Wireframe shader: How to display quads and not triangles?

Question

I have a wireframe shader that displays triangles such as on the left cube, and wanted to update it so that it would only display quads such as on the right cube.

Here's the code:

Shader "Custom/Wireframe"
{
    Properties
    {
        _WireColor("WireColor", Color) = (1,0,0,1)
        _Color("Color", Color) = (1,1,1,1)
    }
SubShader
{
    Pass
    {
        CGPROGRAM
        #include "UnityCG.cginc"
        #pragma target 5.0
        #pragma vertex vert
        #pragma geometry geom
        #pragma fragment frag

        half4 _WireColor, _Color;

        struct v2g
        {
            float4  pos : SV_POSITION;
            float2  uv : TEXCOORD0;
        };

        struct g2f
        {
            float4  pos : SV_POSITION;
            float2  uv : TEXCOORD0;
            float3 dist : TEXCOORD1;
        };

        v2g vert(appdata_base v)
        {
            v2g OUT;
            OUT.pos = mul(UNITY_MATRIX_MVP, v.vertex);
            OUT.uv = v.texcoord;
            return OUT;
        }

        [maxvertexcount(3)]
        void geom(triangle v2g IN[3], inout TriangleStream<g2f> triStream)
        {

            float2 WIN_SCALE = float2(_ScreenParams.x/2.0, _ScreenParams.y/2.0);

            //frag position
            float2 p0 = WIN_SCALE * IN[0].pos.xy / IN[0].pos.w;
            float2 p1 = WIN_SCALE * IN[1].pos.xy / IN[1].pos.w;
            float2 p2 = WIN_SCALE * IN[2].pos.xy / IN[2].pos.w;

            //barycentric position
            float2 v0 = p2-p1;
            float2 v1 = p2-p0;
            float2 v2 = p1-p0;
            //triangles area
            float area = abs(v1.x*v2.y - v1.y * v2.x);

            g2f OUT;
            OUT.pos = IN[0].pos;
            OUT.uv = IN[0].uv;
            OUT.dist = float3(area/length(v0),0,0);
            triStream.Append(OUT);

            OUT.pos = IN[1].pos;
            OUT.uv = IN[1].uv;
            OUT.dist = float3(0,area/length(v1),0);
            triStream.Append(OUT);

            OUT.pos = IN[2].pos;
            OUT.uv = IN[2].uv;
            OUT.dist = float3(0,0,area/length(v2));
            triStream.Append(OUT);

        }

        half4 frag(g2f IN) : COLOR
        {
            //distance of frag from triangles center
            float d = min(IN.dist.x, min(IN.dist.y, IN.dist.z));
            //fade based on dist from center
             float I = exp2(-4.0*d*d);

             return lerp(_Color, _WireColor, I);              
        }
        ENDCG
    }
}

Someone has mentioned that one way to do it would be to compare the normals of the neighboring triangles. If the dot product of the two normals is close to 1, the edge can be skipped. But I don't know how to implement it as I don't have any knowledge on geometry shaders.

Can you please help me edit this shader?

Thanks.

Answer 1

Someone posted a question where one of the pieces that they started with was...a diagonal-less wireframe shader! Being on the lookout for one of these for some time (and remembering this question) I figured it deserved an answer.

The shader below was found at this site.

Shader "Custom/Geometry/Wireframe"
{
    Properties
    {
        [PowerSlider(3.0)]
        _WireframeVal ("Wireframe width", Range(0., 0.5)) = 0.05
        _FrontColor ("Front color", color) = (1., 1., 1., 1.)
        _BackColor ("Back color", color) = (1., 1., 1., 1.)
        [Toggle] _RemoveDiag("Remove diagonals?", Float) = 0.
    }
    SubShader
    {
        Tags { "Queue"="Geometry" "RenderType"="Opaque" }

        Pass
        {
            Cull Front
            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag
            #pragma geometry geom

            // Change "shader_feature" with "pragma_compile" if you want set this keyword from c# code
            #pragma shader_feature __ _REMOVEDIAG_ON

