1.

float remap2(float a,float b,float t)
{
    return (t - a) / (b - a);
}

float remap(float a,float b,float c,float d,float t)
{
    return (t - a) / (b - a) * (d - c) + c;
}

vec4 Eye(vec2 uv)
{
    vec4 col = vec4(0.0);

    return col;
}

vec4 Mouth(vec2 uv)
{
    vec4 col = vec4(0.0);

    return col;
}

vec4 Head(vec2 uv)
{
    vec4 col = vec4(0.9,0.65,0.1,1.0);
    float d = length(uv);

    //uv坐标到中点距离，小于0.49的透明为0，大于0.5的为1，中间差值过度
    col.a = smoothstep(0.5,0.49,d);
    float edge = remap2(0.35,0.5,d);

    col.rgb *= edge;

    return col;
}

vec4 Smile(vec2 uv)
{
    vec4 col = vec4(0.0);
    vec4 head = Head(uv);
    col = mix(col,head,head.a);
    return col;
}

void mainImage( out vec4 fragColor, in vec2 fragCoord )
{
    // Normalized pixel coordinates (from 0 to 1)
    vec2 uv = fragCoord/iResolution.xy;
    uv -= 0.5;

    //将uv的x坐标进行放大
    uv.x *= iResolution.x / iResolution.y;

    vec4 col = Smile(uv);
    fragColor = col;
}

2

float remap2(float a,float b,float t)
{
    return clamp((t - a) / (b - a),0.0,1.0);
}

float remap(float a,float b,float c,float d,float t)
{
    return (t - a) / (b - a) * (d - c) + c;
}

vec2 withIn(vec2 uv,vec4 rect)
{
    return (uv - rect.xy) / (rect.zw - rect.xy);
}

vec4 Eye(vec2 uv)
{
    uv -= 0.5;
    vec4 col = vec4(1.0);
    float d = length(uv);
    col.a = smoothstep(0.5,0.4,d);

    return col;
}

vec4 Mouth(vec2 uv)
{
    vec4 col = vec4(0.0);

    return col;
}

vec4 Head(vec2 uv)
{
    vec4 col = vec4(0.9,0.65,0.1,1.0);
    float d = length(uv);

    //uv坐标到中点距离，小于0.49的透明为0，大于0.5的为1，中间差值过度
    col.a = smoothstep(0.5,0.49,d);
    float edge = remap2(0.35,0.5,d);
    edge *= edge;

    col.rgb *= 1.0 - edge * 0.5;
    col.rgb = mix(col.rgb,vec3(0.6,0.3,0.1),smoothstep(0.47,0.48,d));

    float highLight = smoothstep(0.41,0.405,d) * 0.75;
    highLight *= remap(0.41,-0.1,0.75,0.0,uv.y);
    col.rgb = mix(col.rgb,vec3(1.0),highLight);

    d = length(uv - vec2(0.25,-0.2));
    float cheek = smoothstep(0.2,0.01,d) * 0.4;
    cheek *= smoothstep(0.17,0.16,d);
    col.rgb = mix(col.rgb,vec3(1.0,0.1,0.1),cheek);

    return col;
}

vec4 Smile(vec2 uv)
{
    vec4 col = vec4(0.0);
    uv.x = abs(uv.x);
    vec4 head = Head(uv);
    vec4 eye = Eye(withIn(uv,vec4(0.03,-0.1,0.37,0.25)));
    col = mix(col,head,head.a);
    col = mix(col,eye,eye.a);
    return col;
}

void mainImage( out vec4 fragColor, in vec2 fragCoord )
{
    // Normalized pixel coordinates (from 0 to 1)
    vec2 uv = fragCoord/iResolution.xy;
    uv -= 0.5;

