This is a mockup of the planned UI. When I designed this we had no UI tools at all, so there was very limited interactivity and no ability to construct the UI from smaller tilable sections. Eventually the UI improvements were shelved so we didn't get a chance to fully implement it.
Below are a few of the icons I created for compiling into the core engine content file. The major icons have on and off states, both for rollover and for deactivation. The original sheet is 2048x2048 so the icons render cleanly when shown on high resolution displays.
Later on I created icons for commonly-used functions like the cursor, highlighting, scrolling, etc. These also had parallax and specular maps so the UI could have depth and reflectivity. However these rendering improvements were later dropped in favor of other priorities, so I baked the lighting in.
