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--- wiki:motionscope_app2 [2022/05/18 11:59]
vizycam [Capturing non-parallel motion (homography to the rescue!)]
+++ wiki:motionscope_app2 [2022/05/18 15:52] (current)
vizycam [MotionScope]
@@ Line 8: / Line 8: @@
 {{wiki:motionscope3.mp4|816x540|loop,autoplay}}
+Check out the above video on [[https://youtu.be/z8bPvyXYJOw|YouTube]].
 ======Quickstart======
@@ Line 164: / Line 166: @@
 MotionScope provides plots of the motion data for easy visualization, but sometimes you want the raw numbers. At the bottom of the ''Analyze'' tab is the dropdown menu ''Export data''.
-{{wiki:image_865.jpg?250}}
+{{wiki:image_865.jpg?210}}
 Selecting one of these types will either download the data file or display it in a separate browser tab.
@@ Line 180: / Line 182: @@
 {{wiki:image_662.jpg}}
-For example, motion directed 45 degrees with respect to the wall won't be fully captured.  About 70% of the motion will be captured, 30% will be lost. (Cosine of 45 degrees is 0.7071 or ~70%.)  Many times we can't position Vizy such that its image plane is parallel to the motion.  For example, imagine capturing the motion of a ball falling from a building.  Here, we typically can only position the camera on the ground, so we necessarily need to point Vizy up at an angle (not parallel to the motion -- doh!)
+For example, motion directed 45 degrees with respect to the wall won't be fully captured.  About 70% of the motion will be captured, 30% will be lost. (Cosine of 45 degrees is 0.7071 or ~70%.)  Many times we can't position Vizy such that its image plane is parallel to the motion.  For example, imagine capturing the motion of a ball falling from a building.  Here, we typically can only position the camera on the ground, so in order to capture the motion, we necessarily need to point Vizy up at an angle (not parallel to the motion -- doh!  But we deal with this problem in the next section.)
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 These controls are fairly self-explanatory -- adjusting the controls changes the perspective of the live camera view.  The ''Shear'' controls are used less often, which is why they aren't normally displayed.
-Back to the "ball falling from a building" example, or in our case, ball falling from a parking garage: Vizy is pointed up at an angle to capture the motion of the ball as it falls.  Vizy is tilted 90 degrees to capture more of the vertical motion.
+Back to the "ball falling from a building" example, or in our case, ball falling from a parking garage: Vizy is pointed up at an angle to capture the motion of the ball as it falls.  Vizy is rotated 90 degrees to capture more of the vertical motion -- check out the picture of Vizy below (Vizy is on its side looking up).  Note also, it's hooked up to a [[wiki:powering_vizy#powering-vizy-through-the-usb-c-connector|portable charger for power]].
-{{wiki:image_908.jpg?300}}
+{{wiki:image_908.jpg?360}}
-{{wiki:image_906.jpg?300}}
+{{wiki:image_906.jpg?350}}
-We can adjust the perspective controls (the tilt in particular) during analysis within the ''Analyze'' tab.  Note, how the building sides become parallel and the ''y velocity'' graph becomes a straight line, which is what you'd expect from an object experiencing constant acceleration.  Note also that we enable the ''Show grid'' overlay so we can line things up more easily (vertical lines should be parallel with respect to each other and the y axis.)
+We can adjust the perspective controls during analysis within the ''Analyze'' tab.  Note, how the sides of the parking garage stairwell become parallel -- it's as if we're looking at the plane of motion head-on instead of up at an angle.  Note also that we enable the ''Show grid'' overlay so we can line things up more easily (vertical lines should be parallel with respect to each other and the y axis.)
-{{wiki:perspective_2.mp4|800x450|loop,autoplay}}
+{{wiki:perspective3.mp4|800x450|loop,autoplay}}
+Before we adjust the perspective the ''x velocity'' graph is curved, but after the perspective is corrected, the ''y velocity'' graph becomes a straight line, which is what you'd expect from an object experiencing constant acceleration.  (Not to confuse things, but the ''x velocity'' graph essentially becomes the ''y velocity'' graph after we rotate (roll) the perspective 90 degrees, hoo boy, this was supposed to be a simple example...)
+{{wiki:image_922.jpg?350}}
+{{wiki:image_921.jpg?350}}
+By changing the camera perspective in this way, we are able to accurately measure the acceleration of the ball at close to 9.8 m/s<sup>2</sup>
+, although given the nature of acceleration (a double time-derivative of position) we need to average over lots of measurements to reduce the noise introduced by differentiation -- below, we adjusted the ''Spacing'' so that we averaged over all measurement points to get the overall average acceleration.
+{{wiki:image_923.jpg?350}}
+For arbitrary yaw and pitch angles, you can use a "calibration grid" of squares in the image such that it's oriented parallel with the plane of motion.  With such a grid in the environment, you can make sure that you capture both the correct perspective and calibration information. When you've corrected the perspective, you're presented with an image of squares that are parallel with the grid overlay. (Yay tomography!)
-In the example above the building offers natural geometry markers for vertical lines and for calibration.  For arbitrary yaw and pitch angles, you can use a "calibration grid" of squares in the image such that it's oriented parallel with the plane of motion.  With such a grid in the environment, you can make sure that you capture both the correct perspective and calibration information. When you've corrected the perspective, you're presented with an image of squares that are parallel with the grid overlay. (Yay homography!)
-By changing the camera perspective in this way, we are able to accurately measure the acceleration of the ball at 9.8 m/s<sup>2</sup>
-, although given the nature of acceleration (a derivative of a derivative) we need to average over lots of points to reduce the measurement noise.
 ======Motion extraction======

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