            #include "UnityCG.cginc"

            struct v2g {
                float4 worldPos : SV_POSITION;
            };

            struct g2f {
                float4 pos : SV_POSITION;
                float3 bary : TEXCOORD0;
            };

            v2g vert(appdata_base v) {
                v2g o;
                o.worldPos = mul(unity_ObjectToWorld, v.vertex);
                return o;
            }

            [maxvertexcount(3)]
            void geom(triangle v2g IN[3], inout TriangleStream<g2f> triStream) {
                float3 param = float3(0., 0., 0.);

                #if _REMOVEDIAG_ON
                float EdgeA = length(IN[0].worldPos - IN[1].worldPos);
                float EdgeB = length(IN[1].worldPos - IN[2].worldPos);
                float EdgeC = length(IN[2].worldPos - IN[0].worldPos);

                if(EdgeA > EdgeB && EdgeA > EdgeC)
                    param.y = 1.;
                else if (EdgeB > EdgeC && EdgeB > EdgeA)
                    param.x = 1.;
                else
                    param.z = 1.;
                #endif

                g2f o;
                o.pos = mul(UNITY_MATRIX_VP, IN[0].worldPos);
                o.bary = float3(1., 0., 0.) + param;
                triStream.Append(o);
                o.pos = mul(UNITY_MATRIX_VP, IN[1].worldPos);
                o.bary = float3(0., 0., 1.) + param;
                triStream.Append(o);
                o.pos = mul(UNITY_MATRIX_VP, IN[2].worldPos);
                o.bary = float3(0., 1., 0.) + param;
                triStream.Append(o);
            }

            float _WireframeVal;
            fixed4 _BackColor;

            fixed4 frag(g2f i) : SV_Target {
            if(!any(bool3(i.bary.x < _WireframeVal, i.bary.y < _WireframeVal, i.bary.z < _WireframeVal)))
                 discard;

                return _BackColor;
            }

            ENDCG
        }

        Pass
        {
            Cull Back
            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag
            #pragma geometry geom

            // Change "shader_feature" with "pragma_compile" if you want set this keyword from c# code
            #pragma shader_feature __ _REMOVEDIAG_ON

            #include "UnityCG.cginc"

            struct v2g {
                float4 worldPos : SV_POSITION;
            };

            struct g2f {
                float4 pos : SV_POSITION;
                float3 bary : TEXCOORD0;
            };

            v2g vert(appdata_base v) {
                v2g o;
                o.worldPos = mul(unity_ObjectToWorld, v.vertex);
                return o;
            }

            [maxvertexcount(3)]
            void geom(triangle v2g IN[3], inout TriangleStream<g2f> triStream) {
                float3 param = float3(0., 0., 0.);

                #if _REMOVEDIAG_ON
                float EdgeA = length(IN[0].worldPos - IN[1].worldPos);
                float EdgeB = length(IN[1].worldPos - IN[2].worldPos);
                float EdgeC = length(IN[2].worldPos - IN[0].worldPos);

                if(EdgeA > EdgeB && EdgeA > EdgeC)
                    param.y = 1.;
                else if (EdgeB > EdgeC && EdgeB > EdgeA)
                    param.x = 1.;
                else
                    param.z = 1.;
                #endif

                g2f o;
                o.pos = mul(UNITY_MATRIX_VP, IN[0].worldPos);
                o.bary = float3(1., 0., 0.) + param;
                triStream.Append(o);
                o.pos = mul(UNITY_MATRIX_VP, IN[1].worldPos);
                o.bary = float3(0., 0., 1.) + param;
                triStream.Append(o);
                o.pos = mul(UNITY_MATRIX_VP, IN[2].worldPos);
                o.bary = float3(0., 1., 0.) + param;
                triStream.Append(o);
            }

            float _WireframeVal;
            fixed4 _FrontColor;

            fixed4 frag(g2f i) : SV_Target {
            if(!any(bool3(i.bary.x <= _WireframeVal, i.bary.y <= _WireframeVal, i.bary.z <= _WireframeVal)))
                 discard;

                return _FrontColor;
            }

            ENDCG
        }
    }
}

Do note the #pragma shader_feature __ _REMOVEDIAG_ON lines, which will cause two shaders to get compiled internally, so if you only need it to do the One Thing, you will need to remove this line and adjust any other references appropriately.