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Stereo Parallel camera setup Stereo Converging camera setup Camera calibration Carry adjustments Putting and chipping IBS image processing Technical help

Stereo Camera Calibration

 

CX Stereo ball tracking

Calibration

Note that the CX Surround's camera are pre-calibrated

so this calibration procedure is only required for the CX Stereo which is using individual mounted cameras

Mounting the cameras sideways means the direction of play is then in the vertical direction

i.e. from down to up in the camera images instead of from right to left.

You will thus have to move the "Ball Launch position" to the bottom center of the camera image.

The above shows the converging camera calibration table that is automatically loaded when "Converging cameras" option is selected

Each row in the table consists of 5 values that are automatically calculated when the system is in calibration mode and new images are grabbed.

Note that if both cameras are perfectly aimed at the same center line then disparity at the converging point at floor level will be zero.

This will probably prove to be difficult to achieve but at least the disparity number at floor level should be a low number.

The above table shows a disparity (Disp) of 24 at floor level.

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A graph plot of disparity to golf ball height shows a non-linear exponential curve.


Calibration

Note: please use Control Panel version 8.8.0.5 or greater for this procedure.

In order the raise the calibration black card and white strip, either a tripod can be used

or you attach the card to 2 magnets that slide directly up the metal tape measure

Calibration procedure

Should re-calibration be required then the following calibration procedure should be made:

Calibration setup

In order to calibrate, we need to raise a 20cm long white strip mounted on a black card from the floor up to around 170cm in 10cm steps on the center line.

You can use any method you like to raise the strip off the ground in the 10cm steps.

Attaching the strip to a metric tape measure that is suspended from the center line of the enclosure and extended to the floor is one method.

The above image shows a white strip card directly attached to a tape measure that can simply be moved up or down to the desired height.

You could also use an extend-able camera tripod.

Note that the calibration table consists of 17 row entries at set 10cm distances from 0 cm to 170 cm.

i.e. 0, 10, 20 ... to 170 cm

17 Calibration steps

Calibration mode new features

- V 9.4.3.6 Beta

To help assist the calibration process, a couple of new features:

1. Pressing the SPACE bar on the keyboard switches the calibration mode on and off so that the images captured can be inspected before entering the values

2. The active row is now bright white

3. A beep and message is shown after the "Ins" key has been pressed showing that the values have been entered into the calibration table

This version of the CP includes the calibration table for the 260cm or 8ft 6" mounting height of the Stereo IR panel cameras.


  • To start the calibration entry process, click the "Calibration Mode" option in one of the two
  • camera windows (stereo left or stereo right) and press the * key.
  • This allows you to make entries into the calibration table.
  • Using the up down keyboard arrow keys (ensure that no other parameter setting is active)
  • select the first table entry
  • Tthis will be height 0 i.e. floor level
  • Step 1: Place the card at the floor level directly under the cameras center point
  • i.e. between the two cameras.
  • Ensure that the white strip on the card is pointing directly forward. i.e. from bottom to top of the screen
  • Step 2: The user triggers both cameras (by pressing the Enter key) to capture images from both cameras
  • so that a basic height, a disparity value, a height and a scaling factor for the known length of the white strip
  • is calculated. An audible beep will then sound.
  • Note that the white tape strip represents a ball trace with 2 3D points at either end of the strip
  • so ensure that system detects both ends of the tape strip in both cameras.
  • Check both camera frames with every step to verify that both ends of the white strip are detected
  • as it is quite possible that the tape measure or parts of the tripod may be being picked up inadvertently.
  • Use the blue border lines to confine the valid FOV so that only the white strip is detected.
  • Step 3. The user preses the "Ins" key to store the values.
  • --
  • Step 4. Now move the card up to the first or next 10 cm marking on the tape - using either the camera tripod or lab stand -
  • and select the next row height step (arrow up key) in the calibration table.
  • i.e. if you started at 0 cm height, then next entry is the 10 cm high entry
  • Step 5: The user repeats these steps 12 to 17 times for a total height of 120 cm to 170 cm or as high as possible before
  • the card's tape strip goes out of the FOV of the cameras..
  • When raising the black card with white strip on it, make sure to keep the card and strip level.
  • With every step, ensure that both ends of the white strip on the card are detected in both cameras
  • (i.e. the green cross hairs are exactly at both ends of the white strip in cameras 1 and 2)
  • Also ensure that the tape measure or any part of the Tripod or Lab Stand is not being picked up.
  • Either use the blue valid FOV borders block these out of the valid FOV of the cameras or cover the stands with a black material.

The above image shows the white strip connected to the tape measure with the slider.

Using the calibration slider on the tape measure, a complete calibration can be done in 10 to 15 minutes.

When testing, set both RAW Path and RAW Launch angle to ON

Note that the above image is from the parallel setup page where the strip orientation is from right to left.

The converging camera strip oprientation is vertical i.e from down to up.

You can view the complete calibration table by selecting the "Show calibration table" option.

 

Although the real distance (d) the ball travels (and thus the ball speed) within the camera exposure time is always measured using RAW LA and Path

(i.e. as measured from the both ends of the ball trace)

it will be more accurate to measure LA and path using fixed launch positions for chipping as by the time the ball is in the FOV of the cameras with slow shots

the ball may well be descending and thus launch angles will be less or even negative if measured from trace ends instead of a known launch position height.

Note that when using a fixed launch position (i.e. when both the RAW LA and RAW path options are switched off)

then the fixed "Ball launch position" has to be set in both the left and right cameras separately

as the left and right cameras will see the launch position in a different places.

Switch RAW Path and RAW LA on/off in camera 2 of the Control Panel

 


Pros and Cons of Stereoscopic vs H cam V cam setups

  • Stereoscopic Pros
  • 1. Increased ball speed detection accuracy
  • Unless calibrated correctly , the V an H cam setup is prone to ball speed measurement errors and inconsistencies when
  • balls are hit left or right and at varying launch angles
  • With the stereoscopic setup, measured ball speed is consistent and accurate no matter in what direction the ball is
  • going in (left or right) and at what launch angle.
  • Note that V and H cam calibration is quite a long and involved process compared to the simple 12 or so step stereo calibration process.
  • 2. Both cameras and lighting are mounted out of the way on the ceiling
  • The V and H cam setups have additional lighting and a camera mounted at floor level which can be inconvenient
  • 3. No light ghosting issues
  • With the stereoscopic setup the lighting is not in view of the cameras so there are no light ghosting issues.
  • Whereas lighting in the V an H cam setup is both on the floor and ceiling - as are the cameras - and thus ghosting light can get into the camera lenses
  • and cause big problems with the image processing
  • Stereoscopic Cons
  • 1. Camera images of the ball make no sense to the viewer
  • Unlike the V and H cam setups where the user can easily see the launch angle and path of the ball,
  • the ball images in a stereoscopic system show no direct relationship to LA and path to the average user.

 

Stereoscopic vision explained

The above diagram explains the basic stereoscopic principles.

Note: the above diagram shows the stereo cameras for the VisTrak IRV Stereo where the direction of play is from right to left and not from bottom to top as with the regular VisTrak Stereo system.

If the two 2 stereo cameras are aimed precisely at the center line and a ball is placed at floor level, the images of the will appear on top of each other.

The disparity of the ball is then zero (or near zero) and this is known as the "Converging point"

When the ball is elevated, the images of the ball in the camera frames will start to separate. The distance the ball images are apart is the "Disparity".

Using a "disparity to ball height" table and with weighted calculations, the exact height of the ball can be determined.

Knowing where the ball was before ball strike (frame 1) and the trigger delay time on the stereo cameras, the ball LA, speed and path can be determined.

Ball path is simply derived from the divergence the center of the disparity distance is from the center line.

i.e. a perfectly straight shot would show that the 2 ball images are the exact same distance from the center line.

Converging camera test shots

The above screen shots are from customer Kevin showing the left and right camera images os a full shot using the new Converging stereo setup.


Editing the stereo calibration table

  • To edit the calibration table, click the "Calibration Mode" option in one of the two stereo camera windows
  • Using the up/down keyboard arrow keys, select the row you'd like to edit.
  • The table values will then be shown in the Height, Scale, LineLen and Disparity fields.
  • To To change any of these values, click the desired value radio button and
  • - while holding the Ctrl Key down - use the up down keyboard arrow keys to adjust

The values again are:

  • 1. Disparity - the difference between the cameras x position of the object at a particular height off ground
  • 2. Line Len PX - the length of the line or strip measured in pixels
  • 3. The Line Len measured in cm - this should always be 20cm i.e. the length of the white strip
  • 4. Scale - the scaling factor used to convert the LineLen pixel value to 20 cm
  • 5. Height - The real height off ground that corresponds to the disparity value.
  • Why edit the table?

    A reason would be to verify that the table entries are correct.

    e.g. in the above image the disparity of a white card placed at a height of 100 cm is 517 for P1

    (P1 is the upper end point of the ball trace. P2 is the lower end point of the ball trace.

    These two points are shown by the two green cross hairs on either end of the ball trace)

    In this example, the height for a disparity of 517 is calculated to be 107.78.

    If, for example, the white card that represents the ball trace was placed at a height of 100 cm off the ground and not 107 cm,

    then you would adjust the current disparity of 468 for a height of 100cm to 517.

    The scaling factor - used to convert trace lengths measured in pixels to real distances in cm - can also be adjusted.

    Using this method, you can verify and adjust values for the complete 19 row table.

    Note that you have to place your white card at the desired height (0 to 190 cm) and hit the

    Soft Trigger in both left and right cameras to grab new frames for every height you are checking.

    Using stereo cameras with VisTrak

    If using stereo cameras with the VisTrak, the only function the cameras will have will be to determine the launch angle of the ball

    which is based on the height of the P1 point of the trace above ground at its user defined launch position.

    i.e. the VisTrak camera measures ball speed and direction and not the stereo cameras.

    In this respect, the scaling factor entries in the stereo calibration table are not being used.


 

 

 

 

 

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