#version 100

precision mediump float;

#define MAX_SPOTS   3

struct Spot {
    vec2 pos;        // window coords of spot
    float inner;    // inner fully transparent centre radius
    float radius;    // alpha fades out to this radius
};

uniform Spot spots[MAX_SPOTS];  // Spotlight positions array
uniform float screenWidth;      // Width of the screen

void main()
{
    float alpha = 1.0;

    // Get the position of the current fragment (screen coordinates!)
    vec2 pos = vec2(gl_FragCoord.x, gl_FragCoord.y);

    // Find out which spotlight is nearest
    float d = 65000.0;  // some high value
    int fi = -1;        // found index

    for (int i = 0; i < MAX_SPOTS; i++)
    {
        for (int j = 0; j < MAX_SPOTS; j++)
        {
            float dj = distance(pos, spots[j].pos) - spots[j].radius + spots[i].radius;

            if (d > dj)
            {
                d = dj;
                fi = i;
            }
        }
    }

    // d now equals distance to nearest spot...
    // allowing for the different radii of all spotlights
    if (fi == 0)
    {
        if (d > spots[0].radius) alpha = 1.0;
        else
        {
            if (d < spots[0].inner) alpha = 0.0;
            else alpha = (d - spots[0].inner)/(spots[0].radius - spots[0].inner);
        }
    }
    else if (fi == 1)
    {
        if (d > spots[1].radius) alpha = 1.0;
        else
        {
            if (d < spots[1].inner) alpha = 0.0;
            else alpha = (d - spots[1].inner)/(spots[1].radius - spots[1].inner);
        }
    }
    else if (fi == 2)
    {
        if (d > spots[2].radius) alpha = 1.0;
        else
        {
            if (d < spots[2].inner) alpha = 0.0;
            else alpha = (d - spots[2].inner)/(spots[2].radius - spots[2].inner);
        }
    }

    // Right hand side of screen is dimly lit,
    // could make the threshold value user definable
    if ((pos.x > screenWidth/2.0) && (alpha > 0.9)) alpha = 0.9;

    // could make the black out colour user definable...
    gl_FragColor = vec4(0, 0, 0, alpha);
}