See more related videos: User-Defined Fitting Functions
In Origin, all fitting functions -- both built-in and user-defined -- are organized by category in the Fitting Function Organizer (FFO). You can open the FFO by selecting Tools: Fitting Function Organizer from the menu, or by pressing F9.
Every fitting function is defined by in an FDF (Function Definition File). An .FDF saves all properties about the fitting function, such as category information(only for Origin 2018 and further), the fitting parameters, function expression, constraints and other information. The .FDFs for the built-in functions are located in \FitFunc subfolder of your Origin program folder, while the user-defined functions will be saved in the \FitFunc subfolder of your Origin User Files Folder. These .FDF files can be opened and edited with any text editor, such as Windows Notepad.
The FFO is a convenient tool to organize and edit .FDF files. Every section in the .FDF file has a corresponding set of controls in the Fitting Function Organizer. You can create, edit and save a function directly. Note, however, that in the case of built-in functions, some sections are not editable. These controls will be grayed out in the FFO.
For specific information on built-in functions, see this list of Origin curve-fitting functions, by category.
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