   Parabola
  Hi, Given a 3D point P and a direction vector D how can i trace a parabola from P going in the direction of D. I want the parabola to go up the Y axis first and then down it. I know that in the 2D case parabolas are easy to calculate but i''m having problems in 3D.  
  well a vector has a direction and the direction of a parabola changes...so therefore the direction of the vector must change as well or you are no longer drawing a parabola in the direction of the vector..........but i digressi know what you are doing and what kind of game you are making just from that questionor at least a best guestyou are making some type of 3d scortched earth gameand you have a yaw and pitch that the firing mechanism is facing.....now you want to fire with an amount of force in the direction you are facing...hence the vectornow you want the particle you fire to initially be travelling in the direction of the vector but of course gravity   is bringing it back down at a fixed rate of acceleration.now what you want to do is google something like "physics position equations"" or "equations of gravity"they will give you equations to calculate positions based on initial position, initial velocity and acceleration based in terms of time elapsedie where you are, initial velocity of the projectile, and gravity.good luck on your game  "A soldier is a part of the 1% of the population that keeps the other 99% free"  - Lt. Colonel Todd, 1/38th Infantry, Ft. Benning, GA ;  Actually its the path i plan on making a grenade follow when fired from a grenade launcher in a FPS. But now i digress. I want the parabola to have its first point at position P and be plotted moving away from P in the direction of D. I really don''t see a need for any heavy physics caluclations here I just need to know how to keep the parabola in the plane along which the  direction vector is pointing. Any ideas? ;  no physics equations?  that is exactly what the flight path of the grenade is following.....seriously you have to look up the equations with position and accelerationlooking for an equation of gravity will make it easier on youor do thisfloat time_elapsed_since_last_frame = figure_it_out_yourself;pos_x += velocity_x * time_elapsed_since_last_frame;pos_y += velocity_y * time_elapsed_since_last_frame;velocity_y -= SOME_GRAVITY_CONSTANT *    time_elapsed_since_last_frame;// constantly check that the pos_y is < terrain_height and// if true then it has hit the ground   "A soldier is a part of the 1% of the population that keeps the other 99% free"  - Lt. Colonel Todd, 1/38th Infantry, Ft. Benning, GA ;r = (1/2)at2 + v0t + r0a = (0, 0, -9.81 m/s2), assuming the z-axis is vertical.v0 is the initial velocity.r0 is the initial position. ;  Thx Terran, BTW NCSU, gravity has nothing to do with plotting a parabola and neither do physical forces such as acceleration etc  ;  hahahha you gotta be kidding me manwhat do you think thea = (0, 0, -9.81 m/s2), assuming the z-axis is verticalis in the equations in the post above thisthose are physics equations for a body in motion   and the a is acceleration and the -9.81 m/s^2 is the acceleration of gravity   but you are right about in the math world you can have a parametric equation or other equation for a parabola that doesnt directly have "forces" but i guarantee you that everyone uses those physics equations for stuff like thrown grenades in their game as in the real world it follows those equations and not the equation of a parabola.....although they are nearly the same"A soldier is a part of the 1% of the population that keeps the other 99% free"  - Lt. Colonel Todd, 1/38th Infantry, Ft. Benning, GA   [edited by - ncsu121978 on February 6, 2004 10:16:34 AM]