#include <fontconfig/fontconfig.h> double FcWeightFromOpenTypeDouble (doubleot_weight);
FcWeightFromOpenTypeDouble returns an double value to use with FC_WEIGHT, from an double in the 1..1000 range, resembling the numbers from OpenType specification's OS/2 usWeight numbers, which are also similar to CSS font-weight numbers. If input is negative, zero, or greater than 1000, returns -1. This function linearly interpolates between various FC_WEIGHT_* constants. As such, the returned value does not necessarily match any of the predefined constants.