Even though the RealCap software module is simple and easy to use many thing can go wrong in the capture process. Sometimes it is not on the user side and sometimes it is. To give you a better understanding of capturing video this small guide will explanation about the various aspects of the capturing process. Maybe this could solve the problem you might experience.
One of the most important issues in
real-time video capturing is that the PC system is capable of storing the amount of data
onto the harddrive. In full size and full speed (NTSC mode) the amount of data that has to
be streamed onto the harddrive is approximately 27Mb (24bit color * 640 * 480 * 30
frames/sec). This amount of data can only be streamed onto a harddrive using very
expensive equipment. Not only has the harddrive to be very fast but also the data bus on
the mainboard. I know that today's PCI bus and harddrive with Ultra DMA is capable of up
to 33Mb/sec of data through put. But it must be remembered that is this in extreme cases
where the system does not interfere on the bus, which is hardly the case for any Windows
operating system. The real through put on these system (in situation where the system has
been optimized) is about 8-9Mb/sec which is still pretty much. In order to stream the
video data onto the harddrive the amount of data much be reduced. In our system the
reduction is done using Motion JPEG format. There is also the MPEG standard but
unfortunately the equipment for full speed and size video capturing is still to expensive.
When displaying the video data on the
monitor there is two options that can do the trick. First option is called preview which
means that the computer forcibly extracts a video image from the video stream each .5 sec
and renders the image on the screen/monitor. This is off course time consuming because the
processor is part of the whole process. Also when searching on the video it is not always
easy to find the location from where you want to capture using only 2 Images/sec. The
second option is called overlay mode and is a special no processor depended operation
where the capture card is capable of streaming the video data directly to the display card
memory. This is a very powerful option and leaves a lot of idle processor time for other
programs and the current running application - The RealCap.
After the video has been captured it is unlikely that the start position of the captured video will mach any other captured video sequence from another angle/view. In 3D biomechanical analysis it is required to use more than 2 view is order for transform image coordinates to spatial coordinates. Even if taking advantage of VCR synchronization the VCR mechanism will not always respond with the same time delay. This means that the start position will have to be modified/trimmed in order for them to correspond. After the video sequence has been captured the RealCap proceeds directly to a trimming process which enables you to modify the video sequence, selecting a synchronization image and the amount of pre and post images.