prepare method
Prepare the object to be written to the stream
Implementation
@override
void prepare() {
super.prepare();
params['/FunctionType'] = const PdfNum(3);
params['/Functions'] = PdfArray.fromObjects(functions);
params['/Order'] = const PdfNum(3);
params['/Domain'] = PdfArray.fromNum(<num>[domainStart, domainEnd]);
params['/Bounds'] = PdfArray.fromNum(bounds);
params['/Encode'] = PdfArray.fromNum(
List<int>.generate(functions.length * 2, (int i) => i % 2));
}