    //将uv的x坐标进行放大
    uv.x *= iResolution.x / iResolution.y;

    vec4 col = Smile(uv);
    fragColor = col;
}

3

float remap2(float a,float b,float t)
{
    return clamp((t - a) / (b - a),0.0,1.0);
}

float remap(float a,float b,float c,float d,float t)
{
    return (t - a) / (b - a) * (d - c) + c;
}

vec2 withIn(vec2 uv,vec4 rect)
{
    return (uv - rect.xy) / (rect.zw - rect.xy);
}

vec4 Eye(vec2 uv)
{
    uv -= 0.5;
    float d = length(uv);
    vec4 irisCol = vec4(0.3,0.5,1.0,1.0);
    vec4 col = mix(vec4(1.0),irisCol,smoothstep(0.1,0.7,d));
    col.rgb = mix(col.rgb,vec3(0.0),smoothstep(0.3,0.28,d));

    col.a = smoothstep(0.5,0.48,d);

    return col;
}

vec4 Mouth(vec2 uv)
{
    vec4 col = vec4(0.0);

    return col;
}

vec4 Head(vec2 uv)
{
    vec4 col = vec4(0.9,0.65,0.1,1.0);
    float d = length(uv);

    //uv坐标到中点距离，小于0.49的透明为0，大于0.5的为1，中间差值过度
    col.a = smoothstep(0.5,0.49,d);
    float edge = remap2(0.35,0.5,d);
    edge *= edge;

    col.rgb *= 1.0 - edge * 0.5;
    col.rgb = mix(col.rgb,vec3(0.6,0.3,0.1),smoothstep(0.47,0.48,d));

    float highLight = smoothstep(0.41,0.405,d) * 0.75;
    highLight *= remap(0.41,-0.1,0.75,0.0,uv.y);
    col.rgb = mix(col.rgb,vec3(1.0),highLight);

    d = length(uv - vec2(0.25,-0.2));
    float cheek = smoothstep(0.2,0.01,d) * 0.4;
    cheek *= smoothstep(0.17,0.16,d);
    col.rgb = mix(col.rgb,vec3(1.0,0.1,0.1),cheek);

    return col;
}

vec4 Smile(vec2 uv)
{
    vec4 col = vec4(0.0);
    uv.x = abs(uv.x);
    vec4 head = Head(uv);
    vec4 eye = Eye(withIn(uv,vec4(0.03,-0.1,0.37,0.25)));
    col = mix(col,head,head.a);
    col = mix(col,eye,eye.a);
    return col;
}

void mainImage( out vec4 fragColor, in vec2 fragCoord )
{
    // Normalized pixel coordinates (from 0 to 1)
    vec2 uv = fragCoord/iResolution.xy;
    uv -= 0.5;

    //将uv的x坐标进行放大
    uv.x *= iResolution.x / iResolution.y;

    vec4 col = Smile(uv);
    fragColor = col;
}

4

float remap2(float a,float b,float t)
{
    return clamp((t - a) / (b - a),0.0,1.0);
}

float remap(float a,float b,float c,float d,float t)
{
    return (t - a) / (b - a) * (d - c) + c;
}

vec2 withIn(vec2 uv,vec4 rect)
{
    return (uv - rect.xy) / (rect.zw - rect.xy);
}

vec4 Eye(vec2 uv)
{
    uv -= 0.5;
    float d = length(uv);
    vec4 irisCol = vec4(0.3,0.5,1.0,1.0);
    vec4 col = mix(vec4(1.0),irisCol,smoothstep(0.1,0.7,d) * 0.5);
    //乘法是且
    col.rgb *= 1.0 - smoothstep(0.45,0.5,d) * 0.5 * clamp(-uv.y - uv.x,0.0,1.0);
    col.rgb = mix(col.rgb,vec3(0.0),smoothstep(0.3,0.28,d));

    irisCol.rgb *= 1.0 + smoothstep(0.3,0.05,d);
    col.rgb = mix(col.rgb,irisCol.rgb,smoothstep(0.28,0.25,d));
    col.rgb = mix(col.rgb,vec3(0.0),smoothstep(0.16,0.14,d));

    float highLight = smoothstep(0.1,0.09,length(uv - vec2(-0.15,0.15)));
    //加法是或
    highLight += smoothstep(0.07,0.05,length(uv + vec2(-0.08,0.08)));
    col.rgb = mix(col.rgb,vec3(1.0),highLight);
    col.a = smoothstep(0.5,0.48,d);

