Easiest way to measure angle in picture ?

Question

I am doing some physics research for school and I need to measure angle between two points in pictures on my PC. How to do this? The only app I found was Screen Protractor but it costs 30$.

Answer 1

Install the GIMP (the freeware GNU Image Manipulation Program): http://www.gimp.org/



That has a built-in Measure tool that will find angles from an image (nominally to 0.01 degrees).



See help for Measure Tool: http://docs.gimp.org/en/gimp-tool-measure.html

Answer 2

1

Answer 3

Two points does not define an angle. Three points does....



Anyway, if you are trying to determine angles, and you can't use a protractor, use the law of cosines:



c² = a² + b² - 2abcosγ, where γ is the angle opposite side "c." So, if you can measure the length of two sides that define the angle you are looking for, then you can figure out the angle.



Got it?

Answer 4

Why not put it into paint?



Then note points on the two lines and the point where the lines meet.



This gives you two vectors - you can then use the dot product method to work out the angle.



(The vectors have two values y = Y1-Yo and x = X1-Xo for the two lines. You then multiply the two y values together and the two x values together and divide the sum by the product of the lengths of each of the two lines (given by sqrt([Y1-Yo]^2 + [X1-Xo]^2)) for each of them. The ratio is the cosine of the angle between the lines.



If you don't know vectors then you can work out the three lengths of a triangle, again by selecting appropriate points and use the cosine rule (a^2 = b^2 +c^2-2bc cos(a))