Question

On an iOs device I have drawn a line between a startPoint and a endPoint. After drawing that line I want to proceed drawing with 90 degrees from the endPoint. So I have to create a Point on the Screen which I don’t know, because the line could be in any angular.

All was programmed using the CGContextAddLineToPoint.

What is the right direction to look?

Answer 1

Call the points you know A and B, and the one you don’t C.

Then the vector from A to B is:

vec.x = B.x - A.x;
vec.y = B.y - A.y;

To rotate a 2d vector by 90 degrees, switch the components and negate one. So you could create:

rightVec.x = vec.y;
rightVex.y = -vec.x;

And then position C at :

C = B + t*rightVec;

For some non-zero t.