    return col;
}

vec4 Mouth(vec2 uv)
{
    uv -= 0.5;
    vec4 col = vec4(0.5,0.18,0.05,1);
    uv.y *= 1.5;
    uv.y -= uv.x * uv.x * 2.0;
    float d = length(uv);
    col.a = smoothstep(0.5,0.48,d);

    float td = length(uv - vec2(0.0,0.6));
    vec3 toothCol = vec3(1.0) * smoothstep(0.6,0.35,d);
    col.rgb = mix(col.rgb,toothCol,smoothstep(0.4,0.37,d));

    td = length(uv + vec2(0.0,0.5));
    col.rgb = mix(col.rgb,vec3(1.0,0.5,0.5),smoothstep(0.5,0.2,td));

    return col;
}

vec4 Head(vec2 uv)
{
    vec4 col = vec4(0.9,0.65,0.1,1.0);
    float d = length(uv);

    //uv坐标到中点距离，小于0.49的透明为0，大于0.5的为1，中间差值过度
    col.a = smoothstep(0.5,0.49,d);
    float edge = remap2(0.35,0.5,d);
    edge *= edge;

    col.rgb *= 1.0 - edge * 0.5;
    col.rgb = mix(col.rgb,vec3(0.6,0.3,0.1),smoothstep(0.47,0.48,d));

    float highLight = smoothstep(0.41,0.405,d) * 0.75;
    highLight *= remap(0.41,-0.1,0.75,0.0,uv.y);
    col.rgb = mix(col.rgb,vec3(1.0),highLight);

    d = length(uv - vec2(0.25,-0.2));
    float cheek = smoothstep(0.2,0.01,d) * 0.4;
    cheek *= smoothstep(0.17,0.16,d);
    col.rgb = mix(col.rgb,vec3(1.0,0.1,0.1),cheek);

    return col;
}

vec4 Smile(vec2 uv)
{
    vec4 col = vec4(0.0);
    uv.x = abs(uv.x);
    vec4 head = Head(uv);
    vec4 eye = Eye(withIn(uv,vec4(0.03,-0.1,0.37,0.25)));
    vec4 mouth = Mouth(withIn(uv,vec4(-0.3,-0.4,0.3,-0.1)));
    col = mix(col,head,head.a);
    col = mix(col,eye,eye.a);
    col = mix(col,mouth,mouth.a);
    return col;
}

void mainImage( out vec4 fragColor, in vec2 fragCoord )
{
    // Normalized pixel coordinates (from 0 to 1)
    vec2 uv = fragCoord/iResolution.xy;
    uv -= 0.5;

    //将uv的x坐标进行放大
    uv.x *= iResolution.x / iResolution.y;

    vec4 col = Smile(uv);
    fragColor = col;
}

完整shader

Shader "Custom/Smile"
{
    Properties
    {
        _MainTex ("Texture", 2D) = "white" {}
    }
    SubShader
    {
        Tags { "RenderType"="Opaque" }
        LOD 100

        Pass
        {
            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag

            #include "UnityCG.cginc"

            struct appdata
            {
                float4 vertex : POSITION;
                float2 uv : TEXCOORD0;
            };

            struct v2f
            {
                float2 uv : TEXCOORD0;
                float4 vertex : SV_POSITION;
            };

            sampler2D _MainTex;

            float remap2(float a,float b,float t)
            {
                return clamp((t - a) / (b - a),0.0,1.0);
            }

            float remap(float a,float b,float c,float d,float t)
            {
                return (t - a) / (b - a) * (d - c) + c;
            }

            float2 withIn(float2 uv,float4 rect)
            {
                return (uv - rect.xy) / (rect.zw - rect.xy);
            }

            float4 Eye(float2 uv)
            {
                uv -= 0.5;
                float d = length(uv);
                float4 irisCol = float4(0.3,0.5,1.0,1.0);
                float4 col = lerp(float4(1.0,1.0,1.0,1.0),irisCol,smoothstep(0.1,0.7,d) * 0.5);
                //乘法是且
                col.rgb *= 1.0 - smoothstep(0.45,0.5,d) * 0.5 * clamp(-uv.y - uv.x,0.0,1.0);
                col.rgb = lerp(col.rgb,float3(0.0,0.0,0.0),smoothstep(0.3,0.28,d));

