http://imagej.273.s1.nabble.com/Perimeter-measurement-tp3696992p3696993.html
Create Mask, Create Selection). This will give you a traced
algorithm as the roi of the original particle.
> Hi Michael,
>
> Thank you very much for your explanation.
> Actually I would like to calculate convexity of particles (convex
> hull perimeter to particle perimeter). The values I have got are
> sometimes higher than 1, whereas in theory convex hull perimeter is
> not longer than the particle`s one. Perhaps, to get values not
> larger than one I have to recalculate back from improved particle
> perimeter to perimeter along the pixel boundaries?
>
> Thank you for investing your time
>
> Regards,
> Olga
>
>
> Michael Schmid-3 wrote:
>>
>> Hi Olga,
>>
>> there are many ways how to define the perimeter.
>>
>> For rectangular selections, ImageJ does take the (rectangular)
>> outline of a rectangle, so a rectangular selection of 1x1 pixel
>> has a perimeter of 1. The same for a polygon that happens to
>> be a 1x1 pixel square (e.g. the convex hull).
>>
>> For a circle with diameter 1, the outline is pi=3.14159...
>> (even though one pixel is selected and the pixel has a square
>> outline).
>>
>> If another type of selection happens to have only one pixel
>> selected, e.g. the result of clicking with the Wand tool,
>> ImageJ cuts the corners because such an operation will usually
>> result in a weird shape. If you follow the outline of such a
>> shape along the pixel boundaries, you would get a perimeter
>> that is too large for objects having sides not parallel to
>> the x or y axes.
>>
>> ---
>>
>> Apart from this, I have the impression that the perimeter for
>> composite selections is not given correctly:
>>
>> A single 10x10 square has a perimeter of 40 (rectangular) or
>> 37.65 (traced by wand). Two separate 10x10 squares have a
>> perimeter of 60, three of them have 90, etc.
>>
>> So it seems that there is a problem with the perimeter, but not
>> that of a single pixel.
>>
>> Michael
>> ________________________________________________________________
>>
>> On 5 Mar 2008, at 12:37, OlgaFishkis wrote:
>>
>>> Hello,
>>> Could someone explain me please why if I measure a square particle
>>> 1x1 sq.
>>> px the values of the area is correct (1 sq px) but the perimeter is
>>> 2.8
>>> instead of 4 px?
>>> If I apply a macro to measure a convex hull of the same square
>>> particle I
>>> get the right value of the perimeter - 4.
>>> Thank a lot
>>> Olga
>>
>>
> Quoted from:
>
http://www.nabble.com/Perimeter-measurement-tp15848177p15850553.html>