[vox-tech] count pixels

[Alex Mandel]

Wade Pinkston wrote:
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> Hello,  I need to process some images and count the number of pixes of a
> given color.
>
> A little background.  I want to take pix of leaves against a mat
> background.  The leaves are then subjected to damage.  I'll then take a
> duplicate photo.  Now what I want to do is subtract out the colors of the
> leaf leaving only a two tone image.
>
> I was hoping to find a way to then use ImageMagick or Gimp to count the
> pixels of color so I can then get a percent change between the two images.
>
> Does anybody know how to get ether of these two programs to give this type
> of information.  Actually any software would be help full.
>
>
>
> - --
> Wade Pinkston
>
> Ipsa scientia potestas est
>
> Windows,a
> 32 bit graphical interface for a
> 16 bit patch to an
> 8 bit operating system internally coded for a
> 4 bit processor written by a
> 2 bit company that can't stand
> 1 bit of competition
>
>   
This might not be the method you were thinking but here's an example 
using ImageMagick
http://ecocam.evsc.virginia.edu/change_detection_description/change_detection_description.htm

What you're doing is called Change Detection and is pretty common in GIS 
applications.
It's probably overkill to suggest doing it in GRASS but if you did it 
would be a simple subtraction of the two photos and then you could 
summarize the table of pixels. But this would involve some method of 
importing the images to a RASTER data set.

Alex