Frame Grabbing

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Frame Grabbing

Frame Grabbing


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6.0 FRAME-GRABBING   The first step for analysis after filming is transferring the video image into the system for digitizing. Select Grab from the
Main APAS Menu to enter the Frame-Grabbing Main Menu.

Frame-Grabbing provides a method of copying the image currently displayed on the video monitor and combining it with the image on the graphic display.

At the top center of the screen is the Image Capacity (images). This is an approximate number of images that the memory can store at this time. If this value is not large enough for the video images then "File Maintenance" must be performed before continuing. The following two lines contain information on the file. When a new file is being created this data will be filled in by the various sub-menus of this module.

The middle window contains the commands that are used to grab or review the images. Each will be explained in the following chapter.

6.1 FILENAME   After the VCR is primed and images have been selected, select Filename from the Frame Grabbing Main Menu (Figure 6-1). A small screen will ask whether the file is New or Old. If Old is selected a list of grabbed files available on the current directory is displayed. These may be opened for viewing or editing if necessary. To abort the selection F10 is used to return to the Filename screen.

If New is selected the system asks for the filename that the images will be stored under, but before a file name is give the Compress function should be selected.

6.1.1 F1-Compression   Upon pressing F1-Compression, a new screen, the Compression Menu (Figure 6-2), appears. Compression allows the image to be saved with the use of less memory, but at the cost of lower resolution. If the event being filmed doesn't require a detailed picture, then a higher resolution isn't necessary but if the requirement is for a detailed clear picture then the resolution should be at its peak. If the video tape recording is of a high quality, data compression should be used. Data compression produces smaller data files and allows the user to grab more frames. The amount of compression which should be chosen is a function of the quality of the video recording. The poorer the quality of the video recording the higher the resolution is needed thus the lower compression used. There are five choices listed in this screen.

bulletNone - selects no data compression. This selection is desired for the highest possible resolution.
bulletVery_High_Res - for approximately 2:1 data compression of the video image.
bulletHigh_Res - for approximately 2.75:1 data compression.
bulletMedium_Res - for approximately 3.5:1 data compression.
bulletLow_Res - for approximately 4.25:1 data compression of the video image. This is the lowest resolution but smallest memory storage. Only video recordings of the highest quality should use this compression.
After the type of compression has been chosen, ENTER is selected. The Root File name screen will reappear. Notice the Compress field now displays the compression selection for this file.

Enter a Filename and press ENTER. The Main Frame Grabbing Menu (Figure 6-1) will reappear and the Filename will be displayed.

It is suggested if more than one camera is used that each of the filenames be either the name of the subject or the event, with a addition of the number of the camera or its position (right, left, side or front). This will help organize the analysis later when digitizing and transforming. For example:
bulletJohn-F - Subject is John and these images are from the front camera.
bulletGolf-R - golf is the event from the right camera view.
bulletbasebal2 - the second camera of a baseball event.
6.2 CAPTURE   Once the compression and filename have been chosen, select Capture from the Frame Grabbing Main Menu (Figure 6-1). This will display the Grab Menu (Figure 6-3).

6.2.1 Control Point Selection   Select VCR to display the video image of the control points. These are either the cube, tripod or even points in the area, that will help determine the three-dimensional measurement of the field. Control points are discussed in more detail in Chapter 5. Filming and Appendix C, Sample Measurements and Control Points. Pressing F10 will return the screen to the Grab Menu (Figure 6-3).

Select Control from the Grab Menu. The computer will automatically grab and store the image.

6.2.2 Parameter Selection   Select VCR again to position the video at the point that is to be the start of the digitizing sequence. Pressing F10 will return the screen to the Grab Menu (Figure 6-3). Select Parms. The Parameter Screen (Figure 6-4) lists the parameters the computer will use to store the images.

Enter each of the fields as follows:
bulletFile descriptors - the name, event, position, data of filming
bulletVCR - scrolling up/down chooses whether the VCR will be run Manually, at the VCR itself, or Automatic, through the computer screen.
bullet1st Frame offset - indicates that the file contains frames that are a continuation of a sequence of frames that were grabbed into a previous file.
bullet#Saved frames - number of frames to be saved.
bulletSkip Factor - If the event is slow speed then every frame may not be necessary and possibly every other frame or even every 3rd frame would be sufficient for analysis. This will also effect the film speed. The calculation for #Saved_frames = the number of frames grabbed, from beginning to end, divided by the Skip Factor + 1. Example: With a skip factor of 1, the number of grabbed frames would be divided by 2; a factor of 2, the number frames would be divided by 3.
Pressing F10 will accept the parameters entered and F1 will abort the entire sequence and return to the Grab Menu.

When the Grab Menu reappears notice that the parameters in the upper screen are now displayed with the information entered.

