Shader Outline

Shader "Hidden/Roystan/Outline Post Process"
{
    SubShader
    {
        Cull Off ZWrite Off ZTest Always

        Pass
        {
            // Custom post processing effects are written in HLSL blocks,
            // with lots of macros to aid with platform differences.
            // https://github.com/Unity-Technologies/PostProcessing/wiki/Writing-Custom-Effects#shader
            HLSLPROGRAM
            #pragma vertex Vert
            #pragma fragment Frag
            #include "Packages/com.unity.postprocessing/PostProcessing/Shaders/StdLib.hlsl"

            TEXTURE2D_SAMPLER2D(_MainTex, sampler_MainTex);
            // _CameraNormalsTexture contains the view space normals transformed
            // to be in the 0...1 range.
            TEXTURE2D_SAMPLER2D(_CameraNormalsTexture, sampler_CameraNormalsTexture);
            TEXTURE2D_SAMPLER2D(_CameraDepthTexture, sampler_CameraDepthTexture);

            // Data pertaining to _MainTex's dimensions.
            // https://docs.unity3d.com/Manual/SL-PropertiesInPrograms.html
            float4 _MainTex_TexelSize;

            float _Scale;
            float4 _Color;

            float _DepthThreshold;
            float _DepthNormalThreshold;
            float _DepthNormalThresholdScale;

            float _NormalThreshold;

            // This matrix is populated in PostProcessOutline.cs.
            float4x4 _ClipToView;

            // Combines the top and bottom colors using normal blending.
            // https://en.wikipedia.org/wiki/Blend_modes#Normal_blend_mode
            // This performs the same operation as Blend SrcAlpha OneMinusSrcAlpha.
            float4 alphaBlend(float4 top, float4 bottom)
            {
                float3 color = (top.rgb * top.a) + (bottom.rgb * (1 - top.a));
                float alpha = top.a + bottom.a * (1 - top.a);

                return float4(color, alpha);
            }

            // Both the Varyings struct and the Vert shader are copied
            // from StdLib.hlsl included above, with some modifications.
            struct Varyings
            {
                float4 vertex : SV_POSITION;
                float2 texcoord : TEXCOORD0;
                float2 texcoordStereo : TEXCOORD1;
                float3 viewSpaceDir : TEXCOORD2;
            #if STEREO_INSTANCING_ENABLED
                uint stereoTargetEyeIndex : SV_RenderTargetArrayIndex;
            #endif
            };

            Varyings Vert(AttributesDefault v)
            {
                Varyings o;
                o.vertex = float4(v.vertex.xy, 0.0, 1.0);
                o.texcoord = TransformTriangleVertexToUV(v.vertex.xy);
                // Transform our point first from clip to view space,
                // taking the xyz to interpret it as a direction.
                o.viewSpaceDir = mul(_ClipToView, o.vertex).xyz;

            #if UNITY_UV_STARTS_AT_TOP
                o.texcoord = o.texcoord * float2(1.0, -1.0) + float2(0.0, 1.0);
            #endif

                o.texcoordStereo = TransformStereoScreenSpaceTex(o.texcoord, 1.0);

                return o;
            }

            float4 Frag(Varyings i) : SV_Target
            {
                float halfScaleFloor = floor(_Scale * 0.5);
                float halfScaleCeil = ceil(_Scale * 0.5);

                // Sample the pixels in an X shape, roughly centered around i.texcoord.
                // As the _CameraDepthTexture and _CameraNormalsTexture default samplers
                // use point filtering, we use the above variables to ensure we offset
                // exactly one pixel at a time.
                float2 bottomLeftUV = i.texcoord - float2(_MainTex_TexelSize.x, _MainTex_TexelSize.y) * halfScaleFloor;
                float2 topRightUV = i.texcoord + float2(_MainTex_TexelSize.x, _MainTex_TexelSize.y) * halfScaleCeil;
                float2 bottomRightUV = i.texcoord + float2(_MainTex_TexelSize.x * halfScaleCeil, -_MainTex_TexelSize.y * halfScaleFloor);
                float2 topLeftUV = i.texcoord + float2(-_MainTex_TexelSize.x * halfScaleFloor, _MainTex_TexelSize.y * halfScaleCeil);

                float3 normal0 = SAMPLE_TEXTURE2D(_CameraNormalsTexture, sampler_CameraNormalsTexture, bottomLeftUV).rgb;
                float3 normal1 = SAMPLE_TEXTURE2D(_CameraNormalsTexture, sampler_CameraNormalsTexture, topRightUV).rgb;
                float3 normal2 = SAMPLE_TEXTURE2D(_CameraNormalsTexture, sampler_CameraNormalsTexture, bottomRightUV).rgb;
                float3 normal3 = SAMPLE_TEXTURE2D(_CameraNormalsTexture, sampler_CameraNormalsTexture, topLeftUV).rgb;

                float depth0 = SAMPLE_DEPTH_TEXTURE(_CameraDepthTexture, sampler_CameraDepthTexture, bottomLeftUV).r;
                float depth1 = SAMPLE_DEPTH_TEXTURE(_CameraDepthTexture, sampler_CameraDepthTexture, topRightUV).r;
                float depth2 = SAMPLE_DEPTH_TEXTURE(_CameraDepthTexture, sampler_CameraDepthTexture, bottomRightUV).r;
                float depth3 = SAMPLE_DEPTH_TEXTURE(_CameraDepthTexture, sampler_CameraDepthTexture, topLeftUV).r;

                // Transform the view normal from the 0...1 range to the -1...1 range.
                float3 viewNormal = normal0 * 2 - 1;
                float NdotV = 1 - dot(viewNormal, -i.viewSpaceDir);

                // Return a value in the 0...1 range depending on where NdotV lies
                // between _DepthNormalThreshold and 1.
                float normalThreshold01 = saturate((NdotV - _DepthNormalThreshold) / (1 - _DepthNormalThreshold));
                // Scale the threshold, and add 1 so that it is in the range of 1..._NormalThresholdScale + 1.
                float normalThreshold = normalThreshold01 * _DepthNormalThresholdScale + 1;

                // Modulate the threshold by the existing depth value;
                // pixels further from the screen will require smaller differences
                // to draw an edge.
                float depthThreshold = _DepthThreshold * depth0 * normalThreshold;

                float depthFiniteDifference0 = depth1 - depth0;
                float depthFiniteDifference1 = depth3 - depth2;
                // edgeDepth is calculated using the Roberts cross operator.
                // The same operation is applied to the normal below.
                // https://en.wikipedia.org/wiki/Roberts_cross
                float edgeDepth = sqrt(pow(depthFiniteDifference0, 2) + pow(depthFiniteDifference1, 2)) * 100;
                edgeDepth = edgeDepth > depthThreshold ? 1 : 0;

                float3 normalFiniteDifference0 = normal1 - normal0;
                float3 normalFiniteDifference1 = normal3 - normal2;
                // Dot the finite differences with themselves to transform the
                // three-dimensional values to scalars.
                float edgeNormal = sqrt(dot(normalFiniteDifference0, normalFiniteDifference0) + dot(normalFiniteDifference1, normalFiniteDifference1));
                edgeNormal = edgeNormal > _NormalThreshold ? 1 : 0;

                float edge = max(edgeDepth, edgeNormal);

                float4 edgeColor = float4(_Color.rgb, _Color.a * edge);

                float4 color = SAMPLE_TEXTURE2D(_MainTex, sampler_MainTex, i.texcoord);

                return alphaBlend(edgeColor, color);
            }
            ENDHLSL
        }
    }
}