Lesson 3

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Lesson 3

Step by step procedures to video capturing with the RealCap APAS Module

Make sure you have a video source connected to your capture card. In this case it is the I-Omega Buz. This video source can be coming from a VCR, a camera, CD or any video output. This video output should be connected by S-VHS cable to the I-Omega Buz video input. You can use also RCA cables for Composite video input. However, the resolution is not as good.  The APAS digitizing system can enhance the markers or the image. But, in all cases, S-VHS is a better video input.

The following video file (2.3MB) which you can download, will show you all the following procedures. It is recommended that you download the file to your computer and then run it with the Microsoft Media Player that can be found in the Windows directory. The name is exactly: Mplayer.exe I made a link here to the Mplayer.exe so you can link to it directly. But this will work only if your Windows directory is on your c:\windows\Mplayer.exe sub-directory.  This way you can advance frame at the time on the video file and see the present text at the same time. Make sure you download the video Lesson3.avi (2.3MB) file first.

Run the RealCap program: 

realcap_icon.gif (1794 bytes)

The first screen you see will look as following:

realcap1.gif (14921 bytes)

The Menu bar on the top shows the:  File ; View; Options; Capture; and Help.

Menu_bar.gif (8391 bytes)

Under the File options you have the following selections:

realcap_file.gif (9811 bytes)


You can save the video in any location on your hard disks for latter use.

You can go to trimming so you can select the precise sequence and synchronize the number of frames and starting and end fields to digitize. However, you can do the trimming later on.

You can trim an old video file that you save before.


realcap_view.gif (9811 bytes)

In the View menu you should select all and the Overlay rather then the Preview. Detailed about the difference between Preview and Overlay modes are explained in previous lesson. Always select the Overlay Mode.


realcap_options.gif (6842 bytes)

Under the options you can select Video Format and Video Source.  These are two important options to make sure that you receive the correct video format whether it is PAL or NTSC and also selecting the proper size and resolution.

Video format

The video format dialog is activated using the menu item Video Format in the Option menu.

videoformat.jpg (15737 bytes)

Using the video format it is possible to setup the image size, format (compression type) and amount of compression which is to be used under the capturing process.

To enable capturing of video images in full size and speed it is absolutely necessary to capture in Motion JPEG format. If this is omitted your system will not be able to do it.

compression.jpg (8829 bytes)

Compression (Quality)

Pressing the Compress... button the amount of Motion JPEG can be adjusted. The amount of compression is inversely proportional with the image quality. Meaning that the higher the compression (less space for each image) the more information about the image you will have to throw away. Also, it is important to keep in mind, that higher quality images,  requires higher throughput of the PC bus and hard drive. Just to see the differences between the different amount of compression observe the video at the Lesson3.avi (2.3MB), which you download before at 33 kb/frame to the following 10 seconds of video capturing (600 fields)  of the gait video data  study at 10 kb/frame (2MB). Notice the degradation in the video quality. This file was highly compressed to save disk space and show the full raw data without trimming. However, normally the video is compressed to 70 kb/frame which produce a very good image.

In order to compare the degradation in image quality due to compression please download the following files.

  1. Gait data taken at 76KB per frame for 3 seconds, resulted in .7MB file size.

  2. Gait data taken at 10KB per frame for 10 seconds, resulted in 2MB file size.

  3. Gait data taken at 50KB per frame for 5 seconds, resulted in 2MB file size.

One can observe that the file taken at 50 KB per frame resulted in a very efficient file size and the image is very clear and will meet the automatic digitizing requirement. The file taken at 10KB per frame is not acceptable since the image degraded too much. The best quality file is obtained at the 76 KB per frame. However, the file size increase significantly to 26 MB for 5 second. But the image is great. With large hard disks of today, there is no reason not to use this level of compression.

Video Format:


realcap_options_Video_format.gif (28541 bytes)

In the video format, always select the Motion JPEG and Image Size of 720 x 480. However, in some conditions when you need a long time data collection, such as 5 minutes, you may use the 352 x 240 window size which will reduce the file size but also the image size.

