Easiest way to map 0 to 360 degrees with 0 at 12 o’clock

I want to make an interface for rotation when mouseX, mouseY are rotated around a circle it will output 0 - 360 degrees with 0 being at 12 o’clock. I have tried a few ways to do this but it’s not working.

This is the closest I have gotten but it puts 0 degrees at 9 o’clock.

Can someone please help me? At the map function I’d like angleDeg to be assigned 0-360 degrees, with 0 becoming the value when the line is at 12 o’clock.

I’ve been searching the forums and the internet for a few days and can not find an explanation of how to do this.

void draw() {
   
   PVector mouse = new PVector(mouseX, mouseY);
   PVector center = new PVector(width/2, height/2);
   mouse.sub(center);
   
   float angleRad = mouse.heading();
   angleDeg = map(degrees(angleRad), -180, 180, 0, 360);
   println(angleDeg);
   
   mouse.normalize();
   mouse.mult(100);
   
   pushMatrix();
   translate(width/2, height/2);
   line(0, 0, mouse.x, mouse.y);
   popMatrix();
}

Hi ddownn,

You want the values to increase clockwise, so 9 o’clock is 270 degrees, right? Floor modulo (as opposed to truncation modulo, the %) may help you simplify the angle values you receive from heading or atan2, since it takes the negative values out of the calculation. Since 0 degrees rotation is 3 o’clock, you have to take an extra HALF_PI (90 degrees) into account.

blah

void draw() {
  background(255);
  PVector mouse = new PVector(mouseX, mouseY);
  PVector center = new PVector(width * 0.5, height * 0.5);
  PVector diff = PVector.sub(mouse, center);

  float angleRad = diff.heading();
  float angleDeg = degrees(floorMod(angleRad + HALF_PI, TWO_PI));
  
  pushMatrix();
  translate(center.x, center.y);
  rotate(angleRad);
  stroke(127);
  line(0, 0, 100, 0);
  fill(0);
  text(angleDeg, 15, 0);
  popMatrix();
}

static float floorMod(float a, float b) {
  return a - b * floor(a / b);
}

Hope that helps!

1 Like

Yes, it really does help, thank you. And thanks for the link. It’s going to take me a while to wrap my mind around why it works, but at least I know where to start. I knew I had to do something with HALF_PI.

Couldn’t you also add 90 degrees to the angle to “convert” it to “0 degrees is 12:00” space?

That’s the first thing I tried, adding 90 degrees counts from 0 at 12:00 to 270 at 9:00, but leaves 9:00 to 12:00 counting down from -90 to 0.
Then I tried the map function to get a continuous 0-360 count, but couldn’t find a way to change the 0 position.

void setup() {
   size(500, 500);
}

void draw() {
   background(255);
   PVector mouse = new PVector(mouseX, mouseY);
   PVector center = new PVector(width * 0.5, height * 0.5);
   PVector diff = PVector.sub(mouse, center);
   float angleRad = diff.heading();
   float angleDeg = degrees(angleRad);
   
   angleDeg = addNinety(angleRad);
   
   //angleDeg = mapFirst(angleRad);
   
   //angleDeg = degrees(floorMod(angleRad + HALF_PI, TWO_PI));
   
   pushMatrix();
   translate(center.x, center.y);
   rotate(angleRad);
   stroke(127);
   line(0, 0, 100, 0);
   fill(0);
   textSize(24);
   text(angleDeg, 25, 0);
   popMatrix();
}
float addNinety(float a) {
   a = degrees(a) + 90;
   return a;
}
float mapFirst(float a) {
   a = map(degrees(a), -180, 180, 0, 360);
   return a;
}

static float floorMod(float a, float b) {
   return a - b * floor(a / b);
}

A question about this, what does adding the “static” to this function do?

Processing.org/reference/static.html

It means floorMod() can be directly called by other classes w/o needing to instantiate the class which floorMod() method belongs to. :calling:

It also means method floorMod() can’t directly access any non-static members of its own class. :door:

Thanks, that makes sense