6.2.3 VCR   The VCR selection has already been covered in Section 6.5. No matter what menu, FrameGrabbing Main menu (Figure 6-1), Grab Menu (Figure 6-3) or the Review Control Screen, the VCR is accessed from, it will operate the same.

6.2.4 Data   Selecting the Data function will grab the images to be stored into memory. If the Toggle_Confirm is "turned on" then each frame will have to be confirmed by pressing the ENTER key. If Toggle_Confirm is "off" then the computer will automatically grab and store the selected images.

6.2.5 Toggle_Confirm Selection   Toggle_Confirm allows each frame to be viewed, then either skipped, selected or reselected, before the computer grabs and stores the image. The default setting is "On".

6.3 RESTORE   Restores a frame from a video file. The desired frame may be entered or F1 and F2 may used to sequentially view the frames in forward and reverse directions. F3 allows the user to view data from the video tape, thus, for a comparison of the grabbed data with video taped data. F10 is used to finish the sequence.

6.4&bnsp; DELETE   The delete function deletes any unwanted file from the current directory. This may be used to remove files if the Image Capacity does not have enough space to store the new file.

The File to be deleted must be "closed" first, if it has been active.
6.5 &nbspVCR   It is suggested that the video images be viewed before starting the frame grabbing. This allows the user to check the quality of the image and prime the picture close to the starting point. If the quality is not sufficient then no time is wasted on setting up the sequence file and the activity can be shot over.

Selecting VCR from the main menu at this point will display the VCR Control Screen (Figure 6-5) which allows operation of the VCR from the computer screen using the mouse. When the screen appears the cursor will be flashing in the Play box. Clicking the ENTER button on the mouse will turn on the VCR and start playing the tape. The search button should then be "pressed". The current picture will freeze on the screen and the VCR will be in the Pause mode.

The control bar on the right side of the screen controls the speed of the picture on the VCR. The tape may be forwarded or reversed by placing the cursor above or below the STILL position and clicking the mouse. When the mouse is clicked and the tape is running the cursor will return to the STILL position, therefore, it is not necessary to move the mouse to stop the tape, only to press ENTER.
bulletF1-Adjust - will bring up the Picture Control Screen (Figure 6-6). This screen allows the user to adjust the contrast, brightness, color or saturation for a clearer, sharper picture for digitizing. By using the cursor, each of these functions may be adjusted from 0 to 99. Pressing F10 - Done will return to the VCR Control Screen.
bulletF2-Search - forwards or reverses the tape any number of frames by entering the number in the requested field. A positive number advances forward while a negative number reverses.
bulletF3-Reset - resets the Frame Counter, at the top of the screen, to zero.
bulletF4-Freeze - grabs the digitized frame and freezes it in position. The screen at this point displays a digital picture temporarily saved in the computer memory (RAM).
bulletF10-Done - exits the VCR function and returns to the Frame-Grabbing Main Menu (Figure 6-1).
6.7 TIMECODES   TimeCodes is selected to determine if time codes have been placed on the video tape. The time code function does not work with the Panasonic AG-6750 VCR. Video tapes which have time codes may be more quickly and efficiently controlled by the analysis system.

6.7.1 Enable   Enable is selected to activate the Time_Code function. The current status of the Time_Code function is labeled in the upper right corner of the menu box. When the time codes are enabled, advance/reverse commands are controlled by reading information stored on the videotape. CTL30 indicates the VCR being used is capable of 30 Hz analysis while CTL60 indicates 60 Hz capabilities. No indicates the Time_Code function has been turned off.

6.7.2 Disable   Disable is selected to turn the timecode function off. The current status of the Time_Code function is labeled in the upper right corner of the menu box. When the Time_Code function is "No", the software controls movement of the tape using frame advance/reverse commands. The tape can still be advanced and reversed, however, this method is more time consuming.

6.8 EXIT   The exit command is selected to exit the Grab Module and return to the Main APAS/CES menu. When this option is selected, the Exit Confirmation Screen will appear (Figure 6-8).

6.8.1 Yes   Yes is selected to exit the Grab Module and return to the Main APAS/CES menu.

6.8.2 No   No is selected when you wish to abort the exit command. You will return to the Main Grabbing menu.

6.9 FRAME GRABBING QUICK REFERENCE   Digitizing takes a vast amount of time and computer memory therefore, it is best to grab only the frames that are needed for digitizing. Though it is a good idea to select 3 or 4 extra frames at either end to allow for synchronization and data smoothing effects. Keep also in mind that not every grabbed frame must be digitized.

A quick sequence of frame grabbing:

  1. Insert tape.
  2. Activate VCR and view desired sequence.
  3. Create or recall Filename.
  4. Determine number of frames or fields for sequence using frame shift buttons.
  5. Enter sequence and number of frames information in Params.
  6. Use VCR function to display cube on monitor.
  7. Save or capture control points.
  8. Save or capture Data.
  9. Repeat for additional views.

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