Unity3D研究院之优化Graphics.DrawMeshInstanced

如果项目中使用GPU Instancing的话，很多人都会用底层API Graphics.DrawMeshInstanced在Update中绘制。

我们先来做个极端的测试，如下代码所示。

 void Update()
    {
        for (int i = 0; i < 1024; i++)
        {
            Graphics.DrawMeshInstanced(mesh, 0, material, m_atrix4x4s, 1023);
        }
    }

如下图所示，能看出来在Update每帧都有一个比较高的耗时。

using UnityEngine;
using UnityEngine.Rendering;
public class G1 : MonoBehaviour
{
    //GPU instancing材质
    public Material material;
    //GPU instancing网格
    public Mesh mesh;
    //随便找个位置做随机
    public Transform target;
    //是否使用cammandBuffer渲染
    public bool useCommandBuffer = false;
    //观察摄像机
    public Camera m_Camera;
 
    private Matrix4x4[] m_atrix4x4s = new Matrix4x4[1023];
    void Start()
    {
       
        CommandBufferForDrawMeshInstanced();
    }
 
 
    private void OnGUI()
    {
        if (GUILayout.Button("<size=50>当位置发生变化时候在更新</size>"))
        {
     
            CommandBufferForDrawMeshInstanced();
        }
    }
 
    void Update()
    {
 
        if (!useCommandBuffer)
        {
            GraphicsForDrawMeshInstanced();
        }
 
    }
 
 
    void SetPos()
    {
        for (int i = 0; i < m_atrix4x4s.Length; i++)
        {
            target.position = Random.onUnitSphere * 10f;
            m_atrix4x4s[i] = target.localToWorldMatrix;
        }
 
    }
 
 
    void GraphicsForDrawMeshInstanced()
    {
        if (!useCommandBuffer)
        {
            SetPos();
            Graphics.DrawMeshInstanced(mesh, 0, material, m_atrix4x4s, m_atrix4x4s.Length);
        }
    }
 
    void CommandBufferForDrawMeshInstanced()
    {
        if (useCommandBuffer)
        {
 
            SetPos();
            if (m_buff != null)
            {
                m_Camera.RemoveCommandBuffer(CameraEvent.AfterForwardOpaque, m_buff);
                CommandBufferPool.Release(m_buff);
            }
 
            m_buff = CommandBufferPool.Get("DrawMeshInstanced");
 
            for (int i = 0; i < 1; i++)
            {
                m_buff.DrawMeshInstanced(mesh, 0, material, 0, m_atrix4x4s, m_atrix4x4s.Length);
            }
            m_Camera.AddCommandBuffer(CameraEvent.AfterForwardOpaque, m_buff);
        }
    }
 
    CommandBuffer m_buff = null;
   
}

如果每帧的调用Graphics.DrawMeshInstanced的位置矩阵和MaterialPropertyBlock参数都是一致，我就在想能否进行优化。

其实CommandBuffer也有DrawMeshInstanced方法，这样就可以不用在Update里每帧调用了。当位置矩阵或者MaterialPropertyBlock参数发生变化时在调用DrawMeshInstanced，放入CommandBuffer中渲染。如下图所示，使用CommandBuffer.DrawMeshInstanced只有当元素位置改变时在刷新，这样明显效率高了很多。

结合实际项目，比如之前做过的地面的草、石头、栅栏一旦创建出来以后都不会去改变位置，更适合用commandbuffer来做。当然裁剪还需要自己来做，GPU instancing不支持自动裁剪