Let me have three points, and the two lines from one of them, the vertex point, to the others two.

How do I calculate the angle between the two lines?

I am trying not to use PVectors.

use atan2 (remark below)

```
//https://discourse.processing.org/t/how-do-i-measure-the-angle-of-a-vertex/34511
//https://processing.org/reference/atan2_.html
void setup() {
size(999, 999);
}
void draw() {
background(204);
checkPoint(100, 100);
checkPoint(555, 120);
noLoop();
}
//------------------------------------------------------------------------
void checkPoint(float x1, float y1) {
// 450,450 is the same for all angles.
// x1,y1 is the point we want get the angle from
ellipse(450, 450, 19, 19);
ellipse(x1, y1, 19, 19);
line(x1, y1,
450, 450);
pushMatrix();
translate(450, 450);
float angle = atan2(y1-450, x1-450); // use y,x here, see https://processing.org/reference/atan2_.html
println(degrees(angle));
rotate(angle);
rect(-30, -5, 60, 10);
popMatrix();
//
}
```

**Remark**

Ah, I just realized, `atan2()`

sucks. And maths too.

- atan2 gives you often negative values imho
- processing has it’s 0° degrees to the east (and NOT north)

here is a quick work-around:

```
//https://discourse.processing.org/t/how-do-i-measure-the-angle-of-a-vertex/34511
//https://processing.org/reference/atan2_.html
void setup() {
size(999, 999);
}
void draw() {
background(204);
pushMatrix();
translate(width/2, height/2);
float a = atan2(mouseY-height/2, mouseX-width/2);
rotate(a);
rect(-30, -5, 60, 10);
if (a<0) {
//a=a-PI;
a=map(a, -PI, 0, PI, TWO_PI);
}
a+=HALF_PI;
if (a>TWO_PI) {
a-=TWO_PI;
}
popMatrix();
text(degrees(a), mouseX+13, mouseY);
}
```

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Yes, nice advise. I’ll let you know. Thanks. Cheers.

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Dor Product of normed vectors is cos of angle.

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