Sharpen

The sharpen effect increases the contrast at the edges of the image. This comes in handy when your graphics are bit too soft.

  // ...

  float amount = 0.8;

  // ...

You can control how sharp the result is by adjusting the amount. An amount of zero leaves the image untouched. Try negative values for an odd look.

  // ...

  float neighbor = amount * -1;
  float center   = amount * 4 + 1;

  // ...

Neighboring fragments are multiplied by amount * -1. The current fragment is multiplied by amount * 4 + 1.

  // ...

  vec3 color =
        texture(sharpenTexture, vec2(gl_FragCoord.x + 0, gl_FragCoord.y + 1) / texSize).rgb
      * neighbor

      + texture(sharpenTexture, vec2(gl_FragCoord.x - 1, gl_FragCoord.y + 0) / texSize).rgb
      * neighbor
      + texture(sharpenTexture, vec2(gl_FragCoord.x + 0, gl_FragCoord.y + 0) / texSize).rgb
      * center
      + texture(sharpenTexture, vec2(gl_FragCoord.x + 1, gl_FragCoord.y + 0) / texSize).rgb
      * neighbor

      + texture(sharpenTexture, vec2(gl_FragCoord.x + 0, gl_FragCoord.y - 1) / texSize).rgb
      * neighbor
      ;

  // ...

The neighboring fragments are up, down, left, and right. After multiplying both the neighbors and the current fragment by their particular values, sum the result.

    // ...

    fragColor = vec4(color, texture(sharpenTexture, texCoord).a);

    // ...

3D Game Shaders For Beginners

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