GPT答：

7i24.cn

以下是一个基本的PBR着色器示例，使用Unity ShaderLab编写：

Shader "Custom/PBR" {
    Properties {
        _MainTex ("Albedo (RGB)", 2D) = "white" {}
        _Metallic ("Metallic", Range(0,1)) = 0.0
        _Smoothness ("Smoothness", Range(0,1)) = 0.5
        _NormalMap ("Normal Map", 2D) = "bump" {}
    }
 
    SubShader {
        Tags {"Queue"="Transparent"}
        LOD 100
 
        CGPROGRAM
            #pragma surface surf Standard
 
            sampler2D _MainTex;
            sampler2D _NormalMap;
 
            struct Input {
                float3 worldPos;
                float3 worldNormal;
                float4 screenPos;
                float3 viewDir;              
            };
 
            half4 LightingStandard (Input i, SurfaceOutputStandard o, half3 lightDir, half atten) {                
                // Diffuse lighting with Lambertian BRDF
                half NdotL = max(dot(i.worldNormal.xyz, lightDir), 0);
                
                // Specular lighting with Cook-Torrance BRDF and Schlick's approximation for Fresnel term.
                half roughnessSquared = o.Smoothness * o.Smoothness;               
                
				half metallicFactor = o.Metallic;

				half F90 = pow(clamp(1 - dot(lightDir.xyz,-i.viewDir),0.001f ,1),5);

				half Fd90= pow(clamp(1 - dot(lightDir.xyz,i.worldNormal.xyz),0.001f ,1),5);

				half Fr=lerp(Fd90,F90,o.Metallic);

				float kS=Fr;

			   	float kD=1-kS;

			   	kD*=(1-o.Metallic);

                half3 H = normalize(lightDir + i.viewDir);
                half NdotH = max(dot(i.worldNormal.xyz, H), 0);
                half D = GGX(NdotH, roughnessSquared);                
                half G = SmithGGXVisibilityTerm(NdotL, NdotH, roughnessSquared);                
				half specularFactor=D*G*kS/(4*(max(0.001f,NdotL)*max(0.001f,NdotV)));

				half diffuseFactor=(1-kS)*o.Albedo.rgb/(3.14159f);

				return (diffuseFactor+specularFactor) * atten;
            }
 
            void surf (Input IN, inout SurfaceOutputStandard o) {
                // Albedo comes from a texture tinted by color
                o.Albedo = tex2D (_MainTex, IN.screenPos.xy).rgb;
 
                // Metallic and smoothness come from slider variables
                o.Metallic = _Metallic;
                o.Smoothness = _Smoothness;               
 
                 // Normal map encoding is stored in the alpha channel.
                 float4 normalMapValue=tex2D(_NormalMap ,IN.screenPos.xy);
                 normalMapValue.a *= 2.0;   // Amplify the effect of the normal map to make it more visible.
                 float3 normalEncoded=normalMapValue.xyz * 2 - 1;   // Decode the RGB values into a vector direction.
                 float3 worldNormal=normalize(mul(normalEncoded,(float3x3)_WorldSpaceCameraTransform.inverse));   // Transform the vector into world space.

				 IN.worldNormal=worldNormal;

				 IN.viewDir=_WorldSpaceCameraPos-IN.worldPos;

				 fixed4 c=float4(0,0,0,0);

				 c.rgb+=LightingStandard(IN,o,_WorldSpaceLightPos[0].xyz,_WorldSpaceLightPos.w);

				 o.Emission=c.rgb;

            }
 
            // GGX distribution function
            half GGX (half NdotH, half roughnessSquared) {
                half alpha = roughnessSquared;
                half alphaSquared = alpha * alpha;
                half denominator = NdotH * NdotH * (alphaSquared - 1) + 1;
                return alphaSquared / (3.14159f * denominator * denominator);
            }
 
            // Smith's method for calculating the visibility term in the Cook-Torrance BRDF.
            // This is a simplified version that assumes that both V and L are pointing in the same hemisphere.
			half SmithGGXVisibilityTerm(half No,half Nh,half roughness)
			{
				half a=roughness*roughness;

				half k=a/2;

				return No/(No*(1-k)+k*Nh);
			}
        ENDCG
    } 
    FallBack "Diffuse"
}

该着色器使用SurfaceOutputStandard结构体来存储表面输出，包括漫反射颜色、金属度和光滑度。它还使用了Cook-Torrance BRDF来计算镜面反射，并使用Schlick的近似公式来计算菲涅尔效应。在光照函数中，我们将漫反射和镜面反射相加，并乘以衰减因子（atten）。

注意，在surf函数中，我们需要将法线从纹理中解码为世界空间向量，并计算视线向量。我们还需要在光照函数中使用世界空间的光源位置和相机位置。

最后，我们将该着色器设置为透明队列，并提供了一个回退选项（Fallback），以便在不支持PBR的设备上仍然能够正常工作。