// 양면 종이 셰이더 (URP)
// - 양면 렌더링: 얇은 종이 메시(Quad/Plane)의 앞뒤가 모두 보인다
// - _BaseColor: 앞뒤 공통 종이 색
// - _FrontTex: 앞면(메시의 노멀 방향 면)에만 표시되는 텍스처(인쇄물). 비우면 앞면도 종이 색 그대로
// - 하프-램버트 + 앰비언트 조명이라 그늘에서도 새까매지지 않음. 그림자 드리우기 포함
Shader "DinoLove/Paper"
{
    Properties
    {
        _BaseColor ("Base Color (양면 종이 색)", Color) = (1, 1, 1, 1)
        _FrontTex  ("Front Texture (앞면 인쇄물)", 2D) = "white" {}
    }

    SubShader
    {
        Tags { "RenderType" = "Opaque" "Queue" = "Geometry" "RenderPipeline" = "UniversalPipeline" }

        // 종이는 얇아서 뒷면도 그린다
        Cull Off

        Pass
        {
            Name "ForwardLit"
            Tags { "LightMode" = "UniversalForward" }

            HLSLPROGRAM
            #pragma vertex vert
            #pragma fragment frag
            #pragma multi_compile_instancing

            #include "Packages/com.unity.render-pipelines.universal/ShaderLibrary/Core.hlsl"
            #include "Packages/com.unity.render-pipelines.universal/ShaderLibrary/Lighting.hlsl"

            struct Attributes
            {
                float4 positionOS : POSITION;
                float3 normalOS   : NORMAL;
                float2 uv         : TEXCOORD0;
                UNITY_VERTEX_INPUT_INSTANCE_ID
            };

            struct Varyings
            {
                float4 positionCS : SV_POSITION;
                float2 uv         : TEXCOORD0;
                float3 normalWS   : TEXCOORD1;
                UNITY_VERTEX_INPUT_INSTANCE_ID
                UNITY_VERTEX_OUTPUT_STEREO
            };

            TEXTURE2D(_FrontTex);
            SAMPLER(sampler_FrontTex);

            CBUFFER_START(UnityPerMaterial)
                float4 _BaseColor;
                float4 _FrontTex_ST;
            CBUFFER_END

            Varyings vert(Attributes input)
            {
                Varyings output;
                UNITY_SETUP_INSTANCE_ID(input);
                UNITY_TRANSFER_INSTANCE_ID(input, output);
                UNITY_INITIALIZE_VERTEX_OUTPUT_STEREO(output);

                output.positionCS = TransformObjectToHClip(input.positionOS.xyz);
                output.normalWS   = TransformObjectToWorldNormal(input.normalOS);
                output.uv         = TRANSFORM_TEX(input.uv, _FrontTex);
                return output;
            }

            half4 frag(Varyings input, bool isFrontFace : SV_IsFrontFace) : SV_Target
            {
                UNITY_SETUP_INSTANCE_ID(input);
                UNITY_SETUP_STEREO_EYE_INDEX_POST_VERTEX(input);

                // 양면 공통 종이 색. 앞면에만 텍스처를 곱한다 (기본 white 텍스처 = 종이 색 그대로)
                half3 albedo = _BaseColor.rgb;
                if (isFrontFace)
                    albedo *= SAMPLE_TEXTURE2D(_FrontTex, sampler_FrontTex, input.uv).rgb;

                // 뒷면은 노멀을 뒤집어서 조명 계산
                float3 normalWS = normalize(input.normalWS);
                if (!isFrontFace)
                    normalWS = -normalWS;

                // 하프-램버트 + 앰비언트 — 부드러운 종이 음영
                Light mainLight = GetMainLight();
                half ndl = saturate(dot(normalWS, mainLight.direction)) * 0.5h + 0.5h;
                half3 lighting = mainLight.color * ndl + SampleSH(normalWS);

                return half4(albedo * lighting, 1);
            }
            ENDHLSL
        }

        // 그림자 드리우기 (없으면 종이가 공중에 뜬 것처럼 보임)
        Pass
        {
            Name "ShadowCaster"
            Tags { "LightMode" = "ShadowCaster" }

            ZWrite On
            ZTest LEqual
            ColorMask 0
            Cull Off

            HLSLPROGRAM
            #pragma vertex shadowVert
            #pragma fragment shadowFrag
            #pragma multi_compile_instancing

            #include "Packages/com.unity.render-pipelines.universal/ShaderLibrary/Core.hlsl"
            #include "Packages/com.unity.render-pipelines.universal/ShaderLibrary/Shadows.hlsl"

            float3 _LightDirection;

            struct ShadowAttributes
            {
                float4 positionOS : POSITION;
                float3 normalOS   : NORMAL;
                UNITY_VERTEX_INPUT_INSTANCE_ID
            };

            float4 shadowVert(ShadowAttributes input) : SV_POSITION
            {
                UNITY_SETUP_INSTANCE_ID(input);

                float3 positionWS = TransformObjectToWorld(input.positionOS.xyz);
                float3 normalWS   = TransformObjectToWorldNormal(input.normalOS);
                float4 positionCS = TransformWorldToHClip(ApplyShadowBias(positionWS, normalWS, _LightDirection));

                #if UNITY_REVERSED_Z
                    positionCS.z = min(positionCS.z, UNITY_NEAR_CLIP_VALUE);
                #else
                    positionCS.z = max(positionCS.z, UNITY_NEAR_CLIP_VALUE);
                #endif
                return positionCS;
            }

            half4 shadowFrag() : SV_Target
            {
                return 0;
            }
            ENDHLSL
        }
    }

    FallBack "Hidden/Universal Render Pipeline/FallbackError"
}