[GRASSLIST:2017] Re: Image classification

[Markus Neteler]

On Tue, Dec 09, 2003 at 01:43:00PM +0000, Miha STAUT wrote:
> Hi,
> 
> I have got 485 8-bit TIFF ortophotos that I would like to merge in a unique 
> image or maybe 3 to 5 images. The procedure I used was r.in.gdal --> r.patch 
> - NO PROBLEM WITH THAT. The problem is that the images are not of a very 
> good quality and have very different values for the same land class, i.e. 
> some are darker and some lighter + they are shot at different times of the 
> day (hilly region). That means the shadows extend differently.
> 
> To get a better view of the area I tried to use r.colors col=grey.eq, but 
> the edges of the original images are still very visible. Then I thought 
> about i.gray.scale. As I understood i.gray.scale changes the values in the 
> image. In that way I would manage to get together the images with the values 
> much more alike.
> 
> But there is no command like that any more. ???
> 
> Any better idea? I am not really and expert in image analysis. The aim is to 
> transform the images in a way the same values would indicate the same land 
> classes. After that get some spectral signatures and try to classify the 
> image in a more or less correct way.
> 
> If anything is not clear enough, please ask for clarification.
> 
> Thanks, Miha Staut

Probably you need (to implement) "histogram matching":
calculating an overall histogram from the individual histograms, then
modify all individual histograms to that overall histogram.

I would need that functionality as well :-)

Here is some ref (googled):
http://www.cnr.berkeley.edu/~bingxu/geo7366/Lecture10/HistogramAdjust.pdf

Markus