# Physics 2D in program

- it can be used in?
- games (most)
- micro interactions
- creative code.

- 遊戲/模擬物理與真實物理不同
- 在兼顧效能與模擬細節的考量下，許多真實運算或需考量的部分會被**省略**。
- simulate is the keypoint in programming
- Newton's laws of motion (古典力學)

- physics tips
- gravity (重力)
- speed/velocity (速度)
- accelerate (加速度)
- friction (摩擦力)
- *collision (碰撞)

- push item in ground
- 第一運動定律
- 靜止的物體會保持靜止狀態，除非有外力施加於這物體。
- 運動中的物體不會改變其運動速度，除非有外力施加於這物體。注意到速度是向量，物體運動速度的大小與方向都不會改變。
- 第二運動定律
- f = ma
- 外力(F)與加速度(a)成正比，而 m 為物體的質量
- https://www.youtube.com/watch?v=WHEeGO9HVPc
- speed ＆ friction
- ios scroll effect
- touchStart: record mouse currentY
- time interval 500ms to update mouse currentY
- touchEnd: endY - startY (speed)

- drop or rain
- gravity: 雨水降落的速度 (or 9.8)
- collision: 雨水碰撞到地面
- tricky: just check if rain/drop over than ground position

- collision detection
- penetrate
- if body1 speed is 100, body2 speed is 10
- why collision so hard?
- https://qwe321qwe321qwe321.github.io/2021/01/11/%E5%88%9D%E5%AD%B8%E9%81%8A%E6%88%B2%E7%89%A9%E7%90%86-%E7%B0%A1%E4%BB%8B/
- https://davidhsu666.com/archives/gamecollisiondetection/
- https://gamedevelopment.tutsplus.com/tutorials/collision-detection-using-the-separating-axis-theorem--gamedev-169
- https://www.youtube.com/watch?v=7_nKOET6zwI
- Box2D CCD (Continue Collision Detection)

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- physics engine
- why Box2D
- performance is best
- can do more complex case.
- matter.js (easy to use but prefomrance & limited)

- Box2d
- Vector
- World
- Body (and BodyDef)
- RigidBody 剛體
- Shape
- circle
- polygon (rect)
- chain shape (complex ground)
- Fitxture
- density (密度)
- friction (摩擦力)
- restitution (恢復係數)
- Constraint (& collision)
- rule of physics world
- contact constraint
- 防止剛體被穿透, 以及模擬摩擦和恢復係數
- Joint (關節)
- b2DisatnceJointDef
- O-----O
- b2RevoluteJointDef
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- Vector
- https://juejin.cn/post/6844903859689619469

- World

const gravity = new b2Vec2(0, 10);
const world = new b2World(gravity)

- Body(BodyDef) & Shape
- create BodyDef (tell the box2d i have a obejct)
- init position
- define type: dynamic / static / kinematic
- create a body object (geometry)
- create shape (attach shape to body)
- create fixture

// define a body
const bd = new b2BodyDef();
bd.set_type(b2_dynamicBody);
bd.set_position(new b2Vec2(0, 0));

// create a body via world
const body = world.CreateBody(bd);

// create a shape
const circle = new b2CircleShape();
circle.set_m_radius(1);

// create a fixture
const fixture = body.CreateFixture(circle, 1);
fixture.SetFriction(200)

body.SetLinearVelocity(new b2Vec2(2, 0));
body.SetAwake(1);
body.SetEnabled(1);

- box 2d tutorials
- https://www.youtube.com/watch?v=MsRROjQJxuo&list=PLRqwX-V7Uu6Zy4FyZtCHsZc_K0BrXzxfE
- https://zhuanlan.zhihu.com/p/452417496
- https://gwb.tencent.com/community/detail/107985
- https://blog.csdn.net/piglite/article/details/72730105
- http://www.ladeng6666.com/blog/?paged=5&cat=8

others
- https://www.zhihu.com/question/56669299
- https://www.youtube.com/watch?v=sf4gtlM0mYs