Snapshots from a live video source in gstreamer

EDIT: A helpful reader pointed out that using videorate in the pipeline should do the trick, something like:

gst-launch -v v4l2src ! tee name=t ! queue ! xvimagesink t. 
! queue ! videorate ! video/x-raw-yuv, width=640, height=480, framerate=1/1 ! jpegenc ! 
filesink location=test.jpeg

I'm fairly sure I tried this before with no success, but it works fine now. There have been some major bugfixes to videorate since I wrote this. In any case, I'll leave the rest of the article up in the hope that it is still useful.

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As part of the propulse[ART] project, we wanted to add a "preview" feature to the existing audio/video engine. This involves writing a frame of video every second to a jpeg file, while displaying the video at a full 30 fps in a window. I was surprised to discover that this feature was not already implemented in gstreamer for live sources (to the best of my knowledge).
This should probably be implemented by a real gstreamer element, but for our purposes a relatively straightforward hack prototyped in streamshot.cpp was sufficient. The video pipeline consists of a video4linux source, a capsfilter to enforce some properties on the video, an ffmpegcolorspace element, and an xvimagesink. We attach a callback to the source pad of the video4linux source that is called every time our source has a new buffer of data (i.e. a frame). Since timing is not a big concern, the cb_have_data callback knows to only write a file every second by checking a boolean value that is periodically set to true by a separate callback. The cb_have_data function makes a copy of the buffer, swaps its red bytes with its blue bytes (swapping the red_mask and blue_mask in the caps did not work for some reason) and writes the modified buffer to a jpeg, line-by-line. The jpeg writing code was based on Andrew White's Xlib screenshot example. Thanks also to Koya Charles for help debugging this example, as well as the byte swapping.
The final version of the preview feature will probably happen entirely in a separate thread and not involve file-writing, but rather be streamed as these frames will be part of our web-interface.


Update: To make up for this entry's lack of flashiness, enjoy this code_swarm video of the propulse[ART] software's (codename miville) development so far (code_swarm instructions courtesy of amix.dk):

Live animation with pygame and video4linux

 EDIT: I've uploaded the code (and merged it into one file) on gitorious.

Last Friday, the SAT saw the premiere of a new work by Alexandre Quessy and Isabelle Caron called Motifs Urbains. The work was part of the experimentation phase of the propulse[art] project. The demo involved Alexandre generating live, multichannel audio and looping it, while at the same time in a separate room Isabelle did live stop-motion animation and looped it using Alexandre's Processing-based software, ToonLoop. Both rooms featured simultaneous playback of the audio and animation.
Inspired by the demo and my recent tinkering with pygame, I decided to try and write a prototype for a simple stop motion animation looper like ToonLoop. The result is cam_loop.py. Thanks to Nirav Patel's 2008 Google Summer of Code project, pygame (subversion revision 1744 or later) now supports video4linux camera input. The camLoop program shows the camera playback in the left hand region of the window, and the animation in the right hand of the video. The user can then grab frames live, and the accumulated frames will be played back in the right hand side of the window:

If you run the program during the winter months, you'll be subjected to a nauseating holiday theme overlayed on the incoming video. This example was simply to demonstrate how it's possible to easily draw arbitrary objects on live video. From Nirav's blog, it would appear that CoreVideo support is also forthcoming.