[游戏开发]UPR Lit Shader解析（2）—前向渲染Pass

//  Forward pass. Shades all light in a single pass. GI + emission + Fog
        Pass//前向渲染的Pass，所有光照效果都在该Pass中计算，包括全局光照、自发光和雾效
        {
            // Lightmode matches the ShaderPassName set in UniversalRenderPipeline.cs. SRPDefaultUnlit and passes with
            // no LightMode tag are also rendered by Universal Render Pipeline
            Name "ForwardLit"//Pass的名字，给Pass命名，方便复用
            Tags{"LightMode" = "UniversalForward"}//光照模式标签一定要与UniversalRenderPipeline.cs中对应，如果先写Unlit shader，可以不用该标签

            Blend[_SrcBlend][_DstBlend]//混合操作，参数的值来自于 Properties块中的定义
            ZWrite[_ZWrite]
            Cull[_Cull]

            HLSLPROGRAM //Shader代码块不像内置渲染管线一样使用CGPROGRAM--ENDCG包裹，而使用HLSLPROGRAM--ENDHLSL包裹
            //CG表示 C for Graphic,HLSL表示High Level Shader Language
            //使用HLSL中引用任何内容都需要显式添加，而CG会隐式添加，会添加进不需要的东西，导致文件变大
            #pragma only_renderers gles gles3 glcore d3d11 //图形API预编译指令，只为给定的图像API编译该Shader，包括OpenGL 3.x/4.x 、OpenGL ES 2.0/3.x 、D3D 11/12
            #pragma target 2.0 //指定shader program应用的最低等级的shader model，2.0适用于 Unity 支持的所有平台。DX9 着色器模型 2.0。 有限数量的算术和纹理指令；8 个插值器；没有顶点纹理采样；片元着色器中没有衍生指令；没有显式的 LOD 纹理采样。
            //

            //--------------------------------------
            // GPU Instancing
            #pragma multi_compile_instancing//着色器变体编译指令，使该Shader支持GPU Instancing功能，
            #pragma instancing_options renderinglayer//指定所有实例化后的对象的渲染层级

            // -------------------------------------
            // Material Keywords 
            //#pragma shader_feature_local表示定义local keywords，后缀_fragment表示关键字用于片元着色器阶段，这些关键字与材质面板上的参数有关，遇到的时候再解释
            //关键字的名字前一般要带下划线
            #pragma shader_feature_local _NORMALMAP
            #pragma shader_feature_local _PARALLAXMAP
            #pragma shader_feature_local _RECEIVE_SHADOWS_OFF
            #pragma shader_feature_local _ _DETAIL_MULX2 _DETAIL_SCALED
            #pragma shader_feature_local_fragment _SURFACE_TYPE_TRANSPARENT
            #pragma shader_feature_local_fragment _ALPHATEST_ON
            #pragma shader_feature_local_fragment _ALPHAPREMULTIPLY_ON
            #pragma shader_feature_local_fragment _EMISSION
            #pragma shader_feature_local_fragment _METALLICSPECGLOSSMAP
            #pragma shader_feature_local_fragment _SMOOTHNESS_TEXTURE_ALBEDO_CHANNEL_A
            #pragma shader_feature_local_fragment _OCCLUSIONMAP
            #pragma shader_feature_local_fragment _SPECULARHIGHLIGHTS_OFF
            #pragma shader_feature_local_fragment _ENVIRONMENTREFLECTIONS_OFF
            #pragma shader_feature_local_fragment _SPECULAR_SETUP

            // -------------------------------------
            // Universal Pipeline keywords
            // #pragma multi_compile表示global keywords，与shader_feature的区别是， Unity只为shader_feature中使用的关键字编译着色器变体shader variant
            #pragma multi_compile _ _MAIN_LIGHT_SHADOWS _MAIN_LIGHT_SHADOWS_CASCADE _MAIN_LIGHT_SHADOWS_SCREEN//一次声明多个关键字
            #pragma multi_compile _ _ADDITIONAL_LIGHTS_VERTEX _ADDITIONAL_LIGHTS
            #pragma multi_compile_fragment _ _ADDITIONAL_LIGHT_SHADOWS
            #pragma multi_compile_fragment _ _SHADOWS_SOFT
            #pragma multi_compile_fragment _ _SCREEN_SPACE_OCCLUSION
            #pragma multi_compile_fragment _ _DBUFFER_MRT1 _DBUFFER_MRT2 _DBUFFER_MRT3
            #pragma multi_compile_fragment _ _REFLECTION_PROBE_BLENDING
            #pragma multi_compile_fragment _ _REFLECTION_PROBE_BOX_PROJECTION
            #pragma multi_compile_fragment _ _LIGHT_LAYERS
            #pragma multi_compile_fragment _ _LIGHT_COOKIES
            #pragma multi_compile _ _CLUSTERED_RENDERING

            // -------------------------------------
            // Unity defined keywords
            #pragma multi_compile _ LIGHTMAP_SHADOW_MIXING
            #pragma multi_compile _ SHADOWS_SHADOWMASK
            #pragma multi_compile _ DIRLIGHTMAP_COMBINED
            #pragma multi_compile _ LIGHTMAP_ON
            #pragma multi_compile_fog
            #pragma multi_compile_fragment _ DEBUG_DISPLAY

            //定义顶点和片元着色器
            #pragma vertex LitPassVertex 
            #pragma fragment LitPassFragment

            //添加引用文件，该Pass没有变量声明、顶点着色器函数、片元着色器函数，这些代码在引用的两个文件中
            #include "Packages/com.unity.render-pipelines.universal/Shaders/LitInput.hlsl"
            #include "Packages/com.unity.render-pipelines.universal/Shaders/LitForwardPass.hlsl"
            ENDHLSL
        }