Why MagSetImageScalingCallback function fails when is executed in new desktop?

[Der schöne Günther]

1. What does your GetLastError() return?
2. Im sure you saw the

   Zitat:

   The MagSetImageScalingCallback function is deprecated in Windows 7 and later, and should not be used in new applications.

3. Your example is missing your CreateDesktop shenanigans. As far as I recall, additional desktops are very limited. Example: They cannot have Aero features (and probably other DWM stuff).