get width/height of two selected points in canvas

Question

I have a canvas which display a picture that The user has uploaded.What I want to do is to ask the client to give me the width and height between two points that he/she has selected on canvas in cm .For example,The client click on top left on canvas and then select top right .after that he/she should enter the width between those two selected points. How should I do that?

Sorry for my bad English.

Thanks

Answer 1

Just get the coordinates of your two points.

Let us assume the first point is called A and the second point is called B . So their coordinates would be:

A = (xa, ya)
B = (xb, xb)

So the horizontal distance between A and B would be, |xa-xb| ¹ . And the vertical distance between them would be |ya-yb| ¹ .

If we draw a rectangle using A and B as diagonally opposite points of it, that rectangle's width would be |xa-xb| , and it's height would be |ya-yb| .

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1. || represents "Absolute Value". See http://en.wikipedia.org/wiki/Absolute_value .