Compress:  For the Compress Option select around 70 KB/Frame.
This depends on your computer speed. The new Pentium III can go all the way to 200 KB/Frame. However, always remember that the greater the KB/Frame, the larger is the file and the better is the resolution. We found that for digitizing purpose, there is no significant difference between 60 to 200 Kb/Frame rate of data capturing.

                      realcap_options_Video_format_compress.gif (30989 bytes)


Video Source

This part of the documentation is related with the hardware setup of the Iomega Buz. We have not included the setup of other hardware vendors but the basic idea by Video Source configuration is the same but the front end to manipulating the setup is different.

Video source

The video source dialog is activated using the menu item Video Source in the Option menu.

videosource.jpg (25587 bytes)

Using the Video Source dialog it is possible to change various aspects of the video input signal.

Video input is used to setup the connection of the video input i.e. where the video signal from either VCR or camcorder is provided.

The Iomega Buz is capable of detecting the video standard supplied on the input. Clicking the Detect button adjusts the Video standard for the appropriate standard. If the video input is not set correctly the Detection process will not work.

The sliders in the bottom of the dialog (Brightness, contrast, Saturation and Hue) is used to modify the image quality when capturing. The settings of these settings is kept after the Software module ended. These settings are important in setting the amount of contrast and brightness to enhance the capability of the automatic digitizing and detection of markers. However, for manual digitizing you should prefer the brightness to the contrast.

Video Source:                         

realcap_options_video_source.gif (47142 bytes)

The video source is very significant. You must determine if you have NTSC or PAL system and select the proper one.  Also, you must select the proper cable of video input if it is Composite (RGB, RCA cables) or S-Video which has a special cable. Always prefer S-Video when it is possible. You can notice also that you can select the Brightness; Contrast; Saturation and Hue to enhance the image. Always prefer the Contrast to be higher and Brightness lower when using automatic digitizing with reflective passive markers

The following Video illustrates how the various images controls are working: (2.2 MB)


:realcap_capture.gif (8596 bytes)

Capturing Preferences

preference.jpg (20889 bytes)

Capture options

Frame Rate: This parameter is the frame rate of captured video sequence. The frame rate must be given in frames/sec and not fields/sec. !!! However, the APAS software will separate the frame into its two fields components. So, even though you capture at 30 frames per second (25 for PAL), the APAS will allow you to analyze the data at 60 Hz (50hz for PAL).

Capture Time: The amount of sec to be captured. When capturing the process can be halted by hitting the Esc key on the keyboard. You should always select at least one to two seconds before the sequence of interest and one or two seconds after the sequence of interest. This way you will not loose any part of the sequence.  When I was capturing the sequences from the Olympics, I set the capturing time to 10 seconds before and 10 seconds after. In this way I never lost any frames of interest. Just remember, that if you lost some frames you can always capture again from the original video tape. This what is so great about video digitizing. You never loose the original tape.

Enable Time Limit: Enables the automatic time limit capturing. If this is unchecked the capturing process will continue until the Esc key is pressed or the hard drive is full! You must do that or you will fill your hard disk very quick. Why you would ever want it to work without checking? When you capture in real time and do not know the end of the sequence, then you use the escape key to terminate the capturing.

Delay: When starting the capturing process a small delay can be invoked just before the actual capturing process is started. This delay can be used in concert with VCR equipment which requires different time delays because of the difference in mechanical construction. This delay should be used only with a VCR where it takes time for the VCR to start. Make sure you add this time to the totally time. So, if the delay is 1 second and the capture time is 5 second, you will loose the first second. So, select 6 seconds to compensate for the delay. I do not use this option since I always start the capturing ahead of the point of interest. In this case I do not care if the first few frames are missing or the VCR did not get the full speed. Since the sequence going to be trimmed latter on anyway, it is sometimes feel more comfortable and time saving just to capture early and then trim.

Enable Delay: Enables the time delay before capture start. In order for the delay to work you must enable it.

