Custom ShaderLab and URP HLSL

How it works and why

ShaderLab declares the Shader, Properties, SubShader and Pass structure. HLSL functions perform the actual vertex and fragment calculations. URP shaders generally need URP shader tags and URP library includes; a Built-In Surface Shader example is not a valid URP replacement.

The example below is an unlit URP pass. It uses the object-to-clip-space transform and returns one configured color. It does not calculate lights, shadows or textures, which keeps the initial learning example focused.

Example you can try

Shader "HCSTech/UnlitColor"
{
    Properties { _BaseColor ("Base Color", Color) = (0.2, 0.8, 0.5, 1) }
    SubShader
    {
        Tags { "RenderPipeline" = "UniversalPipeline" "RenderType" = "Opaque" }
        Pass
        {
            HLSLPROGRAM
            #pragma vertex Vert
            #pragma fragment Frag
            #include "Packages/com.unity.render-pipelines.universal/ShaderLibrary/Core.hlsl"

            struct Attributes { float4 positionOS : POSITION; };
            struct Varyings { float4 positionCS : SV_POSITION; };
            CBUFFER_START(UnityPerMaterial)
                float4 _BaseColor;
            CBUFFER_END
            Varyings Vert(Attributes input)
            {
                Varyings output;
                output.positionCS = TransformObjectToHClip(input.positionOS.xyz);
                return output;
            }
            half4 Frag(Varyings input) : SV_Target { return (half4)_BaseColor; }
            ENDHLSL
        }
    }
}

When to use it and what can go wrong

Why the constant buffer? Keeping material properties in UnityPerMaterial follows the layout URP expects for SRP Batcher compatibility. More advanced shaders need proper pass tags and lighting includes. Pitfall: Test on your selected target platforms, not only in Scene view.

Try it: Create a Shader asset with this code, create a material and change _BaseColor. Compare it to a Lit URP shader.

Further reading and official references