FcWeightToOpenTypeDouble - Convert from fontconfig weight values to OpenType
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#include <fontconfig/fontconfig.h>
double FcWeightToOpenTypeDouble (doubleot_weight);
FcWeightToOpenTypeDouble is the inverse of FcWeightFromOpenType.
If the input is less than FC_WEIGHT_THIN or greater than FC_WEIGHT_EXTRABLACK,
returns -1. Otherwise returns a number in the range 1 to 1000.