SBasis2d partial_derivative(SBasis2d const &f, int dim) {
SBasis2d result;
for(unsigned i = 0; i < f.size(); i++) {
result.push_back(Linear2d(0,0,0,0));
}
result.us = f.us;
result.vs = f.vs;
for(unsigned i = 0; i < f.us; i++) {
for(unsigned j = 0; j < f.vs; j++) {
Linear2d lin = f.index(i,j);
Linear2d dlin(lin[1+dim]-lin[0], lin[1+2*dim]-lin[dim], lin[3-dim]-lin[2*(1-dim)], lin[3]-lin[2-dim]);
result[i+j*result.us] += dlin;
unsigned di = dim?j:i;
if (di>=1){
float motpi = dim?-1:1;
Linear2d ds_lin_low( lin[0], -motpi*lin[1], motpi*lin[2], -lin[3] );
result[(i+dim-1)+(j-dim)*result.us] += di*ds_lin_low;
Linear2d ds_lin_hi( lin[1+dim]-lin[0], lin[1+2*dim]-lin[dim], lin[3]-lin[2-dim], lin[3-dim]-lin[2-dim] );
result[i+j*result.us] += di*ds_lin_hi;
}
}
}
return result;
}
