Upcoming Deadlines Fourth Homework (Tracker Video Analysis) Due Tuesday, February 14 th (Next week) 15 points (10 points if late) Fifth Homework (Analysis of Path of Action) Due Tuesday, February 21 st (In two weeks) 15 points (10 points if late) For full schedule, visit course website: ArtPhysics123.pbworks.com Pick up a clicker but dont turn it on yet. Homework Assignment #4 In this assignment youll use tracking software to analyze the motion of a falling object. The first step is to shoot video reference of an object dropped from a height of roughly four feet. You have to be the person in the video dropping the object so you may need a friend to operate the camera. Homework #4 (cont.) Plan your scene so that the object is clearly visible on each frame; make sure that the background is uncluttered. Stage the scene so that the object is in frame from the moment of release until it hits the ground. Be sure to keep the camera level and steady; the simplest way to do this is to set the camera on a table. Shoot at least 5 takes, even if they are all more or less the same. You can record one long clip or several short clips. Use the highest quality digital camera or camcorder that you can to minimize motion blur effects. Tracker Video Analysis Software For this assignment you will use Tracker, a software tool for video analysis. To download at:Tracker is a free program created by Doug Brown, a physicist at Cabrillo College. Importing Video into Tracker Run the Tracker program on your computer and import your video clip of a falling object. You may need to convert your clip into a format that Tracker can import (either MOV or AVI), if it's not already in that format. Tracking an Object in Tracker In the "Tracks" menu select "New -> Point Mass"; a Track Control window should appear. Press the second button in the Track Control window and select the "Full Trails" option. Go to the first frame in your clip that you want to analyze, hold down the shift key to change the cursor into cross-hairs, then click on the center of your object. The program will mark that position and advance one frame. Tracking an Object (cont.) Continue holding down the shift and marking the position of the object until it hits the ground (which should be after 15 to 20 frames). Due to motion blur the object will "stretch" as it falls and it may be difficult to locate the exact center but just do your best to estimate the center of the blur. Plotting the Position Data Click on the "Plots" button in the upper-right corner and select the 2 plot option. You now have two plots showing the horizontal (x) and vertical (y) positions of the object versus time. Select and drag the bar separating the plots from the data tables and pull the bar down to expand the plots window. Tracking the Ball Drop Horizontal Motion Vertical Motion Homework #4 (cont.) In the "Edit" menu select "Copy Image -> Frame" to copy the whole window to the clipboard. Open your favorite graphics program, such as Paint or PhotoShop, paste the image, then save it to a file. Finally, upload that image to your blog into a post called Tracker video analysis of falling". Visit assignment on course website to see examples. This assignment is due by 8am on Wednesday, February 14 th (next week). 15 points (if late, 10 points) Assignment Checklist * Shoot video of object dropped from a height of about four feet. Do at least 5 takes. * Track the falling object from about the time of release until it hits the ground. You may continue tracking bounces (optional). * Show plots for both horizontal (x) position versus time and vertical (y) position versus time. * Graph of vertical position versus time is roughly a parabolic arc. * Post your screen shot and reference video in a blog entry entitled "Tracker video analysis of falling" Activating your Clicker * Turn on your clicker. * Enter the channel number or letter for joining this class. Hit Enter/Send key. * Clicker should read AP123S1 * Type in your student ID; hit Enter/Send. Clicker is now ready to use. Hit any key to wake the clicker from sleep mode. Survey Question Have you looked at your classmates blogs? A)Not yet B)Looked at a couple C)Looked at many of them Note: You score 1 point of credit for answering survey questions, regardless of your answer. Review Question If the spacings near the apex arent quite right then the ball reverses direction at the top in an unnatural way, as if it hits an invisible ceiling. Which of these spacings is correct? Going up, then down A) B) C) Hitting the Ceiling Spacing A) is correct. Not having enough drawings near the apex looks like hitting an invisible ceiling. Having too many drawings makes the ball seem to hover at the apex. Going up, then down Right Wrong Very Wrong Review Question An example of the Nyquist effect is: A)Weightless freefall B)Pose-to-pose animation C)Wagon wheel illusion D)Perception of motion E)Stretch animation Nyquist Effect The brain tracks the movement of the spokes by looking for the nearest location on each frame. The Nyquist effect limits how fast a wheel can spin in an animation and still be seen as turning. Actual Rotation Illusion #1 #2 Wheel on frames #1 and #2 Wagon wheel illusion is an example of the Nyquist effect. Paths of Action Path of Action The path of action is the trajectory (tracking curve) of a moving object. Complex Path of Action Brick Drop Exercise Path of Action Brick drop exercise illustrates the path of action for falling. Line of Action vs. Path of Action The path of action indicates the trajectory for a sequence of drawings in an animation. Individual drawings have a