How do I detect a clients camera seeing an object?

[Forcoy]

I am currently messing around with studio and I thought it would be interesting to recreate SCP-173 (from the scp foundation) on roblox. Everything is simple for me to do except detecting when the camera sees the SCP mesh. I have done quite a lot of googling and none of the results are really helpful if at all. So can anyone explain to me how this is achieved?

[nicemike40]

You could use WorldToViewportPoint for pretty good results. Add a buffer or sample multiple points on the mesh to mitigate some issues.

Edit: here’s a slightly more interesting (marginally slower) way that computes the AABB of a part on screen, and does the intersection with the viewport (untested):

-- returns true if a part's 2D bounding box on screen intersects with the screen
-- rect's
local function OnScreen(camera, part)
    -- 1. get all eight points in 3D space for the part

    local pos, front, right, up = part.Position, part.CFrame.LookVector, part.CFrame.RightVector, part.CFrame.UpVector
    local hx, hy, hz = part.Size.X / 2, part.Size.Y / 2, part.Size.Z / 2

    local points = {
        pos - front * hz - right * hx - up * hy,
        pos - front * hz - right * hx + up * hy,
        pos - front * hz + right * hx - up * hy,
        pos - front * hz + right * hx + up * hy,
        pos + front * hz - right * hx - up * hy,
        pos + front * hz - right * hx + up * hy,
        pos + front * hz + right * hx - up * hy,
        pos + front * hz + right * hx + up * hy,
    }
    
    -- 2. compute AABB of the part's bounds on screen
    
    local minX, maxX, minY, maxY = math.huge, -math.huge, math.huge, -math.huge
    
    for i = 1, 8 do
        local screenPoint = camera:WorldToViewportPoint(points[i])
        local x, y = screenPoint.X, screenPoint.Y
        
        if screenPoint.Z > 0 then
            if x < minX then minX = x end
            if x > maxX then maxX = x end
            if y < minY then minY = y end
            if y > maxY then maxY = y end
        end
    end
    
    -- 3. AABB collision with screen rect
    
    local w, h = camera.ViewportSize.X, camera.ViewportSize.Y
    
    return minX < w and maxX > 0 and minY < h and maxY > 0
end