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## Average of two vectors

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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?

### #2Oluseyi  GDNet+

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Posted 04 May 2002 - 09:39 AM

Lookup parametric interpolation.

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Thanks to Kylotan for the idea!

### #3LilBudyWizer  Members

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Posted 04 May 2002 - 09:58 AM

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

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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.

### #5davepermen  Members

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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.

"take a look around" - limp bizkit

### #6CGameProgrammer  Members

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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( );

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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]

### #8Oluseyi  GDNet+

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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.

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[ MS RTFM [MSDN] | SGI STL Docs | Boost ]