Title: DirectX.Capture Class Library Author: Brian Low Email: mportobello@hotmail.com Environment: VS.NET, WinXP Keywords: DirectShow, Video, Capture, AVI, Class Library Level: Intermediate Description: A .NET class library for capturing video and audio to AVI files. Section General C# SubSection Multimedia
This article presents a class library for capturing audio and video to AVI files in .NET. Some of the features of this library:
The Capture class is
the core of this library. Here is a simple example:
// Remember to add a reference to DirectX.Capture.dll
using DirectX.Capture
// Capture using the first video
// and audio devices available
Capture capture = new Capture( Filters.VideoInputDevices[0],
Filters.AudioInputDevices[0] );
// Start capturing
capture.Start();
// Stop capturing
capture.Stop();
Remember to add a reference in your project to DirectX.Capture.dll.
This dll requires DShowNET.dll, so make sure they are both in
the same
directory. Once you add the reference, VisualStudio.NET should take care of the copying for you.
This example will capture video and audio using the
first video and audio devices installed on the system. To capture video only,
pass a null as the second
parameter to the constructor.
The class is initialized to a valid temporary file in
the Windows temp folder. To capture to a different file, set the Capture.Filename property
before you begin
capturing.
This next example shows how to change video and audio
settings. Properties such as Capture.FrameRate
and Capture.AudioSampleSize allow you to
programmatically adjust the capture. Use Capture.VideoCaps and Capture.AudioCaps
to determine valid values for
these properties.
Capture capture = new Capture( Filters.VideoInputDevices[0],
Filters.AudioInputDevices[1] );
capture.VideoCompressor = Filters.VideoCompressors[0];
capture.AudioCompressor = Filters.AudioCompressors[0];
capture.FrameRate = 29.997; // NTSC
capture.FrameSize = new Size( 640, 480 ); // 640x480
capture.AudioSamplingRate = 44100; // 44.1 kHz
capture.AudioSampleSize = 16; // 16-bit
capture.AudioChannels = 1; // Mono
capture.Filename = "C:\MyVideo.avi";
capture.Start();
...
capture.Stop();
The example above also shows the use of video and audio
compressors. In most cases you will want to use compressors. Uncompressed video
can easily consume over a 1GB of disk space per minute. Whenever possible, set
the Capture.VideoCompressor and Capture.AudioCompressor
properties as early as
possible. Changing them requires the internal filter graph to be rebuilt which
often causes most of the other properties to be reset to default values.
This project uses 100% DirectShow to capture video. Once a capture is started, DirectShow spawns another thread and handles retrieving/moving all the video and audio data itself. That means you should be able to capture at the same speed and quality as an application written in C.
DirectShow is implemented as a set of COM components and
we use .NET Interop to access them. The pioneering work on this was done
by
NETMaster with the
DShowNET project. This Capture library uses DShowNET
for the interop layer with only a few extensions. This is the DShowNET.dll
mentioned earlier.
Sitting on top of all of this is the Capture class library. The center of any DirectShow app is the filter graph and the filter graph manager. For a good overview see The Filter Graph and Its Components from the MSDN.
The library tries at all times to do the least amount of work possible. The problem is: DirectShow is very flexible, but has few firm standards for driver developers and I have limited hardware to test with. As a result, the class tries to avoid doing any work that may not be necessary, hopefully avoiding potential incompatibilities in the process.
One example is video preview. You can start and stop preview with:
// Start preview
capture.PreviewWindow = myPanelControl;
// Stop preview
capture.PreviewWindow = null;
Hopefully this is simple to use. Internally, DirectShow does a lot of
work: add required upstream filters for WDM devices, search for preview pins,
use the Overlay Manager for video ports (hardware overlays), insert SmartTee
filters when a seperate preview pin is not available and more. Instead of
rendering the preview stream as soon as the class is created, the class waits
until the PreviewWindow propert is set.
For developers who don't need preview, none of this work will ever be done. That means your application is more likely to work on a wider range of hardware. For developers that do need preview, this makes it easier to locate the cause of the problem and fix it or handle it gracefully.
Capture class are retrieved directly from
the underlying DirectShow COM components. If you need to refer to the property repeatedly
in a block of code, take a copy of the value and use your copy.
// AudioSampleSize is retrieved from DirectShow each iteration
for ( int c = 0; c < 32; c++ )
{
if ( c == capture.AudioSampleSize )
MessageBox.Show( "Found!" );
}
// A faster solution
int x = capture.AudioSampleSize;
for ( int c = 0; c < 32; c++ )
{
if ( c == x )
MessageBox.Show( "Found!" );
}
Why doesn't the class simply cache the value internally? We don't know when the filter (device driver) will change this value, so we have to retrieve the value everytime. This means you will always get the real value of the property.
The DirectShow interop layer was developed by NETMaster in the DShowNET project. The MDSN-style documentation was generated from the source code using nDoc.
I have tested this with an Asus v7700 (NVidia GeForce2, reference drivers) and my onboard sound card. I can't guarantee any other hardware will work. However, I expect most video capture cards and sound cards will work. You may have trouble with TV Tuner cards and DV devices (Firewire camcorders) though they should be solvable. If you do have problems, post them in the forum and I'll be happy to help.
Try the AMCap sample from the DirectX SDK (DX9\Samples\C++\DirectShow\Bin\AMCap.exe) or Virtual VCR, a free DirectShow capture application.
This class library uses COM Interop to access the full capabilities of DirectShow, so if there is another application that can successfully use a hardware device then it should be possible to modify this class library to use the device. Please post your experiences, good or bad, in the forum below.
Post comments, questions and stories to the forum below so all can benefit. I will try to keep an eye on the forums. Feel free to send me a copy of the post or reminder if I haven't checked the forums for a while.
Feel free to submit your patches and improvements to me. If they fit within the scope of the project, I'll integrate them, give you credit for your work and post the updated project on this page for everyone.