Search code examples
directshowdirectshow.net

How can I capture a webcam and append to a file?

My application needs to record video interviews with the ability to pause and resume, and have these multiple segments captured to the file.

I'm using directshow.net to capture camera stream to a preview window AND an avi file, and it works, except that whenever I start recording a new segment, I overwrite the avi file instead of appending. The relevant code is:

captureGraphBuilder.SetOutputFileName( ref mediaSubType, Filename, out muxFilter, out fileWriterFilter )

How can I create a capture graph so that the capture is appended to a file instead of overwriting it?

Solution

  • Most media files/formats, and AVI specifically, do not suppose or allow appending. When you record, you populate the media file AND then you finalize it on completion. You typically don't have the option to "unfinalize" and resume recording.

    The symptom of overwriting you are seeing is a side effect of writing filter implementation. There is no append vs overwrite mode you can easily switch to.

    Your options basically are the following (in the order of less-to-more development):

    1. Record new media file each time, then run an external tool (like FFmpeg) which is capable to concatenate media and produce new continuous file out of segments.

    2. Implement a DirectShow filter inserted into the pipeline (esp. in two instances, for video and for audio) which is capable to implement pause/resume behavior. Once you pause the filter would discard new media data, and once you resume it starts again passing them respectively modifying time stamps to mimic continuous stream. The capture graph will be in running state through all segments and pauses.

    3. Implement a custom multiplexer and/or writer filter which is capable to read existing file and append new media so that the file itself is once again finalized on completion with old and new segments, continuous.

    Item #3 above is technically possible to implement, but I don't think such implementation at all exists: workarounds are always easier to do. #2 is a sort of supposed way to address the mentioned task, but since you are doing C# development with DirectShow.NET, I anticipate that it is going to be a bit difficult to address the challenge from this angle. #1 is relatively easy to do and the cost involved is an external tool to use.