world point to texture coordinate + no mesh + arbitrary rotation

.Unity3DPaint.cs.md

Unity3DPaint.cs

// void* src: https://gist.github.com/andrew-raphael-lukasik/3a57c586017a593b6f8ba2a5b22133ff
using UnityEngine;
using UnityEngine.InputSystem;

public class Unity3DPaint : MonoBehaviour
{
	[SerializeField][Min(4)] int _textureWidth = 64;
	[SerializeField][Min(4)] int _textureHeight = 64;
	[SerializeField] Vector2 _regionWorldSize = new Vector2{ x=100 , y=100 };
	Texture2D _texture;
	[SerializeField] Material _dstMaterial = null;

	#if UNITY_EDITOR
	void OnValidate () { if( _texture!=null && ( _texture.width!=_textureWidth || _texture.height!=_textureHeight ) ) { Dispose(); CreateNewTexture(); } }
	void OnDrawGizmos ()
	{
		GetAreaWorldCorners( region:_regionWorldSize , forTransform:transform , TR:out var tr , TL:out var tl , BR:out var br , BL:out var bl );
		Gizmos.color = Color.red;                Gizmos.DrawLine(tl,tr);
		Gizmos.color = new Color(0,1,1,1);        Gizmos.DrawLine(bl,br);
		Gizmos.color = Color.green;                Gizmos.DrawLine(bl,tl);
		Gizmos.color = new Color(1,0,1,1);        Gizmos.DrawLine(br,tr);
	}
	#endif
	
	void OnEnable () => CreateNewTexture();
	void OnDisable () => Dispose();

	void Update ()
	{
		var mouse = Mouse.current;
		if( mouse!=null && mouse.leftButton.isPressed )
		{
			Vector3 origin = transform.position;
			Vector3 normal = transform.forward;
			var camera = Camera.main;
			var plane = new Plane( inNormal:normal , inPoint:origin );
			var ray = camera.ScreenPointToRay(Input.mousePosition);
			if( plane.Raycast(ray,out float hitDist) )
			{
				Vector3 point = ray.origin + ray.direction * hitDist;
				bool isPointInsideRegion = PointRegionProjection( out Vector2 coord , point:point , region:_regionWorldSize , center:origin , horizontal:-transform.right , vertical:transform.up );
				if( isPointInsideRegion )
				{
					Debug.DrawLine( camera.transform.position , point , Color.white , 0.1f );
					_texture.SetPixel( (int)( _texture.width * coord.x ) , (int)( _texture.height * coord.y ) , Color.red );
					_texture.Apply();
				}
				else Debug.DrawLine( camera.transform.position , point , Color.black , 0.1f );
			}
		}
	}

	void CreateNewTexture ()
	{
		_texture = new Texture2D( width:_textureWidth , height:_textureHeight , textureFormat:TextureFormat.ARGB32 , mipCount:3 , linear:true );
		_texture.filterMode = FilterMode.Point;// no smooth pixels
		_texture.SetPixel( 0 , 0 , Color.black );
		_texture.SetPixel( x:_texture.width-1 , y:_texture.height-1 , Color.magenta );
		_texture.Apply();
		if( _dstMaterial!=null ) _dstMaterial.mainTexture = _texture;
	}

	void Dispose () => Dispose( _texture );
	void Dispose ( Object obj )
	{
		#if UNITY_EDITOR
		if( Application.isPlaying ) { UnityEngine.Object.Destroy( obj ); }
		else { UnityEngine.Object.DestroyImmediate( obj ); }
		#else
		UnityEngine.Object.Destroy( thisObject );
		#endif
	}

	/// <summary>Projects world point onto a region of a plane with arbitrary rotation.</summary>
	/// <returns>True when coordinate is inside given region.</returns>
	/// <param name="coordinates">UV-type coordinates.</param>
	/// <param name="point">World point you want to project.</param>
	/// <param name="region">Region absolute world space size.</param>
	/// <param name="center">Region center (world space).</param>
	/// <param name="horizontal">Horizontal axis (world space).</param>
	/// <param name="vertical">Vertical axis (world space).</param>
	public static bool PointRegionProjection ( out Vector2 coordinates , Vector3 point , Vector2 region , Vector3 center , Vector3 horizontal , Vector3 vertical )
	{
		GetAreaWorldCorners( region:region , center:center , horizontal:horizontal , vertical:vertical , TR:out var TR , TL:out var TL , BR:out var BR , BL:out var BL );
		Vector3 pointRelativeToBLCorner = point - BL;
		Vector3 H = Vector3.Project( vector:pointRelativeToBLCorner , onNormal:horizontal );
		Vector3 V = Vector3.Project( vector:pointRelativeToBLCorner , onNormal:vertical );
		Vector2 regionalCoords = new Vector2{
			x = H.magnitude * Mathf.Sign( Vector3.Dot(H,horizontal) ) ,
			y = V.magnitude * Mathf.Sign( Vector3.Dot(V,vertical) )
		};
		coordinates = regionalCoords / region;
		return( coordinates.x>0 && coordinates.y>0 && coordinates.x<1f && coordinates.y<1f );
	}

	/// <param name="region">Region absolute world space size.</param>
	/// <param name="center">Region center (world space).</param>
	/// <param name="horizontal">Horizontal axis (world space).</param>
	/// <param name="vertical">Vertical axis (world space).</param>
	/// <param name="TR">Top-right corner (world space).</param>
	/// <param name="TL">Top-left corner (world space).</param>
	/// <param name="BR">Bottom-right corner (world space).</param>
	/// <param name="BL">Bottom-left corner (world space).</param>
	public static void GetAreaWorldCorners (
		Vector2 region ,
		Vector3 center , Vector3 horizontal , Vector3 vertical ,
		out Vector3 TR , out Vector3 TL , out Vector3 BR , out Vector3 BL
	)
	{
		horizontal = horizontal.normalized;
		vertical = vertical.normalized;
		Vector3 horizontalExtent = horizontal * region.x * 0.5f;
		Vector3 verticalExtent = vertical * region.y * 0.5f;

		TR = center + horizontalExtent + verticalExtent;
		TL = center + -horizontalExtent + verticalExtent;
		BR = center + horizontalExtent + -verticalExtent;
		BL = center + -horizontalExtent + -verticalExtent;
	}
	/// <inheritdoc/>
	/// <param name="forTransform">Transform to source <paramref name="center"/>, <paramref name="horizontal"/> and <paramref name="vertical"/> values from.</param>
	public static void GetAreaWorldCorners (
		Vector2 region ,
		Transform forTransform ,
		out Vector3 TR , out Vector3 TL , out Vector3 BR , out Vector3 BL
	)
	{
		Vector3 center = forTransform.position;
		Vector3 left = -forTransform.right;
		Vector3 up = forTransform.up;
		GetAreaWorldCorners( region:region , center:center , horizontal:left , vertical:up , TR:out TR , TL:out TL , BR:out BR , BL:out BL );
	}

}