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# 3D Vector Math Equations

Should be a simple 500 points for the geometry geeks in here.

I have a point located at a vector in 3D space.  (X,Y,Z)
I have a "direction" for that vector kept in degrees/radians another vector (A,B,C)
I have a second point located in the distance (D,E,F)

I need the formulas to do the following:

1.) What is the distance between XYZ & DEF?
2.) What is the find the "angle" between XYZ and DEF (and thus define ABC so that XYZ is facing DEF).
3.) What is the formula to "move" the point XYZ "forward" a set amount (N) based on it's current "direction" (ABC)?

These aren't for school.  I'm trying to do a simple 3D game, and sucked at Geometry 15 years ago.  More will no doubt follow.

-Javin

Game ProgrammingMath / Science Last Comment
Javin007

8/22/2022 - Mon
TommySzalapski

Distance uses Euclid's formula dist = sqrt( (X-D)^2 + (Y-E)^2 + (Z-F)^2 )
The 'angle between' can be expressed as the vector <D-X, E-Y, F-Z>
So let's say the speed is 5 units per iteration. Then dist/5 gives the number of iterations that should be required so <X,Y,Z> at iteration 1 becomes <X + (D-X)*5/dist, Y + (E-Y)*5/dist, Z + (F-Z)*5/dist>
TommySzalapski

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Javin007

1.) What is the distance between XYZ & DEF?
"Distance uses Euclid's formula dist = sqrt( (X-D)^2 + (Y-E)^2 + (Z-F)^2 )"

So the formula is:

D = Sqrt( (X-D)^2 + (Y-E)^2 + (Z-F)^2 )

I assume this is derived from the Pythagorean theorem? (A^2 + B^2 = C^2)

2.) What is the find the "angle" between XYZ and DEF (and thus define ABC so that XYZ is facing DEF).
"The 'angle between' can be expressed as the vector <D-X, E-Y, F-Z>"

That's not what I need.  I need the angle of ABC to be in Degrees or Radians, as I stated here:
"I have a "direction" for that vector kept in degrees/radians another vector (A,B,C)"

Thus, the formula that follows wouldn't quite work.  Though I'd imagine there's some quick conversion formulas that can take the ABC (in rad/deg, which means C isn't really important, since it's the "roll" angle) and convert it to a vector?

3.) What is the formula to "move" the point XYZ "forward" a set amount (N) based on it's current "direction" (ABC)?
"So let's say the speed is 5 units per iteration. Then dist/5 gives the number of iterations that should be..."

I'm pretty lost on this one.

Assuming I have my point of origin:  XYZ
Assuming I have my current angel in Rad/Deg: ABC
Assuming I have a Speed: S
Assuming I have a "Time" traveled: T

What is the formula to find my new point: DEF
TommySzalapski

Yes, Euclidean distance is derived from the Pythagorean.

I made it easier for you. Trying to express a 3D angle requires two angles, the angle in the xy plane and the angle above the plane. Then you have complex trigonometry to get the angle and to get the movements.

Forget ABC. You have:
Point of origin (X,Y,Z)
Travel vector <D-X, E-Y, F-Z>
Speed S
Time T

Your new point will be (X + (D-X)*S*T/dist, Y + (E-Y)*S*T/dist, Z + (F-Z)*S*T/dist)
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Javin007

I can't "forget ABC."

At some point during the travel, the rotation of, say, the X axis may change.  If it changes, I no longer know what my "look at" point is with your method.
Javin007

The problem here is that you're assuming I know DEF, which I do not.  I would only know DEF for the first two questions, but you've skipped an answer for #2.
TommySzalapski

Yes you will. That's the beauty of it. Travel vector <D-X, E-Y, F-Z> will rotate seamlessly with points (X,Y,Z) and (D,E,F).
Travel vector <D-X, E-Y, F-Z> is simply expressing the angle in Cartesian coordinates instead of spherical
coordinates.
However, if you want it in spherical coordinates, here goes. Angle in xy plane (theta) is arctan((E-Y)/(D-X)). Angle above xy axis (phi) is arctan( (F-Z)/sqrt( (D-X)^2 + (E-Y)^2 ) )
And you just do than in reverse to get the travel vector back to plug into the equation for movement.
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TommySzalapski

If you know ABC you can get DEF and vice-versa.
TommySzalapski

I think I see what you are doing. Take the travel vector and scale it to have a magnitude of 1.

So for question 1 we have the same dist as before.

For question 2 we get the unit vector to face XYZ toward DEF.
<A,B,C> = <(D-X)/dist, (E-Y)/dist, (F-Z)/dist>
Now you just rotate <A,B,C> and (X,Y,Z) with your axis.

Question 3 then is that the position at time T with speed S is
(X + A*S*T, Y + B*S*T, Z + C*S*T)
ozo

How are you storing your (A,B,C) direction?  Are they direction cosines?
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TommySzalapski

No, it's a Cartesian unit vector. Or are you asking Javin?
ozo

A cartesian vector is how I would keep it, but how is that  "kept in degrees/radians"?
TommySzalapski

Like I said, if he really wants it in angles he can use spherical coordinates with
theta = arctan((E-Y)/(D-X))
phi = arctan( (F-Z)/sqrt( (D-X)^2 + (E-Y)^2 ) )
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