How do I make a particle go a certain direction relative to the player or the normal of a collision during c_trace or c_move? I understand the x y z thing, but how do I turn a point into a velocity for a particle, or use the normal of a surface to make a particle go a certain way?

I have a blood effect that I want the blood to come tward the player, and away from the player when the enemy is shot. I also want a smoke-ish particle to come off the normal of a wall, when it is shot by a c_trace.

use vec_add with vel_x and vel_y and vec_diff
about walls... use vec_to_angle(my.pan,normal); //sprite pan

From the manual (look for "effect" or "effect_local"):
Code:

---

// helper function: sets the vector to random direction   and length
function vec_randomize (&vec, range)
{
   vec[0] = random(1) - 0.5;
   vec[1] = random(1) - 0.5;
   vec[2] = random(1) - 0.5;
   vec_normalize(vec,random(range));
}

// helper function: fades out a particle
function part_alphafade()
{
   my.alpha -= time + time;
   if (my.alpha <= 0) { my.lifespan = 0; }
}

// particle function: generates a fading explosion into vel direction
function effect_explo()
{
   vec_randomize(temp,10);
   vec_add (my.vel_x, temp);
   my.alpha = 25 + random(25);
   my.bmap = scatter_map;
   my.flare = on;
   my.bright = on;
   my.move = on;
   my.function = part_alphafade; // change to a shorter, faster function
}

...
vec_scale(normal,10); // produce an explosion into the normal direction
effect(effect_explo,1000,my.x,normal);

---

It uses the last parameter of the "effect" function to set the velocity vector of the particles, to the normal-direction.

Ok, can you make the direction relative the the player part a little more clear?

I don't really understand how to use the vec_add and vec_diff...