DevDoc:HUD

HUD : Head-up display, a visual display technology for fighter airplanes, cars and others

HUD-elements includes object markers such as the edges around objects and the text next to them

such hud elements are 2d graphics aligned to the projected positions of 3d objects

for special hudelements with custom geometry (lines) use Hook_HUDStep and project object pos manually (obj.mvPos, obj.mfBoundingRad, ProjectSizeAndPosEx)

otherwise use autotracking : tp avoid iterating over all objects/hudelements which is expansive in lua, see gfx2D.h

 Vector2					mvTrackPosOffset;	///< in pixels
 Vector2					mvTrackPosTargetSizeFactor;
 Vector2					mvTrackPosOwnSizeFactor;
 Vector2					mvTrackClampMin; ///< left top border, in pixels, clamps projected target pos
 Vector2					mvTrackClampMax; ///< right bottom border
 Vector2					mvTrackSetSizeFactor; ///< for mbTrackSetSize
 bool					mbTrackClamp;
 bool					mbTrackHideIfClamped;		///< warning, overrides manual visibility
 bool					mbTrackHideIfBehindCam;		///< warning, overrides manual visibility
 bool					mbTrackClampMaxXIfBehindCam;
 bool					mbTrackClampMaxYIfBehindCam;
 bool					mbTrackSetSize; ///< see also mvTrackSetSizeFactor
 bool					mbTrackMouse;

mvTrackClampMin,mvTrackClampMax : clamp to border l,t,r,b (can be smaller than screen e.g. menu bar on a side)

example for a simple marker over an object

 function MakeObjectHudMarker (trackgfx,col) 
   local mat = GetPlainTextureMat("raute.png")
   mat = GetHuedMat(mat,1,1,1, 1,1,1,0.5)  -- white color, halftransparent
   local e = 32 -- image-size
   local hudmarker = guimaker.MakePlane(gHudMarkerDialogLayer.panel,mat,-0.5*e,-0.5*e,e,e)
   hudmarker.gfx:SetTrackOffset(-0.5*e,-0.5*e)
   hudmarker.gfx:SetTrackPosSceneNode(trackgfx)
   hudmarker.gfx.mbTrackHideIfClamped = false
   hudmarker.gfx.mbTrackHideIfBehindCam = false
   local vw,vh = GetViewportSize()
   hudmarker.gfx.mbTrackClamp = true
   hudmarker.gfx.mvTrackClampMin = {0,0}
   hudmarker.gfx.mvTrackClampMax = {vw,vh}
   return hudmarker
 end