[GRASS-dev] some detail questions on i.segment

Moritz Lennert mlennert at club.worldonline.be
Wed May 18 09:36:13 PDT 2016

Hi Markus,

I'm working on potentially improbing the i.segment.uspo addon and am 
looking at the possibility of including the goodness of fit output map 
somehow in the evaluation of the quality of the segmentation.

For that, I need to exactly understand the goodness of fit measure.

As a starter: why is the threshold parameter (globals->alpha) squared 
before being used in create_isegs.c (and in the calculation of the 
goodness of fit) ? Is it because i.segment works with the squared 
distance and not the actual distance ?

IIUC, the worst goodness of fit measure (i.e. 1 - difference) is equal 
to the 1 - threshold parameter value. This thus means that if one would 
want to compare segmentations done with different threshold values by 
comparing mean goodness of fit, for example, this would have to be 
scaled taking into account the respective parameter value. Would 
something like

( goodness of fit - (1 - threshold parameter value) )  / threshold 
parameter value

make sense ?

BTW, in write_output.c, in the comments starting at line 82, there is 
mention of a globals->threshold, but there is not threshold in the 
globals structure... I guess this should read globals->alpha or 
threshold->answer, or ?


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