GLSL Metaball shader (ishidex2)

void mainImage( out vec4 fragColor, in vec2 fragCoord )
{
    float cx = 420.0;
    float cy = 200.0;
    float r = 40.0;

    float cx2 = iMouse.xy.x;
    float cy2 = iMouse.xy.y;
    float r2 = 40.0;


    float dist = sqrt(pow(fragCoord.x-cx, 2.0)+pow(fragCoord.y-cy, 2.0));
     float dist2 = sqrt(pow(fragCoord.x-cx2, 2.0)+pow(fragCoord.y-cy2, 2.0));
    fragColor = vec4(0.0,0.0,0.0,1.0);

    fragColor += vec4(1.0/dist*r,1.0/dist*r,1.0/dist*r,1.0);
    fragColor += vec4(1.0/dist2*r2,0.9/dist2*r2,0.5/dist2*r2,1.0);

}