黑白变化很有用，可以用来做颜色融合。裁剪，扰动，变化等。
1.面片着色

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

    // Time varying pixel color
    float d = length(uv);
    float c = d;
    // Output to screen
    fragColor = vec4(vec3(c),1.0);
}

边角的向量模长为 0.5^2 + 0.5^2 = 0.5
越靠近中心，模长越接近0，也就是黑色
效果如下:

shader完整代码:

Shader "Custom/quadCol4"
{
    Properties
    {
        _Color("Main Color",Color) = (1,1,1,1)
    }
    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;
            };

            float4 _Color;

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

            fixed4 frag (v2f i) : SV_Target
            {
                float d = length(i.uv.xy - float2(0.5,0.5));
                fixed4 col = fixed4(d,d,d,1);
                return col;
            }
            ENDCG
        }
    }
}

越接近中点越黑，越接近边角越白。
效果:

没有渐变的:

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

    // Time varying pixel color
    float d = length(uv - vec2(0.5,0.5));
    float c = 0.0;
    if(d < .3){
        c = 1.0;
    }
    fragColor = vec4(vec3(c),1.0);
}

效果: