Re: Area and pixel count in Photoshop vs ImageJ

Posted by Gabriel Landini on Jun 09, 2015; 8:04am
URL: http://imagej.273.s1.nabble.com/Area-and-pixel-count-in-Photoshop-vs-ImageJ-tp5013084p5013086.html

On Monday 08 Jun 2015 16:04:44 you wrote:

> Here are results between Photoshop and ImageJ:
>
> In Photoshop, the measurement log shows that the Area of a 100x100 circle
> is 5720.  In my histogram, it reads "Pixels: 5640 pixels."  The measurement
> scale is set to 1 pixel = 1 pixel.
>
> My project involves counting the number of green pixels in an image and
> dividing that number by the total number of pixels in the ROI, in order to
> get the percentage of vegetation.  The number shown in the histogram of
> Photoshop next to Pixels is the value I've been using as the total number
> of pixels.  Since starting to look into ImageJ, I'm worried that this
> method is not accurate.
>
> In ImageJ, I removed the scale and get the correct area of 7860.
>
> I am hoping to switch over from using Photoshop to ImageJ, but I need to
> first understand why the numbers I get in Photoshop are so different.

Most likely PS is not strictly counting pixels (some area algorithms measure ,
for example, the area inside the boundary pixels), or your image was
calibrated or PS interpolates the boundary.

IJ is correct, I also get 7860 "in number of pixels", but the area inside the
circle I get 7719 "pixels squared "according to the Freeman chain encoding
algorithm implemented in Particles8.
There are more ways of counting lengths and areas to compensate for the
discretisation of the lattice and also various ways of deciding what is the
boundary of an object.
They are not wrong, it just depends what definitions you use.

Cheers
Gabriel

--
ImageJ mailing list: http://imagej.nih.gov/ij/list.html