CODEWITHSUNDEEP
javascriptmathtrigonometry
LeonaTheSun
LeonaTheSun

Reputation: 45

Calculate an angle when there is a log-scale in coordinates

that's problem just a question of basic math but i can't figure out where my mistake is. So i got a logarithmic x axis and a linear y axis. I have two points A(centerX, centerY) and B(x2, y2) in my plot, and I want to calculate the angle between the x-axis and vector AB.

I know the x,y value of both points, but if I compare my solution with a calculator i get the wrong value. I'm using atan2 in Javascript. Do I have to normalize my vectors first ?

function calcAngle(centerX, centerY, x2, y2) {
  var distY = y2 - centerY; 
  var distX = x2 - centerX; 
  var theta = Math.atan2(distY, distX);

  return theta*180/Math.PI; 
}

enter image description here

Upvotes: 0

Views: 260

Answers (1)

Carsten Massmann
Carsten Massmann

Reputation: 28236

If you want to get the angle as seen on your plot you will have to take into account the scaling factors for x and y and - yes - you will also have to calculate the logarithm of the x values.

Something like the following might work:

function calcAngle(centerX, centerY, x2, y2, xscale, yscale) {
  var distX = Math.log(x2) - Math.log(centerX); 
  var distY = y2 - centerY; 
  var theta = Math.atan2(yscale*distY, xscale*distX);

  return theta*180/Math.PI; 
}

Upvotes: 1

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