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Angle from point A to point B

BlitzMax Forums/BlitzMax Beginners Area/Angle from point A to point B

Farflame(Posted 2009) [#1]
Seems strange that whenever I look this topic up on the net, it's often met with some rather sarcastic comments that there is no angle from point A to point B, and then some mathematical jargon :p

Anyway, let's say I have my player at 100,100 and a monster at 200,200. How do I calculate the angle from the player to the monster (I know how to move along that angle once I have it).

I suspect it's something like ATAN2(playerY-monsterY,playerX-MonsterX) but that gives some strange results. It might be correct but they just need adjusting afterwards to 0-359 degrees somehow?


GW(Posted 2009) [#2]
Function AngleTO:Float(X1:Float, Y1:Float, X2:Float, Y2:Float)
         Local dx# = x2 - x1
         Local dy# = y2 - y1
	 Return (ATan2(dy, dx)
End Function


If your getting values outside of 0-360 then use:
Function WrapAngle:Float(angle:Float)
	If angle > 0 Then
		While (angle >= 360 )
			angle :- 360
		Wend
	ElseIf angle < 0 Then
		While(angle < 0)
			angle :+ 360
		Wend
	End If
	Return angle 
End Function



_Skully(Posted 2009) [#3]
You should just be able to use the return value from a=ATAN2 in a deltax=Sin(a)*speed, deltay=Cos(a)*speed scenario


Jesse(Posted 2009) [#4]
if you are planning to shoot from the source to the target do this:

angle = atan2(destination_y - source_y, destination_x - source_x)

directionx = cos(angle)
directiony = sin(angle)

to move it in the direction desired:

x = x + directionx * speed
y = y + directiony * speed

the negative result in the angle don't matter unless you are planning to do other things with the result. if you are just trying to use the returning angle to shoot, this should suffice.

this is the same thing that everybody is saying, I just hope this helps understand it better.


Farflame(Posted 2009) [#5]
Yep, those calculations work fine, thank you :)


Farflame(Posted 2009) [#6]
A bit of a problem with this is that the angles don't work as compass angles, i.e 90 to the right, 180 is down, 270 is left, 0 is up. If I add 90 to the Atan line, it converts to a compass angle, but then the sin/cos lines don't work correctly. For example, try the code below.....

Graphics 800,600

Angle:Float=0


While Not KeyDown(Key_Escape)
	Cls
	
	Angle=AngleTo(GraphicsWidth()/2,GraphicsHeight()/2,MouseX(),MouseY())
	DrawText "Angle to mouse pointer from centre of screen : "+Angle,0,0

	DrawLine (GraphicsWidth()/2,GraphicsHeight()/2,GraphicsWidth()/2+(Cos(angle)*100),GraphicsHeight()/2+(Sin(angle)*100))


	Flip
Wend

Function AngleTO:Float(X1:Float,Y1:Float,X2:Float,Y2:Float)
	Local dx:Float=x2-x1
	Local dy:Float=y2-y1
	Local Angle:Float=(ATan2(dy,dx))+90
	
	If Angle>0 Then
		While(Angle>= 360)
			Angle:-360
		Wend
	ElseIf Angle<0 Then
		While(Angle<0)
			Angle:+360
		Wend
	End If
	Return Angle 
End Function


You'll see the angle to the mouse pointer is a compass angle but the line doesn't point towards it. I've tried every combination of +sin/-cos I can think of but none of them point to the mouse pointer. What's the solution to that?


BladeRunner(Posted 2009) [#7]
You have to do x = sin; y = -cos as the y-axis is opposite to the standard one.



Farflame(Posted 2009) [#8]
Oh, switch Sin and Cos around on X and Y? Thanks, didn't think of that :)


Jesse(Posted 2009) [#9]
ups!