Temporary File Allocation

Filename: This is the filename of the pre allocated video capturing file which is used during the capturing process. The default name is c:\capture.avi.  This file should be allocating at least 10MB of memory and it will be overwritten every time you collect data to this file. It is really a temporary file to allow contiguous disk space to optimize real time transfer of data to the hard disk.

Allocation Size in Mb: This is the amount of hard drive space which is to be used in the pre allocation file. The default is 10Mb which in moderate compression is enough for 4-5 sec of video data. This translate to 300 frames.  If you need more then 300 frames, then increase the default to 20 MB of disk space for the file.  You can increase the size and also if the file that you capture will take more then this amount, the system automatically will allocate more space.

VCR Remote Control

Enable: Enable VCR remote control. Of course, only if you have a compatible VCR with serial port interface. With the APAS system, you can use any video source. So, it is not important anymore to control the VCR from the computer. You just let the VCR or the camera run and you capture all the video and then trim it to the point of interest.

Auto Start: Using this option the RealCap will turn the VCR into play mode just before the capturing is started. This is used as synchronization process between VCR equipment and Video capturing.

Type: Specify the type of VCR you have connected to the computer. If this option is incorrectly setup the synchronization will not work properly.

Serial Port: This option is to specify the serial port to which the VCR is hooked.

Under the Capture option you have the Video Sequence and the Preferences.  The Video Sequence will immediately will start the capturing of the video according to the Preferences options.

The Preferences menu is shown here: 

realcap_capture_preferences.gif (66677 bytes)

This Menu is extremely important since it determine the way the RealCap program is going to capture the video. First you must select the Video type whether it is PAL or NTSC. The PAL mode allow capturing of 50 fields per second where the NTSC standard allow 60 fields per second.  Next is the Capture Time. The Capture Time determine how long you are going to capture the sequence. As a rule of thumb, you should always capture about one second before the view of interest and continue to about 1 second after the view of interest. This will allow you to have more frames than you really need and then you can trim the sequence to exactly the frames to be selected for the analysis.  You should always Enable the time Limit.

The Delay represent the time in Millisecond to delay the capturing. This is important factor to be used only when you use a VCR. Since the VCR is a mechanical device, it take some time for the VCR to reach the proper speed. Normally around 500 Millisecond. Therefore, the delay should be set to around 500 to 800 MS. This means that you should take into consideration the time you want to capture and add to it the mechanical delay.

Lets say that I want to capture 300 fields. Then I will start capturing about 50 fields before and 50 fields after the activity. This will make it 5 seconds at 60 Hz make it 300 fields and to that I must add another second. So, the capture time will be around 6 second. I would set the time to 8 seconds and be sure that I have the sequence captured. Then I will trim it to the proper locations.

Capturing Video

The actual capturing of the video is very straight forward and does not require anything special. Use either the capspeed.jpg (852 bytes) button in the toolbar or the menu item Video Sequence under the Capture menu and you are ready to go.

Attention: If there is no synchronization between the computer (RealCap module) and the VCR/camcorder it is important to capture a video sequence that is large enough to catch the whole sequence including some slack in the beginning and the end. When storing the video sequence to disk the sequence will be trimmed and only the images of interest will be stored.

Important: When capturing video to the hard drive it was previous said that the through put to the hard drive has to be quite high and if this is in some way slowed down the RealCap module will loose some images/frames. If this is the case the RealCap will give a message that frames has been dropped and the sequence can not be used. The first thing to do, is to increase the compression. This will slow the frame data into the hard disk and will allow more time for capturing.

The reasons for the drop of frames can be many, but one thing that could  cause it is the thermal calibration of the hard drive. A thermal calibration of the hard drive is used to adjust the hard drive head for proper positioning. Some hard drives do not do this calibration (special SCSI AV hard drives) but they are pretty expensive. The use of a pre allocated file for the capturing process is done to try to cover for the jumping of the hard drive head when capturing the video. In some cases it is not always enough space to create a contiguous file and the jumping will occur. To avoid this, try to defragment the hard drive using the Windows95/98 tool Disk defragmenter. Also make sure that you have as much space available on the hard drive as possible.

