简单来说就是这张图：

图源：https://forum.unity.com/threads/what-does-the-function-computescreenpos-in-unitycg-cginc-do.294470/

下面的程序可以用来验证：

// Upgrade NOTE: replaced 'mul(UNITY_MATRIX_MVP,*)' with 'UnityObjectToClipPos(*)'

// This shader fills the mesh shape with a color predefined in the code.
Shader "Example/URPUnlitShaderBasic"
{
    Properties
    {}

    SubShader
    {
        Tags { "RenderType" = "Opaque" "RenderPipeline" = "UniversalPipeline" }
        Pass
        {
            HLSLPROGRAM
            #pragma vertex vert
            #pragma fragment frag
            #include "Packages/com.unity.render-pipelines.universal/ShaderLibrary/Core.hlsl"
            #include "Packages/com.unity.render-pipelines.universal/ShaderLibrary/DeclareDepthTexture.hlsl"
            struct Attributes
            {
                // Object Space 模型空间位置
                float4 positionOS   : POSITION;
            };

            struct Varyings
            {
                // Windows Space 窗口空间位置
                float4 positionWS  : SV_POSITION;
                float4 positionHCS  : TEXCOORD1;
                float4 positionOS   : TEXCOORD2;
            };

            Varyings vert(Attributes IN)
            {
                Varyings OUT;
                OUT.positionOS = IN.positionOS;
                // 进行 MVP 变换
                OUT.positionWS = mul(UNITY_MATRIX_MVP,IN.positionOS);
                OUT.positionHCS = OUT.positionWS;
                return OUT;
            }

            // The fragment shader definition.
            half4 frag(Varyings IN) : SV_Target
            {
                // 去掉注释观察模型空间上色情况
                //return half4(IN.positionOS.xyz * 0.5 + 0.5,1);

                // 去掉注释观察 HCS 空间上色情况
                //return half4(IN.positionHCS.x/IN.positionHCS.w * 0.5 + 0.5,0,0,1); // 将物体在 x 轴移动，现象如图 1
                //return half4(IN.positionHCS.y * 0.5 + 0.5/IN.positionHCS.w,0,0,1); // 将物体在 y 轴移动，现象类似图 1，只不过是在 y 轴上变化

                // 去掉注释观察屏幕空间上色情况
                return half4(IN.positionWS.xy/_ScaledScreenParams.xy,0,1); // 现象如图 2
            }
            ENDHLSL
        }
    }
}

图一

图二