                irisCol.rgb *= 1.0 + smoothstep(0.3,0.05,d);
                col.rgb = lerp(col.rgb,irisCol.rgb,smoothstep(0.28,0.25,d));
                col.rgb = lerp(col.rgb,float3(0.0,0.0,0.0),smoothstep(0.16,0.14,d));

                float highLight = smoothstep(0.1,0.09,length(uv - float2(-0.15,0.15)));
                //加法是或
                highLight += smoothstep(0.07,0.05,length(uv + float2(-0.08,0.08)));
                col.rgb = lerp(col.rgb,float3(1.0,1.0,1.0),highLight);
                col.a = smoothstep(0.5,0.48,d);

                return col;
            }

            float4 Mouth(float2 uv)
            {
                uv -= 0.5;
                float4 col = float4(0.5,0.18,0.05,1);
                uv.y *= 1.5;
                uv.y -= uv.x * uv.x * 2.0;
                float d = length(uv);
                col.a = smoothstep(0.5,0.48,d);

                float td = length(uv - float2(0.0,0.6));
                float3 toothCol = float3(1.0,1.0,1.0) * smoothstep(0.6,0.35,d);
                col.rgb = lerp(col.rgb,toothCol,smoothstep(0.4,0.37,d));

                td = length(uv + float2(0.0,0.5));
                col.rgb = lerp(col.rgb,float3(1.0,0.5,0.5),smoothstep(0.5,0.2,td));

                return col;
            }

            float4 Head(float2 uv)
            {
                float4 col = float4(0.9,0.65,0.1,1.0);
                float d = length(uv);

                //uv坐标到中点距离，小于0.49的透明为0，大于0.5的为1，中间差值过度
                col.a = smoothstep(0.5,0.49,d);
                float edge = remap2(0.35,0.5,d);
                edge *= edge;

                col.rgb *= 1.0 - edge * 0.5;
                col.rgb = lerp(col.rgb,float3(0.6,0.3,0.1),smoothstep(0.47,0.48,d));

                float highLight = smoothstep(0.41,0.405,d) * 0.75;
                highLight *= remap(0.41,-0.1,0.75,0.0,uv.y);
                col.rgb = lerp(col.rgb,float3(1.0,1.0,1.0),highLight);

                d = length(uv - float2(0.25,-0.2));
                float cheek = smoothstep(0.2,0.01,d) * 0.4;
                cheek *= smoothstep(0.17,0.16,d);
                col.rgb = lerp(col.rgb,float3(1.0,0.1,0.1),cheek);

                return col;
            }

            float4 Smile(float2 uv)
            {
                float4 col = float4(0.0,0.0,0.0,0.0);
                uv.x = abs(uv.x);
                float4 head = Head(uv);
                float4 eye = Eye(withIn(uv,float4(0.03,-0.1,0.37,0.25)));
                float4 mouth = Mouth(withIn(uv,float4(-0.3,-0.4,0.3,-0.1)));
                col = lerp(col,head,head.a);
                col = lerp(col,eye,eye.a);
                col = lerp(col,mouth,mouth.a);
                return col;
            }

            v2f vert (appdata v)
            {
                v2f o;
                o.vertex = UnityObjectToClipPos(v.vertex);
                o.uv = v.uv;
                return o;
            }

            fixed4 frag (v2f i) : SV_Target
            {
                fixed4 texCol = tex2D(_MainTex,i.uv);
                i.uv -= 0.5;
                float4 c = Smile(i.uv);
                //这个是不透明的，alpha 0和1都无所谓，反正不生效
                fixed4 col = fixed4(c.x,c.y,c.z,1);
                float d = length(fixed3(c.x,c.y,c.z) - fixed3(0.0,0.0,0.0));
                if(d < 1e-6){
                    return texCol;
                }
                return col;
            }
            ENDCG
        }
    }
}

效果: