Acos() not giving angles between PI/2-3PI/2

Processing Coding Questions
1

I am making a tank game, using the following to determine the angle from the tank to the cursor
rotate(atan(((mouseY-height/2)*scalar)/((mouseX-width/2)*scalar)));
then drawing my cannon within it (for shortening purpouses its just a rect)
rect(0,-5,30,10);
Then undoing the rotation with the negative of the previously mentioned rotate

Problem is, whenever the angle is from pi/2 to 3pi/2 (quadrants 2 and 3), it gives me the inverse.
To better show this I have made a pointer from where the tank is, to where the cursor is with a line:
ellipse((mouseX-width/2)*(scalar),(mouseY-height/2)*(scalar), 5,5);
line(0,0,(mouseX-width/2)*(scalar),(mouseY-height/2)*(scalar));
When the line is intersecting the rect, it is working correctly

here is a working copypasta that shows my problem

void setup(){
  fullScreen();
}

void draw(){
  background(255);
  translate(width/2, height/2);
  scale(height/540.0);
  
  ellipse(0,0,20,20);
  rotate(atan(((mouseY-height/2)*(540.0/height))/((mouseX-width/2)*(540.0/height))));
  rect(0,-5,30,10);
  rotate(-atan(((mouseY-height/2)*(540.0/height))/((mouseX-width/2)*(540.0/height))));
  ellipse((mouseX-width/2)*(540.0/height),(mouseY-height/2)*(540.0/height), 5,5);
  line(0,0,(mouseX-width/2)*(540.0/height),(mouseY-height/2)*(540.0/height));
}
1 Like
2

I suggest you try using atan2 rather than atan to get the angle for rotation. The code below seems to work

void setup(){
  fullScreen();
}

void draw(){
  background(255);
  translate(width/2, height/2);
  scale(height/540.0);
  
  ellipse(0,0,20,20);
  rotate(atan2(((mouseY-height/2)*(540.0/height)), ((mouseX-width/2)*(540.0/height))));
  rect(0,-5,30,10);
  rotate(-atan2(((mouseY-height/2)*(540.0/height)), ((mouseX-width/2)*(540.0/height))));
  ellipse((mouseX-width/2)*(540.0/height),(mouseY-height/2)*(540.0/height), 5,5);
  line(0,0,(mouseX-width/2)*(540.0/height),(mouseY-height/2)*(540.0/height));
}
4 Likes
3

In computing we don’t use tan to calculate the angle because it can produce illegal results… Consider your line
rotate(atan(((mouseY-height/2)*(540.0/height))/((mouseX-width/2)*(540.0/height))));

If mouseX == width/2 then we have
rotate(atan(((mouseY-height/2)*(540.0/height))/ 0 ));

i.e. division by zero.

this is not a problem with atan2 because we are not doing the division, we have
rotate(atan2(((mouseY-height/2)*(540.0/height)) , 0 ));

4 Likes