#include<stdio.h>
#include<math.h>
#include<stdlib.h>
 
    /*
       PVI: Perpendicular Vegetation Index
       
       PVI = sin(a)NIR-cos(a)red
       for a  isovegetation lines (lines of equal vegetation) would all be parallel to the soil line therefore a=1
       
     */ 
double p_vi(double redchan, double nirchan) 
{
    double result;

    if ((nirchan + redchan) == 0.0) {
	result = -1.0;
    }
    else {
	result = (sin(1) * nirchan) / (cos(1) * redchan);
    }
    return result;
}