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Changes for page Ball Physics

Last modified by Soeren Domroes on 2025/01/30 12:18
From version 4.1
edited by stu104225
on 2020/03/04 16:26
Change comment: There is no comment for this version
To version 13.1
edited by stu122612
on 2023/07/06 14:48
Change comment: Renamed from xwiki:PP18.Ball Physics

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1 +Project Documentation
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1 +The **ball physics model**'s main objective is to provide a point in time to the controller, when the ball is predicted to be hittable, in order to not let the ball fall into the drain.
2 +The code for what is described in this section can be found in the files "physics.cpp" and "physics.h".
2 2  
3 -== Task ==
4 4  
5 -The ball physics model is meant to do things ...
6 6  
6 +{{toc/}}
7 +
7 7  == Ball trajectory ==
8 8  
9 -The trajectory of a ball can be modeled to be parabolic.
10 +----
10 10  
12 +The trajectory of a ball can be modeled to be parabolic. From two point //p// and //p_old// with corresponding timestamps, the velocity //v// is computed.
13 +For a future point in time //t//, with //t=0 //being the timestamp of// p//, we can then approximately predict the corresponding ball position using the following formulas.
14 +
15 +[[image:attach:Bildschirmfoto von 2020-03-07 12-21-22.png]]
16 +
17 +The incline of the playfield causes a vertical acceleration, which is modeled via a gravitational pull with the factor //g = sin(6°) * 9.81 m/s²//.
18 +The value of 6° is the incline of the playfield  measured on the physical device.
19 +
20 +== Prediction ==
21 +
22 +----
23 +
24 +The controller calls the function //full_prediction// passing it two subsequent ball positions with timestamps. From these a ball trajectory is inferred as described above.
25 +Along the trajectory future times are stepwise tested for whether the corresponding predicted position is in the flipper range.
26 +Once a position inside the flipper range is found the search is continued both forwards and backwards in smaller steps in order to find a good enough estimate of the first and last possible time to hit the ball.
27 +If the trajectory does not intersect the flipper range within a certain prediction frame (see below "**Prediction parameters**") a value far enough into the future is returned, that will be overwritten by subsequent predictions.
28 +
29 +With the current configuration the controller uses the middle point in time between the first and last possible time in order to be most positive to hit the ball.
30 +The computed velocity is also passed to the controller and used is some more advanced  tactical decisions like catching balls.
31 +
11 11  == Configuration ==
12 12  
13 -=== Points of interest ===
34 +----
14 14  
15 -The model of the area, where the the flippers can hit a ball is computed from a set of five points. These points are set to real world coordinates in meter
36 +=== Prediction parameters ===
16 16  
38 +(% class="wrapped" %)
39 +|=(((
40 +Parameter
41 +)))|=(((
42 +Description
43 +)))|=(((
44 +Default value
45 +)))
46 +|(((
47 +PREDICTION_FRAME
48 +)))|(((
49 +window size for prediction - How far
50 +
51 +into the future are predictions calculated at most.
52 +)))|(((
53 +70ms
54 +)))
55 +|(((
56 +PREDICTION_STEP_SIZE
57 +)))|(((
58 +size of the steps for the forward search
59 +)))|(((
60 +10ms
61 +)))
62 +|(((
63 +REFINEMENT_FACTOR
64 +)))|(((
65 +The prediction step size for the reverse search is
66 +PREDICTION_STEP_SIZE divided by REFINEMENT_FACTOR.
67 +)))|(((
68 +10
69 +)))
70 +
71 +When setting the PREDICTION_FRAME keep in mind that predictions, that reach multiple frames into the future, are likely to be overwritten, when new frames arrive and new predictions are calculated.
72 +Nevertheless it is reasonable to calculate predictions beyond the next frame in case a ball is not recognized on the next frame.
73 +
74 +With the default values up to 7 predictions are calculated if the ball is not predicted to be in flipper range roughly within the next two frames.
75 +
76 +The REFINEMENT_FACTOR is also the worst case number of predictions calculated during the reverse search part of the prediction process.
77 +
78 +\\
79 +
80 +=== Flipper range ===
81 +
82 +The model of the area, where the the flippers can hit a ball is computed from a set of five points. These points are set to real world coordinates in meters
83 +
17 17  and should not need further adjustment after the camera is correctly calibrated (see [[doc:World Coordinate System]]).
18 18  
86 +The values for the left flipper are set to be symmetrical.
87 +
19 19  [[image:attach:physics_points.png]]
20 20  
90 +(% class="wrapped" %)
21 21  |=(((
22 22  Point#
23 23  )))|=(((
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51 51  
52 52  \\
53 53  
54 -=== Flipper range ===
124 +From these points an area, where the ball is deemed to be hittable, is computed.
55 55  
56 -From the aforementioned points
57 -
58 -\\
59 -
60 -\\
61 -
62 -\\
63 -
64 64  [[image:attach:physics2.png]]
65 65  
66 66  \\
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67 67  
68 68  === Inlane Area ===
69 69  
70 -\\
132 +Since the trajectory prediction does not consider collisions in general, we need a different kind of predictions for the inlanes,
133 +i.e. the path feeding a falling ball from the playfield to the flippers, behind the slingshot.
134 +A ball is considered to be in an inlane, when it is in a rectangular area, with one edge given by the beginning of the flipper
135 +and the other by a point at highest, outermost position of the angled part of the inlane.
71 71  
72 -[[image:attach:physics_inlane.png]]
137 +In this area the vertical acceleration is deactivated in the prediction, since it would cause the prediction to assume the ball would slide into the wall.
138 +Though in reality there is still some acceleration, a simple linear prediction has shown to be practical in this less critical area.
73 73  
74 -\\
75 -
76 -=== Prediction parameters ===
77 -
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140 +[[image:attach:physics_inlane.png]]
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Confluence.Code.ConfluencePageClass[0]
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1 -71401641
1 +71401594
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1 -https://rtsys.informatik.uni-kiel.de/confluence//wiki/spaces/PP18/pages/71401641/Ball Physics
1 +https://rtsys.informatik.uni-kiel.de/confluence//wiki/spaces/PP18/pages/71401594/Ball Physics