aurora_effect

/*
Aurora Effect Frag Shader

by Eddie Lee (twitter: @eddietree)
http://illogictree.com
Jan 30, 2012
*/

#version 110

varying vec2 uv;

uniform sampler2D tex;
uniform vec4 Misc;
uniform vec4 Color;

const float PI = 3.14159;

void main(void)
{
    float time = Misc.x;        // current time
    float timeOffset = Misc.y;  // if multiple auroras, have time offset
    float direction = Misc.z;   // direction that the effect is fading

    // fetch perlin height
    vec4 perlinTex =  texture2D(tex, vec2(uv.x, time*0.00003+timeOffset) );
    float height = perlinTex.x;
    discard( height < uv.y );

    // gentle fading
    float alphaBuffer = 0.3; // buffer from edge
    float alpha = smoothstep(0.0, height*(1.0-alphaBuffer), uv.y );
    alpha *= (1.0-smoothstep( height*(1.0-alphaBuffer*1.5), height, uv.y ));
    alpha *= smoothstep(0.0, alphaBuffer, uv.x);
    alpha *= 1.0-smoothstep(1.0-alphaBuffer, 1.0, uv.x);
    alpha *= sin( PI*2.0*(uv.x+timeOffset)+direction*time*0.0009)*0.5+0.5;
    alpha *= 0.2f;

    vec3 auroraColor = Color.xyz;
    gl_FragColor = vec4(auroraColor, alpha);
}