3D Objects inside Guis?

I'd like to know more on how this is done. I'm having trouble understanding how ROBLOX Battle did it even while looking at the source code.

What is entailed in getting a 3D object in a Gui? I see it has something to do with Billboard guis in the player gui, but I'm hoping someone can give me a few tips here.

Thanks.

I was originally not going to answer this question, but I feel that enough people are confused about the topic that I must. ROBLOX's Games team used the a function in the following code to convert the object's position into a vector2. They then cloned an ImageLabel to that position. There was never a 3D object in the GUI. Here is the code, please note that it was made by the Games team, not me.

local function PointToScreenSpace(at)
    local point = Workspace.CurrentCamera.CoordinateFrame:pointToObjectSpace(at)
    local aspectRatio = ScreenGui.AbsoluteSize.X / ScreenGui.AbsoluteSize.Y
    local hfactor = math.tan(math.rad(Camera.FieldOfView)/2)
    local wfactor = aspectRatio*hfactor
    --
    local x = (point.x/point.z) / -wfactor
    local y = (point.y/point.z) / hfactor
    --
    return Vector2.new(ScreenGui.AbsoluteSize.X*(0.5 + 0.5*x), ScreenGui.AbsoluteSize.Y*(0.5 + 0.5*y))
end


PlayerGotCoin_Event.Changed:connect(function()
    if PlayerGotCoin_Player.Value == Player then
        local at = PlayerGotCoin_Location.Value
        --
        local pos = PointToScreenSpace(at)
        local pointSet = {
        PointToScreenSpace(at + Vector3.new( 0.75, 0, 0.75));
        PointToScreenSpace(at + Vector3.new(-0.75, 0, 0.75));
        PointToScreenSpace(at + Vector3.new(-0.75, 0, -0.75));
        PointToScreenSpace(at + Vector3.new( 0.75, 0, -0.75));
        }
        local minx, maxx, miny, maxy = math.huge, -math.huge, math.huge, -math.huge
        for _, pt in pairs(pointSet) do
            if pt.x > maxx then maxx = pt.x end
            if pt.x < minx then minx = pt.x end
            if pt.y > maxy then maxy = pt.y end
            if pt.y < miny then miny = pt.y end
        end
        local width = maxx - minx
        local height = maxy - miny
        local avg = (width+height)/2
        width = avg
        height = avg
        
        -- make the coin GUI
        local coinGui = Create'ImageLabel'{
        Name = 'Coin';
        Image = 'http://www.roblox.com/asset/?id=121148238';
        BackgroundTransparency = 1;
        ZIndex = 1;
        }
        if
        at ~= Vector3.new(0,0,0) and
        pos.x >= 0 and pos.y >= 0 and
        pos.x <= ScreenGui.AbsoluteSize.x and pos.y <= ScreenGui.AbsoluteSize.y then
            -- onscreen
            coinGui.Position = UDim2.new(0, pos.x-width/2, 0, pos.y-height/2)
            coinGui.Size = UDim2.new(0, width, 0, height)
            else
            -- offscreen, just show it comming from the center of the screen
            coinGui.Position = UDim2.new(0.5, -20, 0.5, -20)
            coinGui.Size = UDim2.new(0, 40, 0, 40)
        end
        coinGui.Parent = ScreenGui
        
        -- tween the coin to the side display, and show the shownamount
        --Delay(0.5, ShowAmount)
        local function endTweenFunc()
            coinGui:Destroy()
            MyCoinCount.Value = MyCoinCount.Value + 1
        end
        coinGui:TweenSizeAndPosition(UDim2.new(0, 70, 0, 70),
        UDim2.new(0, 9, 0.4, 50),
        'In', 'Quad', 0.9, true,
        endTweenFunc)
    end
end)

Not sure, but I guess its not inside the gui. There is a frame around the 3D character, that is actually a normal model with a part behind as background, they just changed the camera position. If you notice, there is no other 3D space being shown in that screen, what means it's not a gui.

I'll tell you how they did it.

It's not possible to put a 3D object in a GUI because ROBLOX can only render one 3D space at once.

They placed a decal background as the "GUIs" and placed the bricks in front of the decal. They then locked your camera to be looking at it to simulate having a 3D GUI.