Chief Delphi - Angle from X,Y

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Chief Delphi (http://www.chiefdelphi.com/forums/index.php)

- Programming (http://www.chiefdelphi.com/forums/forumdisplay.php?f=51)

- - Angle from X,Y (http://www.chiefdelphi.com/forums/showthread.php?t=27685)

Astronouth7303

10-04-2004 18:41

Angle from X,Y

How do you find the angle from (X, Y)? :confused:

And how do you optimize this for realtime on the FRC? :confused:

And One Of Astronouth's Rare Contensts: Why would I care? :D

Yan Wang

10-04-2004 18:45

Re: Angle from X,Y

Angle from (x,y) (I'm assuming on a normal Cartesian Plane) to what in relation to what as a vertex?

ie. angle between (x,y) and x-axis with origin as vertex.

Astronouth7303

10-04-2004 18:47

Re: Angle from X,Y

The angle between (0,0) and (x,y) in relation to the x axis (A horizantal line, y=0)

FotoPlasma

10-04-2004 19:04

Re: Angle from X,Y

The inverse tangent function (arctan) will give you the angle from the origin to a point. For example: given (x,y), arctan(y/x) = the angle from the origin to the point (x,y).

If you search CD for something like "arctan" or "trig" or "trigonometry", you should get a fair amount of results already covering implementation of trigonometric functions on the FRC controller.

I don't have any experience using these, myself, but I've heard that a few teams have had success with using an arbitrary degree Taylor series approximation of trig functions.

Astronouth7303

10-04-2004 20:28

Re: Angle from X,Y

Doesn't arc tangent return an angle for a given slope? As in: -90 to 90.
I'm talking about 360 degrees. Like a Rectangular to Polar converter thing.

Joe Ross

10-04-2004 20:40

Re: Angle from X,Y

Quote:

Originally Posted by Astronouth7303

Doesn't arc tangent return an angle for a given slope? As in: -90 to 90.
I'm talking about 360 degrees. Like a Rectangular to Polar converter thing.

You then look at the signs of X and Y to see if what quadrant the angle is in.

Astronouth7303

10-04-2004 20:58

Re: Angle from X,Y

:WHAP: Duh

Code:

unsigned int GetAngle(signed int X, signed int Y)

{

   signed int Angle = arctan(Y/abs(X));

 unsigned char OtherSide = IsNegetive(X);

 unsigned int Value = 0;

 

 if (OtherSide)

  Value = 180 - Angle + 360;

 else

  Value = Angle + 360;

 

 Value %= 360;

 return Value;

}

Unfortunately, you have to use a float for Angle because angles between -45 and 45 are as important as the others. I suppose I could flip X and Y around for that.

[Changed that tan to arctan]

michael_obrien

10-04-2004 22:23

Re: Angle from X,Y

I don't know much about programming, but I screwed around with winamp AVS for a while, and on that there was a function known as "atan2(x,y)" which was the same as atan but it automatically corrected for quadrants... isn't there anything simular in C? I mean, C is much more powerful than that AVS junk from what I understand, it should at least have the same capabilities.

The Lucas

10-04-2004 22:51

Re: Angle from X,Y

Quote:

Originally Posted by Astronouth7303

:WHAP: Duh

Unfortunately, you have to use a float for Angle because angles between -45 and 45 are as important as the others. I suppose I could flip X and Y around for that.

[Changed that tan to arctan]

You don't need a float to represent (Y/X). Just try ((Y*100)/X) and scale your arctangent function to reflect this. This works well if you are using a lookup table. However, I would use a float if I was using a McLaurin series. Otherwise you are likely to overload a long the when you start getting into the higher powers (100^5 is way too big). Also you might want to check to make sure X is not zero before dividing by it.

