Rgb color map to a normalized value - c++

Question

I'm using this function to convert a normalized value between 0 and 1 to an RGB value depending on the JET colormap.
std::vector<double> mapJet(double v, double vmin, double vmax)
{
if (v < vmin)
v = vmin;
if (v > vmax)
v = vmax;
double dr, dg, db;
if (v < 0.1242) {
db = 0.504 + ((1.-0.504) / 0.1242)*v;
dg = dr = 0.;
} else if (v < 0.3747) {
db = 1.;
dr = 0.;
dg = (v - 0.1242) * (1. / (0.3747-0.1242));
} else if (v < 0.6253) {
db = (0.6253 - v) * (1. / (0.6253-0.3747));
dg = 1.;
dr = (v - 0.3747) * (1. / (0.6253-0.3747));
} else if (v < 0.8758) {
db = 0.;
dr = 1.;
dg = (0.8758 - v) * (1. / (0.8758-0.6253));
} else {
db = 0.;
dg = 0.;
dr = 1. - (v - 0.8758) * ((1.-0.504) / (1.-0.8758));
}
return std::vector<double> { 255 * dr, 255 * dg, 255 * db };
}
My aim is to find the function double v = mapJet_inv(R,G,B). That is to say, I convert an RGB color to a normalized value between 0 and 1 depending on the colormap. I tried to start from the end of the mapJet function, however I didn't know how to specify the ranges of the R G B components.
Maybe I'm doing this badly. I will appreciate your help.
Thank you

Answer 1

vector<float> colors_to_value(vector<float> colors_tab)
{
double v ,db,dg,dr; v=db=dg=dr=0; vector<float> values_result;
for(int i=0;i<colors_tab.size();i++)
{
dr=colors_tab(i,0)/255.;
dg=colors_tab(i,1)/255.;
db=colors_tab(i,2)/255.;
if ( dg == 0. && dr == 0. ) {
v = (db - 0.504) / (1.-0.504) * 0.1242; // a revoir
}
else if ( db == 1. && dr == 0. ) {
v = dg/4. + 0.1242;
}
else if (db==0 && dr==1)
{
v = 0.8758 -dg*(1. / (0.8758-0.6253)) ;
}
else if ( db==0 && dg ==0)
{
v =( 1. - dr ) / ((1.-0.504) / (1.-0.8758)) + 0.8758;
}
else{
float val1= 0.6253 - db/(1. / (0.6253-0.3747)) ;
float val2=dr/ (1. / (0.6253-0.3747)) + 0.3747;
// v=(val1+val2)/2.;
if (val1>val2)
v=val1;
}
if(v<0) v=0;
values_result.push_back(v);
}
return values_result;
}