They stay in the same position, regardless of screen size/aspect ratio, so when i expand from a phone to a tablet, the tablet’s invisible walls are farther in than the actual edge of the screen is. How can i fix this?
You need to calculate the frustum height/width of the camera at the distance of the walls.
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This will give the width and height of the camera view at the distance the walls are located. Then place the walls according to this width and height.