Unity Platform 01 Gravity Setting / Wall overlap
===
2020.08.05

###### tags: `platform`

## Object
1. Custom script-driven physics for 2D
2. Horizontal movement & cancelable jump
3. Sloped and angled surfaces collision

## LIST
1. [Default Gravity](#SG)
1. [deltaTime](#delta)
2. [Default Setting](#default)
2. [Collision with grounded](#collision)
1. [Check Overlap](#overlap)
2. [ContactFilter2D](#filter)
<br><br>

## CONTENT
### Defualt Gravity<a id="SG"/>
#### deltaTime<a id="delta"/>
`Time.deltaTime` is frame time before. It use for correct time duration with physics moving.
<br><br>

### Defuault Setting<a id="default"/>

using UnityEngine;
using System.Collections;
using System.Collections.Generic;

public class PhysicsObj : MonoBehaviour {
	public float gravityModifier = 1f;

	protected Rigidbody2D    rb2D;
	protected Vector2        velocity;

	private void Start() {
		rb2D = GetComponent<Rigidbody2D>();		
	}

	private void FixedUpdate() {
		velocity += gravityModifier * Physics2D.gravity * Time.deltaTime;
		Vector2 deltaPosition = velocity * Time.deltaTime;
		Vector2 move = Vector2.up * deltaPosition.y;
		Movement(move);
	}

	protected void Movement(Vector2 move)
	{
		rb2D.position += move;
	}

} 

<br><br>

### Collision with Ground<a id="collision"/>
#### Check Overlap<a id="overlap"/>
Overlapping objects are found by casting.<br>Casting is collision check with line and collision space.<br>

int count = rb2D.Cast(move, contactFilter, hitBuffer, distance + shellRaius);
hitBufferList.Clear();
for (int i = 0; i < count; i++)
{
    hitBufferList.Add(hitBuffer[i]);
}

<br><br>

#### ContactFilter2D<a id="filter"/>
ContactFilter2D for collision filtering using layerMask, depth, etc...<br>If you want to use layermask, write below

	contactFilter.useTriggers = false;
    contactFilter.SetLayerMask(Physics2D.GetLayerCollisionMask(gameObject.layer));
	contactFilter.useLayerMask = true;

Filtering layer option set on `"Project Setting" - "Physics2D".`
<br><br>

#### ground check(with slide line)
Use `List<RayCastHit2D>` and each normal for ground check.<br>RayCastHit list is composed of all collision objs<br>If normal is greater than minValue, it is ground<br>And if we move to y pos direction, calculate velocity with normal value.<br><br>

for (int i = 0; i< hitBufferList.Count; i++)
{
    Vector2 currentNormal = hitBufferList[i].normal;
    if (currentNormal.y > minGroundNormalY)
    {
        grounded = true;
    	if (yMovement)
    	{
    	    groundedNormal = currentNormal;
    		currentNormal.x = 0;
    	}
    } 
    float projection = Vector2.Dot(velocity, currentNormal);
    // descnedanct velocity
    if (projection < 0)
    {
        velocity = velocity - projection * currentNormal;
    }
    // Collision distance low and real move, back it
    float modifiedDistance = hitBufferList[i].distance - shellRaius;
    distance = modifiedDistance < distance ? modifiedDistance : distance;
}

Projection is rate loss speed with current force(velocity) and groundupVector(groundNormal)