Qt is throwing an error when calling DirectX, saying that the function Direct3DCreate9() is not defined.

When calling DirectX-related functions in Qt, you need to add the corresponding library files and header file paths in the project file (.pro). The specific steps are as follows:

1. Open the .pro file of the Qt project.
2. Add the following code to the document:

win32: LIBS += -ld3d9

INCLUDEPATH += "C:/Program Files (x86)/Microsoft DirectX SDK (June 2010)/Include"

Please modify according to the installation path of your system and DirectX SDK.

1. Save and rebuild the project.

This way, you can use DirectX-related functions in your Qt project.