VCR Synchronization

When capturing a certain time sequence within a video it is difficult to use the pause button and the PC keyboard to synchronize the capturing process. Further it is important to ensure that most of the sequence of interest is part of the captured sequence. Therefore a lager amount of video data has to be captured even though in practice we can compensate for this approach.
Current VCR equipment using PC remote control is at the moment quite expensive and the software provided with the equipment is also very bad.
At the moment the RealCap module supports the synchronization of the capturing process using Panasonic VCR models AG-7350 and AG-5700. However, you do not need to purchase these expensive VCRs.  You can use a remote control of any VCR to capture any sequence. The only difference is that you will have to give more attention to the trimming module to synchronize the sequences.

The RealCap has just been changed to be compatible with a modified Panasonic AG-2550C with a special BCD-control interface. Support for other models are under development. In the future it will be possible to synchronization the capturing using the playback facilities of normal camcorder.

The basic idea with the VCR synchronization is to automatically start the VCR before the capturing process from the computer is initiated. To make this possible it is necessary to ensure that the VCR is in proper speed before the images is put to disk. If the speed of the video tape is not correct the first images will be incorrect/jittered. The time for the VCR to be in proper speed is very individual. If for example. the VCR has been extensively used the friction in the mechanical parts will be different from a brand new and the time for attain the proper speed different.

To compensate for this time slack of speed development it is possible to setup a delay from VCR play to capturing process before the capturing start. Refer to the Capturing preferences for further details on how to setup the Delay option.

Improve time for build up video tape speed: When searching for the video sequence on the video that needs to be captured it is wise to position the VCR in pause mode instead of stop mode. The time for the VCR to build up the proper speed is much longer in stop mode than in pause mode. This is due to the fact that the video tape is released from the video head in the stop mode. The time for the VCR to grasp the video tape unto the VCR head is in many cases more than 1 sec.

There is no way to check the time it will take before the VCR has build up a proper speed where the images are OK. Try play with the delay time and minimize it to about 500ms. This time is properly too small for the VCR to build up the speed. Capture a sequence and use the trimming dialog to verify the image quality.

After the Capturing process has been configured with Auto start and the proper delay the capturing process is started in exactly the same way as normal capturing process.

Now you ready to capture video. If you already have the capture card, go ahead and practice capturing. You have enough knowledge to do it with no problems. However, if you do not have the capture card and want to continue with the Trimming process, I already capture the files for you.  I capture a gait sequence from two cameras. The right camera and the left camera. The files names are:
bulletGait1_left_camera_4-9-99.avi (1.5 MB highly compressed) - This is the left camera and the file size is, 1.5MB. You can download it from here to your hard disk.
bulletGait1_right_camera_4-9-99.avi (1.5 MB highly compressed) - This is the right camera and the file is large, 1.5MB. You can download it from here to your hard disk.

Both of these files were compressed to save disk space. Therefore, the resolution of the video and the markers reduced significantly, and therefore, it could have a problem in the Automatic Digitizing mode since the contrast between the background and the foreground is essential.  Also, the video speed was reduce form 60 Hz to 15 Hz in order to save space. For demonstration of the digitizing process this is adequate. Of course, in real life you would keep the 60 Hz and will not compress the files.

These two files are already trimmed and ready for analysis. However, you should download them for latter use with the digitizing module.  In the next lesson you will have a  download file that was not trimmed yet, and I will take you step by step through the trimming process.

Make sure you taking a lunch or dinner or a nap while the computer download the files. Both files will take around one hour to download. However, we will use these files to digitize the data and synchronize the sequences in the next lesson.

Previous ] Up ] Next ]

2D AnalysisBasic APAS instructions for 2D analysis
APAS TutorialsHands-on help with running APAS projects
Lesson 1APAS Tutorials - the RealCap module
Lesson 2Capturing video with RealCap
Lesson 3Step by step procedures to video capturing with RealCap
Lesson 4Trimming the Video Sequences
Lesson 5The Digitizing Module
Lesson 6Hands-on experience with the Digitizing Module
Lesson 7Creating a new digitizing view file
Squat JumpAnalysis of a squat jump with a bar

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