Building the DSWebcamCap Filter Graph

Building the DSWebcamCap Filter Graph

Next we need to add a video renderer to the filter graph so that we can watch the video at the same time we are saving it to disk. To do so, invoke the ICaptureGraphBuilder2 method RenderStream to connect the video renderer to the appropriate pins in the filter graph. RenderStream handles all the filter creation and connection necessary to produce a path from an input stream to a particular renderer. In this case, we want to render a preview (monitor) to the display, so we pass a pointer to PIN_CATEGORY_PREVIEW as the first value, followed by MEDIATYPE_Video, indicating that we re interested in a video stream. Then we pass two NULL values. The first NULL value tells RenderStream that no intermediate filter needs to be connected in the filter graph. If there is a filter you want to place into the path of the renderer (such as the Smart Tee filter, an encoder, or some other transform filter), supply a pointer to that filter as this argument. The second NULL value allows the RenderStream method to create a monitor window for the video stream, so we can see the video capture on screen while it is being written to file. RenderStream will determine the best path between the input filter and the renderer filter.

Frequently, calls to RenderStream will result in the addition of transform filters known as Smart Tees to the filter graph. A Smart Tee takes a single stream and converts it into two identical, synchronized streams. One of these streams is designated as the capture stream and presented on the filter s Capture pin, and the other stream is designated as a preview stream and presented on the filter s Preview pin.

What makes the Smart Tee so smart? The Smart Tee understands priorities, and it will never cause the capture stream to lose frames. If the CPU starts getting overloaded with video processing, the Smart Tee will detect the overloading and drop frames from the preview display, sacrificing the preview in favor of capture quality. Using the Smart Tee, you might see a jerky preview, but the captured file should be perfect.

To complete the construction of the filter graph, we make two more calls to RenderStream. The first call connects the video capture filter to the WM ASF Writer that s been created by the ICaptureGraphBuilder2 object. In this case, PIN_CATEGORY_CAPTURE is passed as the first parameter of RenderStream; this parameter tells RenderStream that it should connect the capture stream rather than the preview stream. The Smart Tee filter will be added if needed. After the first call to RenderStream is completed, another call is made to RenderStream, with parameters to specify that the audio stream should be connected to the WM ASF Writer.

That concludes the construction of the filter graph. Because we ve used the Capture Graph Builder to handle the details of wiring the filters together, it s taken only a few lines of code to create a functional filter graph. This filter graph will capture camcorder video and audio streams to a file while providing a video monitor window on the display. When the application invokes the Filter Graph Manager s Run method, a small preview window will open on the display, showing the real-time output of the webcam, even as the webcam s video stream is written to a disk file.