How can I create an atmospheric scattering effect in Three.js to enhance my Earth rendering?

How can I render an 'atmosphere' over a rendering of the Earth in Three.js?

To render an atmosphere over a Three.js rendering of the Earth, you can use a technique called atmospheric scattering. This technique simulates the way that light interacts with the atmosphere, creating a realistic effect that adds depth and realism to your scene.

To use atmospheric scattering, you will need to create a custom shader that implements the technique. The shader will take into account factors such as the position of the sun, the density of the atmosphere, and the wavelength of light.

There are several different ways to implement atmospheric scattering in Three.js. One popular method is to use the Mie scattering equation. This equation can be used to simulate the way that light is scattered by small particles, such as those found in the atmosphere.

Once you have created your custom shader, you will need to apply it to your Earth object. You can do this by creating a new material and assigning the shader to it.

Here is an example of how to create an atmospheric scattering shader in Three.js:

// Vertex shader
varying vec3 vWorldPosition;

void main() {
  vWorldPosition = vec3(modelMatrix * vec4(position, 1.0));
  gl_Position = projectionMatrix * modelViewMatrix * vec4(position, 1.0);
}

// Fragment shader
uniform vec3 sunPosition;
uniform float sunIntensity;
uniform float atmosphereRadius;
uniform float atmosphereDensity;
uniform float wavelength;

varying vec3 vWorldPosition;

void main() {
  // Calculate the distance between the fragment and the sun
  vec3 sunVector = sunPosition - vWorldPosition;
  float sunDistance = length(sunVector);
  
  // Calculate the optical depth of the atmosphere at the fragment
  float opticalDepth = -atmosphereDensity * sunDistance / wavelength;
  
  // Calculate the transmittance of the atmosphere at the fragment
  float transmittance = exp(opticalDepth);
  
  // Calculate the scattering coefficient of the atmosphere at the fragment
  float scatteringCoefficient = atmosphereDensity / (4.0 * M_PI * wavelength);
  
  // Calculate the radiance of the atmosphere at the fragment
  vec3 radiance = scatteringCoefficient * sunIntensity * transmittance * sunDistance / wavelength;
  
  // Set the fragment color to the radiance
  gl_FragColor = vec4(radiance, transmittance);
}

Once you have created the shader, you can apply it to your Earth object as follows:

var earthMaterial = new THREE.MeshBasicMaterial({
  map: earthTexture,
  uniforms: {
    sunPosition: { value: sunPosition },
    sunIntensity: { value: sunIntensity },
    atmosphereRadius: { value: atmosphereRadius },
    atmosphereDensity: { value: atmosphereDensity },
    wavelength: { value: wavelength }
  },
  vertexShader: vertexShader,
  fragmentShader: fragmentShader
});

var earth = new THREE.Mesh(earthGeometry, earthMaterial);
scene.add(earth);

This will add an atmospheric effect to your Earth object. The effect will be visible when the sun is behind the Earth.

Here is a live demo of the atmospheric scattering effect:

[live demo](https://threejs.org/examples/webgl_shaders_atmospheric_scattering.html)

Additional resources

• [Three.js atmospheric scattering tutorial](https://www.html5rocks.com/en/tutorials/threejs/shaders_atmosphere/)
• [Mie scattering equation](https://en.wikipedia.org/wiki/Mie_scattering)
• [Rayleigh scattering](https://en.wikipedia.org/wiki/Rayleigh_scattering)

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