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ZDoom uses a special coordinate system for HudMessages. In 4:3 resolutions, the coordinate system's rectangle matches that of the screen. In aspect ratios other than that, a 4:3 rectangle is put at the center of the screen and the remaining screen space is out of its boundaries.
ACSUtils provides functions to correctly work with the HUD coordinate system.
GetAspectRatio returns the aspect ratio that ZDoom assumes the screen is. ZDoom uses tricky code for that.