Quote:

Originally Posted by michael_obrien

IFI doen not provide us with libraries of functions like this, but C certainly has them. The standard atan function requires more prcessing and floating point math than most FIRST programmers want to put in thier RC

Matt Reiland

12-04-2004 16:29

Re: Angle from X,Y

This is the formula that LeoM posted a while back to get the angle from rectangular coordinates. Worked extremely well back in 2002

Temp1 was the positive distance in X direction
Temp2 was the positive distance in Y direction
temp3 = (temp2*127) / SQR((temp1*temp1)+(temp2*temp2)) 'Make values on a 127 unit circle

ss_degrees = ((1-(temp3/118))*(((temp3+181)*temp3+744)/769))+((temp3/118)*(((((3*temp3-707)*temp3+42611)/20))+((temp3-115)/3)))

ss_degrees = (ss_degrees*180)/128 'Convert to Degrees

Astronouth7303

13-04-2004 07:33

Re: Angle from X,Y

Ay. The problem is I don't get what it's doing.

Astronouth7303

13-04-2004 07:34

Re: Angle from X,Y

My current code is like this:

Code:

#define abs(Num) ( ((Num) < 0) ? (0 - (Num)) : (Num) )

#define IsNegetive(Num) ( ((Num) < 0) ? 255 : 0 )

#define IsPositive(Num) ( ((Num) > 0) ? 255 : 0 )







/* 360 degrees = 256

  0 =   0 (0x00)

 45 =  32 (0x20)

 90 =  64 (0x40)

135 =  96 (0x60)

180 = 128 (0x80)

225 = 160 (0xA0)

270 = 192 (0xC0)

315 = 224 (0xE0) */



unsigned char GetAngle(signed char X, signed char Y)

{

   signed int  Angle = 0;

 unsigned char OtherSide = (X < 0) ? 255 : 0;

 unsigned char Value = 0;

 unsigned char Simple = (X == 0) | (Y == 0) | (X == Y);

 

 //Perform Simple tests

 if (Simple)

 {

  if (X == 0)

   if (Y < 0)

    Angle = 192; // Down

   else if (Y > 0)

    Angle = 64; //Up

   else // Y == 0

    Angle = 0; //None, default

  

  if (Y == 0)

   if (X < 0)

    Angle = 128; // Left

   else if (X > 0)

    Angle = 0; //Right

   else // X == 0

    Angle = 0; //None, default

  

  if (X == Y)

  {

   switch( (IsNegetive(X) & 2) | (IsNegetive(Y) & 1) )

   {

    case 0: //++

     Angle = 32;

     break;

    case 2: //-+

     Angle = 96;

     break;

    case 3: //--

     Angle = 160;

     break;

    case 1: //+-

     Angle = 224;

     break;

   }

  }

 }



 //Again for goto, no braces

 if (Simple) goto Done; //I hope that's right

 

 if (X > Y) //Preserve accuracy

  Angle = 64 - atan(abs(X)/Y);

 else

  Angle = atan(abs(X)/Y);

 

 Angle = OtherSide ? (127 - Angle) : Angle;

 Value = (unsigned char)((Angle + 255) % 255);

 

Done:

 return Value;

}

Nobody's answered the last question!

Greg Ross

13-04-2004 13:35

Re: Angle from X,Y

Quote:

Originally Posted by Astronouth7303

Nobody's answered the last question! [regarding optimization]

Jamie,

The best advice I've ever heard about optimization is "Don't -- until you need to." You generally want to get your code working, and only then worry about optimizing -- if necessary.

Astronouth7303

13-04-2004 21:22

Re: Angle from X,Y

I was refeing to post 1:

Quote:

Originally Posted by Astronouth7303

And One Of Astronouth's Rare Contensts: Why would I care? :D

Greg Ross

13-04-2004 21:56

Re: Angle from X,Y

Quote:

Originally Posted by Astronouth7303

Quote:

Originally Posted by Astronouth7303

And One Of Astronouth's Rare Contensts: Why would I care? :D

I was refeing to post 1:

OK. And now I'm confused. I don't understand "And One Of Astronouth's Rare Contensts", so I don't know how to answer "Why would I care?" :confused: (I originally thought that line was part of your signature, so I just ignored it.)

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