Fog

Fog (or mist as it's called in Blender) adds atmospheric haze to a scene, providing mystique and softening pop-ins (geometry suddenly entering into the camera's frustum).

// ...

uniform vec4 color;

uniform vec2 nearFar;

// ...

// ...

uniform sampler2D positionTexture;

  // ...

  vec2 texSize  = textureSize(positionTexture, 0).xy;
  vec2 texCoord = gl_FragCoord.xy / texSize;

  vec4 position = texture(positionTexture, texCoord);

  // ...

In addition to the fog's attributes, you'll also need the fragment's vertex position.

  float fogMin = 0.00;
  float fogMax = 0.97;

fogMax controls how much of the scene is still visible when the fog is most intense. fogMin controls how much of the fog is still visible when the fog is least intense.

  // ...

  float near = nearFar.x;
  float far  = nearFar.y;

  float intensity =
    clamp
      (   (position.y - near)
        / (far        - near)
      , fogMin
      , fogMax
      );

  // ...

The example code uses a linear model for calculating the fog's intensity. There's also an exponential model you could use.

The fog's intensity is fogMin before or at the start of the fog's near distance. As the vertex position gets closer to the end of the fog's far distance, the intensity moves closer to fogMax. For any vertexes after the end of the fog, the intensity is clamped to fogMax.

  // ...

  fragColor = vec4(color.rgb, intensity);

  // ...

Set the fragment's color to the fog color and the fragment's alpha channel to the intensity. As intensity gets closer to one, you'll have less of the scene's color and more of the fog color. When intensity reaches one, you'll have all fog color and nothing else.

// ...

uniform sampler2D baseTexture;
uniform sampler2D fogTexture;

  // ...

  vec2 texSize  = textureSize(baseTexture, 0).xy;
  vec2 texCoord = gl_FragCoord.xy / texSize;

  vec4 baseColor = texture(baseTexture, texCoord);
  vec4 fogColor  = texture(fogTexture,  texCoord);

  fragColor = baseColor;

  // ...

  fragColor = mix(fragColor, fogColor, min(fogColor.a, 1));

  // ...

Normally you calculate the fog in the same shader that does the lighting calculations. However, you can also break it out into its own framebuffer texture. Here you see the fog color being applied to the rest of the scene much like you would apply a layer in GIMP. This allows you to calculate the fog once instead calculating it in every shader that eventually needs it.

3D Game Shaders For Beginners

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