Can anyone help?? I am trying to measure the length of a line drawn using c++ between two points. does anyone know an easy way of doing this?/ The result needs to be in centimeters and pixels.

If (x1, y1) are the Cartesian coordinates of point number 1 and (x2, y2) are the Cartesian coordinates of point number 2, then the distance between them is given by the following expression (according to the Pythagorean Theorem):


The result is in the same units as the coordinates: pixels, cm, fathoms, furlongs, etc.

Regards,

Dave

Thanks very much!! I dont know whether you know if there is an easy way to convert pixels to centimeters, or is this not possible??

You can't convert values expressed in one unit system to another without knowing a scale factor that connects the systems.

You keep asking for an "easy way". I don't know how "easy" it is, but there's always a way (assuming you have defined your terms precisely, and that you have enough information to determine a scale factor).


If by "pixels" you mean the dots on a screen, then you must know the screen resolution, usually expressed (in the U.S. at least) in DPI (dots-per-inch).

If by "pixels" you mean dots printed by an inkjet printer, then you must know the print resolution, say, in DPI.

So, for a screen that is 15 inches wide and is operating with a resolution of 1024x768, the DPI can be found by:

ScreenDPI = 1024/15 ~ 68.2 dots/inch

Then a line that is, say, 35 pixels long (from the Pythagorean formula, using x,y coordinates expressed in screen pixels) will have the following length on screen:

LengthInInches = NumberOfDots / DotsPerInch

LengthInInches = 35/68.2 ~ .51 inches

To convert inches to centimeters:

NumberOfCentimeters = NumberOfInches / NumberOfInchesPerCentimeter

or

NumberOfCentimeters = NumberOfInches * NumberOfCentimetersPerInch

Here's a way to look at unit conversions in general.

We used to call this dimensional analysis (I don't know if the expression is still used). Let #cm be the number of cm and #in be the number of inches.

#cm = #in * #cm/#in

where #cm/#in = 2.54 cm/inch

or

#cm = #in / (#in/#cm)

where #in/#cm ~ .3937 inches/cm

Regards,

Dave