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Average of two vectors


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#1 BradDaBug   Members   -  Reputation: 904

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Posted 04 May 2002 - 09:30 AM

I have this sprite, and it has a motion vector. I want it to randomly wander around, so every 500 ms or so it randomly creates a new vector. I want it to slowly move from the current vector to the new vector, so it looks smooth, not erratic. Right now I''m just finding the average of the X and Y components of the vectors and setting the motion vector to that, like this:
  	
if (SDL_GetTicks() - TimeAtLastVectorChange > 200)
{

TimeAtLastVectorChange = SDL_GetTicks();

MovementVector.SetLength(200);	//200 pixels per second!

MovementVector.SetAngle(rand() % 360);

}

//Add the new vector and the old vector

int x = MovementVector.GetX();
int y = MovementVector.GetY();

int oldx = CurrentVector.GetX();
int oldy = CurrentVector.GetY();

CurrentVector.SetX((oldx + x) / 2);
CurrentVector.SetY((oldy + y) / 2);

  
It doesn''t seem to be working. The sprite still jerks around each time the vector changes. What''s wrong with it?

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#2 Oluseyi   Staff Emeritus   -  Reputation: 1678

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Posted 04 May 2002 - 09:39 AM

Lookup parametric interpolation.

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Thanks to Kylotan for the idea!


#3 LilBudyWizer   Members   -  Reputation: 491

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Posted 04 May 2002 - 09:58 AM

Try (1-t)*u+t*v while varying t from 0 to 1.

#4 BradDaBug   Members   -  Reputation: 904

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Posted 04 May 2002 - 10:05 AM

What does that give me? X? Y? i don''t see right off how to use it.

#5 davepermen   Members   -  Reputation: 1032

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Posted 04 May 2002 - 10:12 AM

u is startpos (vector)
v is endpos (vector)
t is time (float)

(1-t)*u + t*v

for t = 0 results in (1-0)*u + 0*v = u => you''re at the startpos
for t = 1 results in (1-1)*u + 1*v = v => you''re at the endpos
for t = .5 results in .5u + .5v => you''re in the middle..

this function is often called lerp

vector lerp(vector start,vector end,float t) {
return (1-t)*start + t*end;
}

lerp is for Linear intERPolation.



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#6 CGameProgrammer   Members   -  Reputation: 640

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Posted 04 May 2002 - 10:24 AM

Those lerp functions look like they deal more with position than velocity - they''re just a way to find what the current position should be when you know the start position, the end position, and the time elapsed. Not real appropriate for your situation, as far as I can tell, Brad.

That is, assuming CurrentVector is the current movement vector and MovementVector is the new movement vector. Right? I think the previous posters assumed CurrentVector was position and MovementVector was movement. Before I go on, you should clarify this.

~CGameProgrammer( );



#7 BradDaBug   Members   -  Reputation: 904

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Posted 04 May 2002 - 10:26 AM

CurrentVector is where the sprite is moving right now. MovementVector is the new direction the sprite should move in.

[edited by - BradDaBug on May 4, 2002 5:27:08 PM]

#8 Oluseyi   Staff Emeritus   -  Reputation: 1678

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Posted 04 May 2002 - 10:28 AM

quote:
Original post by CGameProgrammer
Those lerp functions look like they deal more with position than velocity

You can interpolate vector quantities, which makes them appropriate for the velocity as well.

[ GDNet Start Here | GDNet Search Tool | GDNet FAQ ]
[ MS RTFM [MSDN] | SGI STL Docs | Boost ]
[ Google! | Asking Smart Questions | Jargon File ]
Thanks to Kylotan for the idea!





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