how to find visual angle

I have these three points(red dots) ,and I have to find angle, I am using polyfit function to calculate angle(finding slope first).

Now angle I am getting is near to 87 degrees, which is correct . But the thing is, I want to find the visual angle, which in this case will be near to 30 degrees. Any idea how to proceed with this? Thanks

[Walter Roberson]

What are you calculating the angle between? Is the 87 degrees the angle from the first red dot to the second? From the second to the third? And which is the "visual" angle?

I suspect the "visual angle" might be atan2((y3-y1), (x3-x1)) in radians

[smriti marathe]

I used polyfit function for a-b and b-c, results is, two slopes, from which I can find angle. (For polyfit function I used all the points between a-b and b-c).After getting slops, i used slope formula to find angle. Which gave 87. visual angle is angle which will be seen after plotting an image (meaning, in this image angle is seen near to 30 but when we do actual calculations using data points, it is near to 87, I think I will have to rescale the plot according to screen resolution, but have no idea how to proceed with it)

[Walter Roberson]

Which angle are you calculating as 87? (a-b) to (b-c) ? There is nothing that to me looks almost 90 degrees. But dot a- dot c- dot b might be maybe 115 degrees or so ?

Answer 1

[Roger Stafford]

The true angle between ba and bc would be:

   ang = atan2( abs((xa-xb)*(yc-yb)-(ya-yb)*(xc-xb)) , ...
                   (xa-xb)*(xc-xb)+(ya-yb)*(yc-yb) );

Your plot without an 'axis equal' restriction will distort these angles.