This is the callback that replaces the GetGraphicsMode GDI function.
Handle of the device context passed to the paint function.
Void pointer that you can use to access a variable or structure containing data that your callback function needs. This gives you a way to receive data indirectly from the function that uses this callback function. (This is the same pointer that you pass in the pUserData parameter of the PAINTCALLBACKDATA structure.)
The return values are the same as the return values for the corresponding GDI function.
The internal display engine calls your callback function if you override the corresponding GDI function in the PAINTCALLBACKDATA structure.
pDisplay is the pDisplay member from the PAINTCALLBACKDATA structure that you passed to the paint function. The default callbacks expect it to be a HDC handle. But, if you override all the paint callbacks, it can be a pointer of any type you want.
Please see the Microsoft documentation for the values you are expected to return and for the meaning of the parameters before pUserData.
For an example, refer to L_PaintDCCallback.
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