Unity3d Learning Notes 6 - GPU instantiation (1)

Catalog

• 1. summary
• 2. Detailed discussion
• 3. Reference resources

1. summary

In the previous article , A material corresponds to a drawing call instruction . Even in this case , Two 3D objects use the same material , But they use different material parameters , Then it will still cause two drawing instructions . The reason lies in , Graphics work is a state machine , The state has changed , You must perform a drawing call instruction .

GPU Instantiation is used to solve such problems ： For things like grass 、 Objects like trees , They often have a large amount of data , But at the same time, there are only minor differences, such as location 、 Posture 、 Color, etc. . If you render like a regular object , There must be many drawing instructions used , Resource occupation is bound to be large . A reasonable strategy is , We specify an object that needs to be drawn , And a large number of different parameters of the object , Then draw it in a drawing call according to the parameters —— That's what's called GPU Instantiation .

2. Detailed discussion

First , We create an empty GameObject object , And attach the following script ：

using UnityEngine;

// Instantiate parameters 

public struct InstanceParam

{

    public Color color;

    public Matrix4x4 instanceToObjectMatrix;        // Instantiate to the matter matrix 

}

[ExecuteInEditMode]

public class Note6Main : MonoBehaviour

{

    public Mesh mesh;

    public Material material;

    int instanceCount = 200;

    Bounds instanceBounds;

    ComputeBuffer bufferWithArgs = null;

    ComputeBuffer instanceParamBufferData = null;

    // Start is called before the first frame update

    void Start()

    {

        instanceBounds = new Bounds(new Vector3(0, 0, 0), new Vector3(100, 100, 100));

        uint[] args = new uint[5] { 0, 0, 0, 0, 0 };

        bufferWithArgs = new ComputeBuffer(1, args.Length * sizeof(uint), ComputeBufferType.IndirectArguments);

        int subMeshIndex = 0;

        args[0] = mesh.GetIndexCount(subMeshIndex);

        args[1] = (uint)instanceCount;

        args[2] = mesh.GetIndexStart(subMeshIndex);

        args[3] = mesh.GetBaseVertex(subMeshIndex);

        bufferWithArgs.SetData(args);

        InstanceParam[] instanceParam = new InstanceParam[instanceCount];

        for (int i = 0; i < instanceCount; i++)

        {

            Vector3 position = Random.insideUnitSphere * 5;

            Quaternion q =  Quaternion.Euler(Random.Range(0.0f, 90.0f), Random.Range(0.0f, 90.0f), Random.Range(0.0f, 90.0f));

            float s = Random.value;

            Vector3 scale = new Vector3(s, s, s);

            instanceParam[i].instanceToObjectMatrix = Matrix4x4.TRS(position, q, scale);

            instanceParam[i].color = Random.ColorHSV();

        }

        int stride = System.Runtime.InteropServices.Marshal.SizeOf(typeof(InstanceParam));

        instanceParamBufferData = new ComputeBuffer(instanceCount, stride);

        instanceParamBufferData.SetData(instanceParam);

        material.SetBuffer("dataBuffer", instanceParamBufferData);

        material.SetMatrix("ObjectToWorld", Matrix4x4.identity);

    }

    // Update is called once per frame

    void Update()

    {

        if(bufferWithArgs != null)

        {

            Graphics.DrawMeshInstancedIndirect(mesh, 0, material, instanceBounds, bufferWithArgs, 0);

        }

    }

    private void OnDestroy()

    {

        if (bufferWithArgs != null)

        {

            bufferWithArgs.Release();

        }

        if(instanceParamBufferData != null)

        {

            instanceParamBufferData.Release();

        }

    }

}

This script means , Set a mesh and a material , Through randomly obtained instantiation parameters , Render multiple instances of this mesh ：

GPU The key interface of instantiation is Graphics.DrawMeshInstancedIndirect().Graphics A series of interfaces of an object are Unity The bottom of the API, It needs to be called every frame .Graphics.DrawMeshInstanced() You can also draw instances , But at most, you can only draw 1023 An example . So still Graphics.DrawMeshInstancedIndirect() better .

Instantiate parameters InstanceParam and GPU Buffer parameters bufferWithArgs Are stored in one ComputeBuffer In the object .ComputeBuffe Defined a GPU Data buffer object , Ability to map to Unity Shader Medium StructuredBuffer in . Instantiate parameters InstanceParam Stores the location of each instantiated object , Posture 、 Zoom and color information , adopt Material.SetBuffer(), Pass to shader ：

Shader "Custom/SimpleInstanceShader"

{

    Properties

    {

    }

    SubShader

    {

		Tags{"Queue" = "Geometry"}

		Pass

		{

			CGPROGRAM

			#include "UnityCG.cginc"

			#pragma vertex vert

			#pragma fragment frag

			#pragma target 4.5

			sampler2D _MainTex;

			float4x4 ObjectToWorld;

			struct InstanceParam

			{

				float4 color;

				float4x4 instanceToObjectMatrix;

			};

		#if SHADER_TARGET >= 45

			StructuredBuffer<InstanceParam> dataBuffer;

		#endif

			// Vertex shader input 

			struct a2v

			{

				float4  position : POSITION;

				float3  normal: NORMAL;

				float2  texcoord : TEXCOORD0;

 			};

			// Vertex shader output 

			struct v2f

			{

				float4 position: SV_POSITION;

				float2 texcoord: TEXCOORD0;

				float4 color: COLOR;

			};

			v2f vert(a2v v, uint instanceID : SV_InstanceID)

			{

			#if SHADER_TARGET >= 45

				float4x4 instanceToObjectMatrix = dataBuffer[instanceID].instanceToObjectMatrix;

				float4 color = dataBuffer[instanceID].color;

			#else

				float4x4 instanceToObjectMatrix = float4x4(1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1);

				float4 color = float4(1.0f, 1.0f, 1.0f, 1.0f);

			#endif

				float4 localPosition = mul(instanceToObjectMatrix, v.position);

				//float4 localPosition = v.position;

				float4 worldPosition = mul(ObjectToWorld, localPosition);						

				v2f o;

				//o.position = UnityObjectToClipPos(v.position);

				o.position = mul(UNITY_MATRIX_VP, worldPosition);

				o.texcoord = v.texcoord;

				o.color = color;

				return o;

			}

			fixed4 frag(v2f i) : SV_Target

			{

				return i.color;

			}

            ENDCG

        }

    }

	Fallback "Diffuse"

}

This is an improvement from 《Unity3D Learning notes 3——Unity Shader Preliminary use of 》 Simple instantiation shader . The position of instantiated drawing is often not fixed , It means Shader Model matrix obtained in UNITY_MATRIX_M It is generally incorrect . Therefore, the key of instantiation rendering is to recalculate the model matrix , Otherwise, the drawing position is incorrect . The instantiated data is often located close , So you can first pass in a reference position ( matrix ObjectToWorld), Then the instantiated data can only be passed into the relative matrix of this position （instanceToObjectMatrix）.

The final running results are as follows , Lots of different positions are drawn 、 Different posture 、 Capsules of different sizes and colors , And the performance is basically unaffected .

3. Reference resources

1. 《Unity3D Learning notes 3——Unity Shader Preliminary use of 》
2. Graphics.DrawMeshInstanced

Specific implementation code

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