Unityでお絵かき http://sugi.cc/post/87411110741/

Draw.cs

using UnityEngine;
using System.Collections;

[RequireComponent(typeof(Camera))]
public class Draw : MonoBehaviour
{
	public Color brushColor;
	public float
		brushSize = 10f,
		brushIntencity = 1f;
	RenderTexture canvas;
	Material
		init,
		draw;
	Vector2 prePos;

	// Use this for initialization
	void Start ()
	{
		init = new Material (Shader.Find ("Custom/WhitePaper"));
		draw = new Material (Shader.Find ("Custom/SimpleBrush"));
		canvas = new RenderTexture (Screen.width, Screen.height, 0);
		canvas.Create ();
		Graphics.Blit (null, canvas, init);
		//キャンバスをinitマテリアルで初期化する。
	}
	
	// Update is called once per frame
	void Update ()
	{
		if (Input.GetMouseButtonDown (0))
			DrawToCanvas (Input.mousePosition, true);
		else if (Input.GetMouseButton (0))
			DrawToCanvas (Input.mousePosition);
	}

	void DrawToCanvas (Vector2 pos, bool first = false)
	{
		if (first)
			prePos = pos;
		//マウスダウンの時は、前回の位置=現在の位置にする。そうしないと、前描いた線から繋がっちゃう

		float
		dist = Vector2.Distance (pos, prePos),
		size = brushSize * Mathf.Max (1 - dist / 100f, 0.5f),
		intencity = brushIntencity * Mathf.Clamp01 (dist / 30f);
		//ちょっと、ブラシのサイズと濃さが描くスピードで変わるよう調整。

		//前回のフレームから位置が大きくはなれていたら、sizeの4分の1の距離ずつ、前回の位置から現在の位置まで、ブラシで描く
		for (float f = 0; f < dist; f += size*0.25f) {
			Vector2 p = Vector2.Lerp (pos, prePos, f / dist);
			draw.SetTexture ("_MainTex", canvas);
			draw.SetColor ("_Color", brushColor);
			draw.SetVector ("_Draw", new Vector4 (p.x / Screen.width, p.y / Screen.height, size, intencity));
			//_Draw(x,y,z,w)のVectorプロパティに、位置、ブラシのサイズ、濃さを入れます。
			
			RenderTexture rt = RenderTexture.GetTemporary (canvas.width, canvas.height, 0);
			Graphics.Blit (canvas, rt);
			Graphics.Blit (rt, canvas, draw);
			//ここで、ブラシを適用してる
			RenderTexture.ReleaseTemporary (rt);
		}
		prePos = pos;
		//現在の位置をprePosに入れておく
	}

	void OnRenderImage (RenderTexture s, RenderTexture d)
	{
		Graphics.Blit (canvas, d);
		//カメラ出力にcanvasを表示。
	}
}

SimpleBrush.shader

Shader "Custom/SimpleBrush" {
	Properties {
		_MainTex ("Base (RGB)", 2D) = "white" {}
		_Color ("color", Color) = (0,0,0,1)
		_Draw ("draw prop(pos.xy,size,alpha)", Vector) = (0,0,0,0)
	}
	CGINCLUDE
		#include "UnityCG.cginc"
 
		sampler2D _MainTex;
		half4 _MainTex_TexelSize;
		half4 _Color,_Draw;
			
		//_Draw.xyの位置(0-1座標)に半径_Draw.z(pixel)の丸いブラシを描く
		fixed4 frag(v2f_img i) : COLOR{
			fixed4 canvas = tex2D(_MainTex, i.uv);
			half2 pix = _MainTex_TexelSize.xy;
			half d = distance(i.uv/pix, _Draw.xy/pix)/_Draw.z;
			return lerp(canvas, _Color, pow(saturate(1-d),2)*_Draw.w);
		}
	ENDCG
	
	SubShader {
		ZTest Always Cull Off ZWrite Off
		Fog { Mode off }  
		ColorMask RGB
 
		pass{
			CGPROGRAM
			#pragma fragmentoption ARB_precision_hint_fastest
			#pragma vertex vert_img
			#pragma fragment frag
			ENDCG
		}
	} 
}

WhitePaper.shader

Shader "Custom/WhitePaper" {
	CGINCLUDE
		#include "UnityCG.cginc"
		
		fixed4 frag(v2f_img i) : COLOR{
			return 1;
		}
	ENDCG
	
	SubShader {
		ZTest Always Cull Off ZWrite Off
		Fog { Mode off }  
		ColorMask RGB
 
		pass{
			CGPROGRAM
			#pragma fragmentoption ARB_precision_hint_fastest
			#pragma vertex vert_img
			#pragma fragment frag
			ENDCG
		}
	} 
}