Unity显示一个对象的像素个数

下图显示了该面片占用的像素个数为147456。
因为面片的大小为1，坐标为(0,0.5f,0)。相机为正交视角，OrthoSize为1. 面片完全显示且高度为 (768/2)^2 = 384^2 = 147456。

另外，可以推测，pixel shader渲染会在所有对象的vertex shader计算结束后开始。
即使面片被遮挡，但如果没做处理依然存在渲染的开销。【放置两个面片发现占用的像素个数数量加倍】。disable其中一个对象后，像素个数变回原值。

过程

使用了 https://github.com/cinight/MinimalCompute 项目中的 05_1_UAVInShader 例子。

修改代码如下：

using UnityEngine;
using System.Collections;
using UnityEngine.Rendering;

public class UAVInShader : MonoBehaviour
{
    public Material mat;
    public TextMesh text;

    private int targetID = 6; //match with shader "register(u6)"
    private ComputeBuffer fieldbuf;
    private float[] fdata = new float[3];

    private int ctargetID = 1; 
    private ComputeBuffer countBuf;
    private int[] cData = new int[1];

    public int count;


    void OnEnable()
    {
        Setup();
    }

    void OnDisable()
    {

//重置数值 unity推出Play Mode 不会重置 ComputeBuffer 里面的值
         fieldbuf.SetData(new float[] { 1f, 1f, 1f });
        Graphics.ClearRandomWriteTargets(); //this prevent crash
        if (fieldbuf != null)
        {
            fieldbuf.Release();
            fieldbuf.Dispose();
            fieldbuf = null;
        }

        if(countBuf != null)
        {
            countBuf.Release();
            countBuf.Dispose();
            countBuf = null;
        }
    }

    void Setup()
    {
        if (fieldbuf == null)
        {
            fieldbuf = new ComputeBuffer(4, sizeof(float), ComputeBufferType.Default);
        }

        if(countBuf== null)
        {
            countBuf = new ComputeBuffer(1,sizeof(int), ComputeBufferType.Default);
        }

    }


    void OnRenderObject()
    {
        Graphics.ClearRandomWriteTargets();
        Setup();
        mat.SetPass(0);

        //名字和shader中的RWBuffer无关
        mat.SetBuffer("Field1", fieldbuf);
        mat.SetBuffer("Count2", countBuf);

        Graphics.SetRandomWriteTarget(targetID, fieldbuf);
        Graphics.SetRandomWriteTarget(ctargetID, countBuf);

        fieldbuf.GetData(fdata);
        text.text = "From shader, the RGB value of rainbow color \n";
        for (int i = 0; i < fdata.Length; i++)
        {
            text.text += i + ":  " + fdata[i] + "\n";
        }

        countBuf.GetData(cData);
        text.text += cData[0] + "\n";
    }
}

Shader "UAVTest/VertFrag"
{
	Properties
	{
		_MainTex("_MainTex (RGBA)", 2D) = "white" {}
		_Speed("_Speed",Range(0,0.5)) = 0.5
	}
	SubShader
	{
		Tags{ "RenderType" = "Opaque" }

		Pass
		{
			CGPROGRAM
			#pragma vertex vert
			#pragma fragment frag
			#pragma target 5.0

			#include "UnityCG.cginc"

			struct appdata
			{
				float4 vertex : POSITION;
				float2 uv : TEXCOORD0;
			};

			struct v2f
			{
				float2 uv : TEXCOORD0;
				float4 vertex : SV_POSITION;
			};

			sampler2D _MainTex;
			float4 _MainTex_ST;

			float _Speed;

			#ifdef UNITY_COMPILER_HLSL
				RWStructuredBuffer<float> Field : register(u6); //match with C# script "targetID"
				RWStructuredBuffer<int> Count : register(u1); //match with C# script "targetID"
			#endif

			v2f vert(appdata v)
			{
				Count[0] = 0;
				v2f o;
				o.vertex = UnityObjectToClipPos(v.vertex);
				o.uv = TRANSFORM_TEX(v.uv, _MainTex);

				return o;
			}

			fixed4 frag(v2f i) : SV_Target
			{
				//Rainbow color
				float3 c;
				c.r = frac(sin(_Time.x*_Speed));
				c.g = frac(sin(_Time.z*_Speed));
				c.b = frac(sin(_Time.w*_Speed));

				#ifdef UNITY_COMPILER_HLSL
					Field[0] = c.r;
					Field[1] = c.g;
					Field[2] = c.b;
				#endif

				InterlockedAdd(Count[0],1);

				float4 col = tex2D(_MainTex, i.uv);
				col.rgb *= c;
				return col;
			}
			ENDCG
		}
	}
}

参考

https://github.com/cinight/MinimalCompute
https://forum.unity.com/threads/setting-buffer-data-from-inside-fragment-shader-to-be-read-from-c-script-urp.1229175/
https://zhuanlan.zhihu.com/p/368307575