const float FLAT_RADIUS          = 64.0;
const float WORLD_RADIUS         = 256.0;
const float WORLD_RADIUS_SQUARED = WORLD_RADIUS*WORLD_RADIUS;

void main() {
    vec4 position = gl_ModelViewMatrix * gl_Vertex;

    if (gl_Color.a != 0.8) {
        // Not a cloud.

        float dist = sqrt(position.x*position.x + position.y*position.y + position.z*position.z) - FLAT_RADIUS;

        if (dist > 0.0f) {
            dist *= dist;
            position.y -= WORLD_RADIUS - sqrt(max(1.0 - dist / WORLD_RADIUS_SQUARED, 0.0)) * WORLD_RADIUS;
        }

    }

    gl_FrontColor = gl_BackColor = gl_Color;
    gl_Position = gl_ProjectionMatrix * position;
    gl_TexCoord[0] = gl_MultiTexCoord0;
    gl_FogFragCoord = gl_Position.z;
}