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HTML

              
                <main>
  <h1>WebGL Shader Masking</h1>
  <div class="effect">
    <div class="back">
      Bacon ipsum dolor amet bresaola ground round tail pork belly shankle shank short loin. Tongue ham pork chop flank, fatback pork belly leberkas boudin bacon meatloaf frankfurter jowl tail. Leberkas jerky cow spare ribs tenderloin boudin.
  <shader-doodle>
    vec3 hsv2rgb(vec3 c) {
      vec4 K = vec4(1.0, 2.0 / 3.0, 1.0 / 3.0, 3.0);
      vec3 p = abs(fract(c.xxx + K.xyz) * 6.0 - K.www);
      return c.z * mix(K.xxx, clamp(p - K.xxx, 0.0, 1.0), c.y);
    }

    vec4 mod289(vec4 x)
    {
      return x - floor(x * (1.0 / 289.0)) * 289.0;
    }

    vec4 permute(vec4 x)
    {
      return mod289(((x*34.0)+1.0)*x);
    }

    vec4 taylorInvSqrt(vec4 r)
    {
      return 1.79284291400159 - 0.85373472095314 * r;
    }

    vec2 fade(vec2 t) {
      return t*t*t*(t*(t*6.0-15.0)+10.0);
    }

    // Classic Perlin noise
    float cnoise(vec2 P)
    {
      vec4 Pi = floor(P.xyxy) + vec4(0.0, 0.0, 1.0, 1.0);
      vec4 Pf = fract(P.xyxy) - vec4(0.0, 0.0, 1.0, 1.0);
      Pi = mod289(Pi); // To avoid truncation effects in permutation
      vec4 ix = Pi.xzxz;
      vec4 iy = Pi.yyww;
      vec4 fx = Pf.xzxz;
      vec4 fy = Pf.yyww;

      vec4 i = permute(permute(ix) + iy);

      vec4 gx = fract(i * (1.0 / 41.0)) * 2.0 - 1.0 ;
      vec4 gy = abs(gx) - 0.5 ;
      vec4 tx = floor(gx + 0.5);
      gx = gx - tx;

      vec2 g00 = vec2(gx.x,gy.x);
      vec2 g10 = vec2(gx.y,gy.y);
      vec2 g01 = vec2(gx.z,gy.z);
      vec2 g11 = vec2(gx.w,gy.w);

      vec4 norm = taylorInvSqrt(vec4(dot(g00, g00), dot(g01, g01), dot(g10, g10), dot(g11, g11)));
      g00 *= norm.x;  
      g01 *= norm.y;  
      g10 *= norm.z;  
      g11 *= norm.w;  

      float n00 = dot(g00, vec2(fx.x, fy.x));
      float n10 = dot(g10, vec2(fx.y, fy.y));
      float n01 = dot(g01, vec2(fx.z, fy.z));
      float n11 = dot(g11, vec2(fx.w, fy.w));

      vec2 fade_xy = fade(Pf.xy);
      vec2 n_x = mix(vec2(n00, n01), vec2(n10, n11), fade_xy.x);
      float n_xy = mix(n_x.x, n_x.y, fade_xy.y);
      return 2.3 * n_xy;
    }

    void main() {
        vec2 uv = gl_FragCoord.xy/u_resolution.xy;
  
        uv.x += sin(u_time * 0.25);
        uv.y += cos(u_time * 0.25);

        float noise = cnoise(uv * 5.);
        noise = noise + sin(u_time * 0.5) * 1.5 + 0.5;
    
        float hue = 1. - noise;
        float sat = 1. - step(0.8, noise);
        float bri = smoothstep(0.799, 0.8, noise);
        float op = 1.;

        vec3 col = hsv2rgb(vec3(hue, sat, bri));
    
        col = 1. - col;
        
        gl_FragColor = vec4(col, op);
    }
  </shader-doodle>
    </div>
    <div class="fore">
      Beef ribs tri-tip ball tip ham hock cupim hamburger. Doner chicken fatback, cow tenderloin biltong bacon. Jowl flank porchetta sirloin salami biltong turducken buffalo short ribs. Beef ribs cow ground round, beef meatloaf pig pork chop.
  <shader-doodle>
    vec3 hsv2rgb(vec3 c) {
      vec4 K = vec4(1.0, 2.0 / 3.0, 1.0 / 3.0, 3.0);
      vec3 p = abs(fract(c.xxx + K.xyz) * 6.0 - K.www);
      return c.z * mix(K.xxx, clamp(p - K.xxx, 0.0, 1.0), c.y);
    }

