockRect() is one way of accessing the captured image content on IDirect3DSurface9, we have another more sophisticated method defined for IDirect3DSurface9, the GetDC() method. We can use the IDirect3DSurface9::GetDC() method to get a GDI compatible device context for the DirectX image surface, which makes it possible to directly blit the surface contents to our application defined DC. Interested readers can explore this alternative.
The sample source code provided with this article implements the
technique of copying the contents of an off-screen plain surface onto a
user created bitmap for capturing the screen contents at regular
intervals, and creates a movie out of the captured image sequences.
However, a point worth noting when using this technique for screen capture is the caution mentioned in the documentation: The GetFrontBufferData() is a slow operation by design,
and should not be considered for use in performance-critical
applications. Thus, the GDI approach is preferable over the DirectX
approach in such cases.
Windows Media 9.0 supports screen captures using the Windows Media Encoder 9 API. It includes a codec named Windows Media Video 9 Screen codec
that has been specially optimized to operate on the content produced
through screen captures. The Windows Media Encoder API provides the
interface IWMEncoder2 which can be used to capture the screen content efficiently.
Working with the Windows Media Encoder API for screen captures is
pretty straightforward. First, we need to start with the creation of an IWMEncoder2 object by using the CoCreateInstance() function. This can be done as:
IWMEncoder2*&g_pEncoder=NULL;&&CoCreateInstance(CLSID_WMEncoder,NULL,CLSCTX_INPROC_SERVER,&&&&&&&&&IID_IWMEncoder2,(void**)&g_pEncoder);&
The Encoder object thus created contains all the operations
for working with the captured screen data. However, in order to perform
its operations properly, the encoder object depends on the settings
defined in what is called a profile. A profile is nothing but a file
containing all the settings that control the encoding operations. We can
also create custom profiles at runtime with various customized options,
such as codec options etc., depending on the nature of the captured
data. To use a profile with our screen capture application, we create a
custom profile based on the Windows Media Video 9 Screen codec. Custom profile objects have been supported with the interface IWMEncProfile2. We can create a custom profile object by using the CoCreateInstance() function as:
IWMEncProfile2*&g_pProfile=NULL;&CoCreateInstance(CLSID_WMEncProfile2,NULL,CLSCTX_INPROC_SERVER,&&&&&&&&&IID_IWMEncProfile2,(void**)&g_pProfile);&
We need to specify the target audience for the encoder in the profile.
Each profile can hold multiple number of audience configurations, which
are objects of the interface IWMEncAudienceObj. Here, we use one audience object for our profile. We create the audience object for our profile by using the method IWMEncProfile::AddAudience(), which would return a pointer to IWMEncAudienceObj which can then be used for configurations such as video codec settings (IWMEncAudienceObj::put_VideoCodec()), video frame size settings (IWMEncAudienceObj::put_VideoHeight() and IWMEncAudienceObj::put_VideoWidth()) etc. For example, we set the video codec to be Windows Media Video 9 Screen codec as:
extern&IWMEncAudienceObj*&pA&#define&VIDEOCODEC&MAKEFOURCC('M','S','S','2')&&&&&&&&long&lCodecIndex=-1;&g_pProfile-&GetCodecIndexFromFourCC(WMENC_VIDEO,VIDEOCODEC,&&&&&&lCodecIndex);&&pAudience-&put_VideoCodec(0,lCodecIndex);&
The fourcc is a kind of unique identifier for each codec in the world. The fourcc for the Windows Media Video 9 Screen codec is MSS2. The IWMEncAudienceObj::put_VideoCodec() accepts the profile index as the input to recognize a particular profile - which can be obtained by using the method IWMEncProfile::GetCodecIndexFromFourCC().
Once we have completed configuring the profile object, we can choose that profile into our encoder by using the method IWMEncSourceGroup :: put_Profile() which is defined on the source group objects of the encoder. A source group is a collection of sources
where each source might be a video stream or audio stream or HTML
stream etc. Each encoder object can work with many source groups from
which it get the input data. Since our screen capture application uses
only a video stream, our encoder object need to have one source group
with a single source, the video source, in it. This single video source
needs to configured to use the Screen Device as the input source, which can be done by using the method IWMEncVideoSource2::SetInput(BSTR) as:
extern&IWMEncVideoSource2*&pSrcV&pSrcVid-&SetInput(CComBSTR(&ScreenCap://ScreenCapture1&);&
The destination output can be configured to save into a video file (wmv movie) by using the method IWMEncFile::put_LocalFileName() which requires an IWMEncFile object. This IWMEncFile object can be obtained by using the method IWMEncoder::get_File() as:
IWMEncFile*&pOutFile=NULL;&g_pEncoder-&get_File(&pOutFile);&pOutFile-&put_LocalFileName(CComBSTR(szOutputFileName);&
Now, once all the necessary configurations have been done on the encoder object, we can use the method IWMEncoder::Start() to start capturing the screen. The methods IWMEncoder::Stop() and IWMEncoder::Pause might be used for stopping and pausing the capture.
While this deals with full screen capture, we can alternately select
the regions of capture by adjusting the properties of input video source
stream. For this, we need to use the IPropertyBag interface of the IWmEnVideoSource2 object as:
#define&WMSCRNCAP_WINDOWLEFT&CComBSTR(&Left&)&#define&WMSCRNCAP_WINDOWTOP&CComBSTR(&Top&)&#define&WMSCRNCAP_WINDOWRIGHT&CComBSTR(&Right&)&#define&WMSCRNCAP_WINDOWBOTTOM&CComBSTR(&Bottom&)&#define&WMSCRNCAP_FLASHRECT&CComBSTR(&FlashRect&)&#define&WMSCRNCAP_ENTIRESCREEN&CComBSTR(&Screen&)&#define&WMSCRNCAP_WINDOWTITLE&CComBSTR(&WindowTitle&)&extern&IWMEncVideoSource2*&pSrcV&int&nLeft,&nRight,&nTop,&nB&pSrcVid-&QueryInterface(IID_IPropertyBag,(void**)&pPropertyBag);&CComVariant&varValue&=&false;&pPropertyBag-&Write(WMSCRNCAP_ENTIRESCREEN,&varValue);&varValue&=&nL&pPropertyBag-&Write(&WMSCRNCAP_WINDOWLEFT,&&varValue&);&varValue&=&nR&pPropertyBag-&Write(&WMSCRNCAP_WINDOWRIGHT,&&varValue&);&varValue&=&nT&pPropertyBag-&Write(&WMSCRNCAP_WINDOWTOP,&&varValue&);&varValue&=&nB&pPropertyBag-&Write(&WMSCRNCAP_WINDOWBOTTOM,&&varValue&);&
The accompanied source code implements this technique for capturing
the screen. One point that might be interesting, apart from the nice
quality of the produced output movie, is that in this, the mouse cursor
is also captured. (By default, GDI and DirectX are unlikely to capture
the mouse cursor).
Note that your system needs to be installed with Windows Media 9.0
SDK components to create applications using the Window Media 9.0 API.
To run your applications, end users must install the Windows Media
Encoder 9 Series. When you distribute applications based on the Windows
Media Encoder SDK, you must also include the Windows Media Encoder
software, either by redistributing Windows Media Encoder in your setup,
or by requiring your users to install Windows Media Encoder themselves.
The Windows Media Encoder 9.0 can be downloaded from:
Conclusion
All the variety of techniques discussed above are aimed at a single
goal - capturing the contents of the screen. However, as can be guessed
easily, the results vary depending upon the particular technique that is
being employed in the program. If all that we want is just a random
snapshot occasionally, the GDI approach is a good choice, given its
simplicity. However, using Windows Media would be a better option if we
want more professional results. One point worth noting is, the quality
of the content captured through these mechanisms might depend on the
settings of the system. For example, disabling hardware acceleration
(Desktop properties | Settings | Advanced | Troubleshoot) might
drastically improve the overall quality and performance of the capture
application.
(P.Gopalakrishna)
本站文章除注明转载外，均为本站原创或编译欢迎任何形式的转载，但请务必注明出处，尊重他人劳动，同学习共成长。转载请注明：文章转载自：罗索实验室 []
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