Is xvidcap able of capturing a running video?
Yes. However, you need to disable hardware acceleration on your graphics adapter. Many video players make use of hardware acceleration. In such a case the individual frames of the video are not actually rendered/drawn within X11 memory. The rendering is delegated to the (faster) hardware of the graphics adapter. Xvidcap needs to capture the rendered image. Because there is no way for xvidcap to get it once it’s on the graphics adapter, you absolutely need to disable hardware acceleration. For mplayer e.g.