    vec4 mod289(vec4 x)
    {
      return x - floor(x * (1.0 / 289.0)) * 289.0;
    }

    vec4 permute(vec4 x)
    {
      return mod289(((x*34.0)+1.0)*x);
    }

    vec4 taylorInvSqrt(vec4 r)
    {
      return 1.79284291400159 - 0.85373472095314 * r;
    }

    vec2 fade(vec2 t) {
      return t*t*t*(t*(t*6.0-15.0)+10.0);
    }

    // Classic Perlin noise
    float cnoise(vec2 P)
    {
      vec4 Pi = floor(P.xyxy) + vec4(0.0, 0.0, 1.0, 1.0);
      vec4 Pf = fract(P.xyxy) - vec4(0.0, 0.0, 1.0, 1.0);
      Pi = mod289(Pi); // To avoid truncation effects in permutation
      vec4 ix = Pi.xzxz;
      vec4 iy = Pi.yyww;
      vec4 fx = Pf.xzxz;
      vec4 fy = Pf.yyww;

      vec4 i = permute(permute(ix) + iy);

      vec4 gx = fract(i * (1.0 / 41.0)) * 2.0 - 1.0 ;
      vec4 gy = abs(gx) - 0.5 ;
      vec4 tx = floor(gx + 0.5);
      gx = gx - tx;

      vec2 g00 = vec2(gx.x,gy.x);
      vec2 g10 = vec2(gx.y,gy.y);
      vec2 g01 = vec2(gx.z,gy.z);
      vec2 g11 = vec2(gx.w,gy.w);

      vec4 norm = taylorInvSqrt(vec4(dot(g00, g00), dot(g01, g01), dot(g10, g10), dot(g11, g11)));
      g00 *= norm.x;  
      g01 *= norm.y;  
      g10 *= norm.z;  
      g11 *= norm.w;  

      float n00 = dot(g00, vec2(fx.x, fy.x));
      float n10 = dot(g10, vec2(fx.y, fy.y));
      float n01 = dot(g01, vec2(fx.z, fy.z));
      float n11 = dot(g11, vec2(fx.w, fy.w));

      vec2 fade_xy = fade(Pf.xy);
      vec2 n_x = mix(vec2(n00, n01), vec2(n10, n11), fade_xy.x);
      float n_xy = mix(n_x.x, n_x.y, fade_xy.y);
      return 2.3 * n_xy;
    }

    void main() {
        vec2 uv = gl_FragCoord.xy/u_resolution.xy;
  
        uv.x += sin(u_time * 0.25);
        uv.y += cos(u_time * 0.25);

        float noise = cnoise(uv * 5.);
        noise = noise + sin(u_time * 0.5) * 1.5 + 0.5;
    
        float hue = 1. - noise;
        float sat = 1. - step(0.8, noise);
        float bri = smoothstep(0.799, 0.8, noise);
        float op = 1.;

        vec3 col = hsv2rgb(vec3(hue, sat, bri));
    
        // col = 1. - col;
        
        gl_FragColor = vec4(col, op);
    }
  </shader-doodle>
    </div>
  </div>
  <h2>What's happening?</h2>
  <p>The background div is blended with the foreground div based on two Shader Doodle masks.</p>
</main>
              
            
!

CSS

              
                @import url('https://fonts.googleapis.com/css2?family=Noto+Sans&family=Noto+Serif&display=swap');

body {
  width: 100vw;
  height: 100vh;
  margin: 0;
  padding: 0;
  display: grid;
  align-items: center;
  justify-content: center;
}

main {
  display: grid;
  align-items: center;
  justify-content: center;
  width: 40vmin;
}

h2 {
  margin-bottom: 0;
}

.effect {
  position: relative;
  width: 40vmin;
  height: 40vmin;
  border: 1px solid black;
  overflow: hidden;
}

div.back, div.fore {
  background-size: cover;
  position: absolute;
  top: 0;
  left: 0;
  bottom: 0;
  right: 0;
  font-size: 1.7em;
  padding: 5px;
}

.back {
  background-color: white;
  font-family: 'Noto Sans', sans-serif;
}

.fore {
  background-color: black;
  font-family: 'Noto Serif', serif;
  color: white;
  mix-blend-mode: multiply;
}

shader-doodle {
  position: absolute;
  top: 0;
  left: 0;
  width:100%;
  height:100%;
  mix-blend-mode: screen;
}

              
            
!

JS

              
                
              
            
!
999px

Console