line of action, which indicates the visual flow of action in that single drawing. Secondary Paths of Action The path of action is usually associated with the primary motion but we can also consider paths of action for secondary motion, such as the motion of a characters hand, arm, foot, etc. Primary (Jump) Secondary (Arm swing) Parabolic Arcs Paths of action of the falling brick and of a bouncing ball are parabolic arcs When gravity is the only force, the path of action is a parabolic arc. Water stream Characters in Motion Thomas Eakins Living beings obey the laws of physics, were just a little more complex than a bouncing ball. Moving & Falling Ball rolling off of a table combines horizontal and vertical motion. Falling starts with key #4, with vertical distances increasing as 1:3:5:7: Horizontal distances equally spaced as with uniform motion. Arc is the combination of uniform horizontal motion and accelerating vertical motion. Rolling off a Table Rolling off a Table, Tracked Video tracking, frame-by-frame, shows that the horizontal motion stays uniform as the vertical motion slows out. Uniform motion (Horizontal) Slowing out (Vertical) Class Demo: Fall and Fire FALL FIRE One ball is released and falls straight down. Other ball is fired horizontally. At all times the balls are at the same height. Hit the ground at the same time. Falling Brick, Video Reference 1 2 Falling Brick The path of action for the falling brick is: Uniform motion in the horizontal (constant velocity). Accelerating motion in the vertical (slowing out as in the ball drop). The center of the brick has a smooth and consistent parabolic path of action Full Parabolic Arcs Horizontal spacings are uniform. The rising and falling motion are the same. Vertical spacings slow in and out following the Odd Rule. Fourth Down at Half Time These drawings are half-way in time between the apex and the lowest points of the arc. Those drawings are a quarter of the way down from the apex and the lowest points of the arc. Sketching a Parabolic Arc (1) Start by picking the lower-left and lower-right points of the arc. Draw a box, with the top line indicating the apex height. Sketching a Parabolic Arc (2) Divide the box into quarters and mark the apex at the center of the upper line. Sketching a Parabolic Arc (3) Divide the upper boxes into quarters and mark the Fourth Down at Half Time points. Sketching a Parabolic Arc (4) Draw a smooth curve connecting the dots to create a parabolic arc. Class Demo: Sketching an Arc Use 4 th Down at Half Time to sketch the path of action for a ball rolling off a ramp. Common Errors in Arcs Wrong Right Most common error in arcs is making them straight. This example is exaggerated to make the error obvious. Nacho Libre A scene in Nacho Libre has one of the worst paths of action in cinematic history. Quiz Question Whats wrong with the spacings in these drawings of a ball thrown in the air? A)Vertical spacings wrong; horizontal spacings are OK. B)Horizontal spacings wrong; vertical spacings are OK C)Both vertical and horizontal spacings are wrong. D)Actually, the spacings are OK for this path of action. Quiz Question C) Both vertical and horizontal spacings are wrong. The vertical spacings are too uniform; the motion should slow in as it rises. The horizontal spacings should be uniform but instead they slow out going left to right. Asymmetric Arcs The arc is skewed; apex is closer to the left than right. This error tends to make the long leg of the arc too straight. Beep Beep (1952)Carefully watch Wile E. Coyotes path of action in this scene. Wile E. Coyotes Path of Action In reality, he would fall in a downward arc similar to his upward path of action. True or False? Beep Beep (1952) Wile E. Coyote travels in a parabolic arc up to the apex, then stops and falls straight downward. Bending the Laws of Physics Wile E. Coyote takes a beating but we dont feel that its animal cruelty because the laws of physics are bent, reminding us that hes in a cartoon universe. Making the action more realistic would change our emotional reaction to the scene. Attack of the Clones Review Staging Staging is one of the principles of animation. Motion is often more interesting when staged to occur with some perspective. Spend half your time planning your scene and the other half animating. Ollie Johnston Brick Drop with Perspective Arcs in Perspective Parabolic arcs look different in perspective but the spacings follow all the same rules as for arcs in profile. Half Arc in Perspective VP HL 1 3 Use the Fourth Down at Half Time rule to help locate points Falling towards foreground Perspective tends to straighten this arc Half Arc in Perspective VP HL 1 3 Use the Fourth Down at Half Time rule to help locate points Falling towards background Perspective tends to increase curvature for this arc Parabolic Arc in Perspective VP HL Ball starts and ends on the ground. Maximum height (ball at midpoint) Parabolic Arc in Perspective (cont.) VP HL 1 3 Add points to the curve by using Fourth Down at Half Time rule. Apex in Perspective Apex needs to be above the half-way point between take-off and landing, which is harder to judge with perspective. Apex of a Jump Lets check if the apex of this jump is correct. Apex of a Jump Correct Apex Hancock (2008) Parabolic arcs seen in perspective are interesting because the timing has texture as motion goes from foreground to background. But the timing in this scene from Hancock is wrong. Can you tell why? Next Lecture Tracking and Motion Graphs Due next Tuesday: Homework #4 (Analyzing Video with Tracker) Please turn off and return the clickers!