在Unity中實時繪制紋理總共需要兩個東西, 一個是RenderTexture, 而如何確定繪制的區(qū)域則需要用到Raycast到MeshCollider從而獲取到指定的textureCoord, 然后使用GL調(diào)用底層函數(shù)把一個Material繪制在RenderTexture上.

需要注意的是:一個有效的可繪制紋理模型,它的Mesh必須有有效的uv數(shù)據(jù),而Unity自帶的各個預(yù)制模型是沒有正常的UV數(shù)據(jù),從而無法在它們身上繪制紋理.在試驗的使用請不要使用類似Unity自帶類似Cube,Sphere等模型.*

第一步: 準(zhǔn)備

定義一個畫筆紋理, 比如一個白色32x32的圓.

畫筆紋理

然后為畫筆紋理創(chuàng)建一個Shader:

Shader "Painting"
{
    Properties
    {
        _MainTex("MainTex (RGB) Trans (A)", 2D) = "white" {}
        _Color("Color", Color) = (1,1,1,1)
    }

    SubShader
    {
        Tags
        {
            "Queue" = "Transparent"
            "IgnoreProjector" = "True"
            "RenderType" = "Transparent"
            "PreviewType" = "Plane"
            "CanUseSpriteAtlas" = "True"
        }

        Cull Off
        Lighting Off
        ZWrite Off
        Fog{ Mode Off }
        Blend One OneMinusSrcAlpha

        Pass
        {
            CGPROGRAM
            #pragma vertex vert  
            #pragma fragment frag  
            #include "UnityCG.cginc"  

            struct v2f
            {
                float4 vertex : SV_POSITION;
                half2 texcoord : TEXCOORD0;
            };

            fixed4 _Color;

            v2f vert(appdata_base IN)
            {
                v2f OUT;
                OUT.vertex = mul(UNITY_MATRIX_MVP, IN.vertex);
                OUT.texcoord = IN.texcoord;
                return OUT;
            }

            sampler2D _MainTex;

            fixed4 frag(v2f IN) : SV_Target
            {
                float4 col = _Color * tex2D(_MainTex, IN.texcoord);
                col.rgb *= col.a;
                return col;
            }
                ENDCG
        }
    }
}

最后創(chuàng)建一個畫筆Material, 將紋理和shader應(yīng)用上. 最終成為這樣

畫筆材質(zhì)

被畫的模型也應(yīng)當(dāng)應(yīng)用一個至少有一個Texture參數(shù)的Shader,比如Unlit/Texture,這樣我們才能夠?qū)⑺瓉淼募y理替換成我們在運行時創(chuàng)建的RenderTexture.

場景搭建完畢以后是這樣:

Game視圖

被繪制的模型的Inspector窗口

第二步: 編寫繪畫代碼

一切盡在代碼中

using UnityEngine;

public class Painter : MonoBehaviour
{
    public const int SIZE = 512;

    public Material mat;
    public float scale;
    public Color col;

    private Renderer rend;
    private RenderTexture rendTex;
    private Texture brushTex;

    private void Start()
    {
        //使用ARGB32的格式創(chuàng)建一個RenderTexture
        rendTex = new RenderTexture(SIZE, SIZE, 24, RenderTextureFormat.ARGB32);
        rendTex.isPowerOfTwo = true;
        rendTex.useMipMap = false;
        rendTex.Create();

        //清空畫布
        Clear();

        //獲取當(dāng)前模型的Renderer把它的紋理替換成剛建立的RenderTexture
        rend = GetComponent<Renderer>();
        rend.material.SetTexture("_MainTex", rendTex);

        //獲取畫筆紋理
        brushTex = mat.mainTexture;
        //設(shè)置我們想要的顏色到畫筆材質(zhì)上.
        mat.SetColor("_Color", col);
    }

    private void Update()
    {
        //當(dāng)鼠標(biāo)按下時發(fā)射射線碰撞模型
        if (Input.GetMouseButton(0))
        {
            var mp = Input.mousePosition;
            var ray = Camera.main.ScreenPointToRay(mp);
            RaycastHit rayHit;
            if (Physics.Raycast(ray, out rayHit))
            {
                //uv坐標(biāo)是0~1,而我們要的是它紋理上的坐標(biāo)是0~SIZE,于是乘以紋理的SIZE
                DrawBrush((int)(rayHit.textureCoord.x * SIZE), (int)(rayHit.textureCoord.y * SIZE), col, scale);
            }
        }
    }

    private void Clear()
    {
        Graphics.SetRenderTarget(rendTex);
        GL.PushMatrix();
        GL.Clear(true, true, Color.white);
        GL.PopMatrix();
    }

    private void DrawBrush(int x, int y, Color col, float scale)
    {
        //計算畫筆居中當(dāng)前位置以后的四個角的坐標(biāo)
        var left = x - brushTex.width * scale / 2f;
        var right = x + brushTex.width * scale / 2f;
        var top = y + brushTex.height * scale / 2f;
        var bot = y - brushTex.height * scale / 2f;

        //將GPU的繪制目標(biāo)轉(zhuǎn)移到當(dāng)前RenderTexture上
        Graphics.SetRenderTarget(rendTex);

        //使用GL圖像庫繪制一個四邊形
        GL.PushMatrix();
        GL.LoadOrtho();
        
        mat.SetPass(0);

        GL.Begin(GL.QUADS);
        GL.TexCoord2(0, 0);
        GL.Vertex3(left / SIZE, bot / SIZE, 0);
        GL.TexCoord2(0, 1);
        GL.Vertex3(left / SIZE, top / SIZE, 0);
        GL.TexCoord2(1, 1);
        GL.Vertex3(right / SIZE, top / SIZE, 0);
        GL.TexCoord2(1, 0);
        GL.Vertex3(right / SIZE, bot / SIZE, 0);

        GL.End();
        GL.PopMatrix();

        //搞定
    }
}

最后將Painter腳本添加到剛才的模型身上, 運行游戲,點擊鼠標(biāo)左鍵就能在模型上畫畫了.是不是很溜!!!