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 : to 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;

(these fields are also accessible from lua) 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