Shaderoo: realtime ballpoint effect(shaderoo.org)

    // calculate velocity change
    vec2 dvel=vec2(0);

    float dir = (float(p.idx%2)*2.-1.); // every 2nd particle is bent left/right
    
    // vel tends towards gradient
    dvel -= 10.*gu*(1.+sqrt(gl*2.))*sc;
    
    // vel tends towards/away from normal to gradient (every second particle)
    dvel -= 20.*N(gu)/(1.+1.*sqrt(gl*100.))*sc*dir;
    
    // vel bends right/left (every second particle)
    dvel += .06*N(p.vel)/(1.+gl*10.)*dir;