angle between 2 points (c++)

[citpes]

is there a function in c/c++ to get the angle between 2 points? if so, what is it? if not, how can i write one? i assume it will use trig but my maths is a bit rusty.
Help?? :???:

[Ninkazu]

two points is a line... you'd need a third to get an actual angle.

Or are you talking about something like this?

Code:

.


              .
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where the line is the base and the two dots are the intersecting line?

You can just take the distance formula of the two dots, and a random dot on the base. Say, the dot near the bottom is A, the dot on the base is B, and the one in the air is C.

Code:

AC = SQRT((Ax - Cx)^2 + (Ay - Cy)^2)
AB = SQRT((Ax - Bx)^2 + (Ay - By)^2)

Angle = 1/cos(AB/AC)

I think...

[xhantt]

I can't understand your question, but the atan y atan2 should be the function you are looking for.
Look at the manual for further examples about this.

[citpes]

as i said, my maths is rusty. i think it would go something like this

Code:

x1=10;y1=10;
x2=100;y2=60;

xdif=x2-x1;
ydif=y2-y1;

i know tan theta = opposite/ajacent
thus theta = arctan opposite/ajacent
thus theta = arctan xdif/ydif

if that correct. the machine im using dosnt have a compiler on it so i cant test it. is the atan function the arc tan im looking for. or is there a easier way to do this (speed is always good!)

EDIT:
Im looking for the equivelent to RelLibs RelAngle. its for calculating x and y velocitys and trajectories in my game.

[Ninkazu]

I'm pretty sure my way would work. I don't know what you're trying to do though.

[SCM]

citpes wrote:

i know tan theta = opposite/ajacent
thus theta = arctan opposite/ajacent
thus theta = arctan xdif/ydif

Your in the right ballpark

Code:

theta = arctan (ydif/xdif)             (the adjacent is xdif)

This will give you an answer from -pi/2 to pi/2. If you want an angle from 0 to pi, add pi to the negative values. If you want values from 0 to 2pi, then add pi when xdif is negative and also when ydif is negative (this totals 2pi when both are negative).

Ninkazu,
try your method with some points. If I understand what you are trying to do, I don't think it will work.
For example if A =(0,0), then if you try it with B = (Bx,By) and with B = (By,Bx) you will get the same answer both times.

[relsoft]

Allegro: Atan2 or was it fatan?


If all else fails...


translate this to c. ;*)

Code:

FUNCTION Rel.Angle% (X1, Y1, X2, Y2)
'Parameters:
'x1,y1= the starting coord
'x2,y2=the target coord
'Returns=the Angle in degrees between the two coords
'useable with SIN and COS so this is useful for calculating the
'vectors between 2 points.

'Rad=Deg*PI/180
'Rad*180=Deg*PI
'Rad*180/PI=Deg
'Deg=Rad*180/PI

IF X1 = X2 THEN
IF Y1 < Y2 THEN
Rel.Angle% = 90
EXIT FUNCTION
ELSEIF Y1 > Y2 THEN
Rel.Angle% = 270
EXIT FUNCTION
ELSE
Rel.Angle% = 0
EXIT FUNCTION
END IF
ELSE
DeltaX = X2 - X1
DeltaY = Y2 - Y1
AngleTemp% = ATN(DeltaY / DeltaX) * 180 / 3.141593
AngleTemp% = AngleTemp% + 90
IF X1 > X2 THEN
AngleTemp% = AngleTemp% + 180
END IF
END IF
AngleTemp% = AngleTemp% - 90
IF AngleTemp% < 0 THEN AngleTemp% = AngleTemp% + 360
Rel.Angle% = AngleTemp%


END FUNCTION

If you want the ASM version, just ask .;*)

BTW, if you're doing homing missiles, ATAN2 and FATAN is not a good choice. Use a modified dot product.

[DrV]

That's just atan2 written out using atan... it's built into the standard C/C++ library...

[citpes]

thanx, rel. ill try it later. the asm source would be useless to me, i hardly understand the qbasic source :lol:
whats all this ruckess bout some atan2 thing. its all gone straight over my head!

[Hard Rock]

This is what you want. Print it out and sleep with it under your pillow, treat it like your best friend, trust me you'll thank me after you read it:

This sine and cosine tut for game coders:
http://www.pixelate.co.za/issues/5/5/art...sincos.htm

A description of the article

This article will explain sine, cosine, vectors, atan2, and some useful special effects such as how to make homing missiles and how bitmap rotation works

'Nuf said.