[BUG] Line visualizations break when one of the ends is culled

[Niterux]

Short Description

When the startpoint or endpoint of a line or a line visualizing an attachment to an entity is out of the view, the rendering flips out!!!

Console Error Logs (Optional)

No errors

How to Reproduce (Optional)

Place a very long line and look away from one of the points

Screenshots (Optional)

Desktop.2026.01.03.-.11.09.09.04-compressed.mp4

Additional Information (Optional)

Tested with all other addons disabled, original precision alignment also seems to struggle with this but only for lines and not for attachment visualizations and the issue seems a lot less severe.

[Pugsworth]

This is a side-effect of not frustum clipping the line. Unfortunately, this is not something that Vector:ToScreen() can ever fix since the purpose is for a single vector. Until a function is added to gmod officially, this has to be done manually.

Here's a function to calculate the two screen points to draw the line between. The general idea is when one point is behind the view, the line is clipped such that a new world vector that lies on the view's znear clipping plane is calculated, then ToScreen will return a valid coordinate.

-- https://wiki.facepunch.com/gmod/Structures/ViewSetup

local function ClippedToScreen(vStart, vEnd)
    -- Not sure if there are any performance penalties with calling this every frame.
    -- Could potentially get the same information needed by just using EyePos() with a cached znear distance if it is.
    local view = render.GetViewSetup()
    local camPos = view.origin
    local camForward = view.angles:Forward()
    
    -- Use the near clipping plane to determine visibility instead of the ToScreen().visible property since that isn't reliable, especially if there was an error in a rendering hook (open issue).
    local epsilon = view.znear 

    local dotA = camForward:Dot(vStart - camPos)
    local dotB = camForward:Dot(vEnd - camPos)

    -- Both points are culled by the near plane
    if dotA < epsilon and dotB < epsilon then return nil end

    -- Partial occlusion: perform clipping
    if dotA < epsilon or dotB < epsilon then
        local visible = dotA >= epsilon and vStart or vEnd
        local behind = dotA < epsilon and vStart or vEnd
        
        local dVisible = camForward:Dot(visible - camPos)
        local dBehind = camForward:Dot(behind - camPos)

        -- Calculate the intersection factor to obtain a new world vector that would sit directly on the camera's znear plane.
        -- This makes the occluded point always on or near enough to the actual screen that ToScreen can give valid results.
        local t = (epsilon - dVisible) / (dBehind - dVisible)
        local clippedPoint = visible + t * (behind - visible)

        return visible:ToScreen(), clippedPoint:ToScreen()
    end

    -- Otherwise fully visible
    return vStart:ToScreen(), vEnd:ToScreen()
end