User blog comment:Scooter8pie/Transparency Bug/@comment-4583259-20111125161748

I put this in a comment in the Chasms article but it really belongs here.

It has been noticed that of all the underground minerals, only Blue_Crystal and occasional empty cubes show up when you are using the transparency bug to look under solid ground. Other minerals you might want to find, like Notchium, do not show up this way. But if these minerals are inside ice, they do show up when viewed by the transparency bug.

This is my theory of why only the Blue Crystal shows up underground.

For efficiency in rendering, the game appears to be treating all opaque materials as the same type, and all transparent materials (except "nothing") as a second type, and air ("nothing") as a third type, before deciding what surfaces to render. Any given square is rendered (displayed) only if it faces the camera (not facing away), and if it is in between two different types of material.

Therefore, most minerals are not drawn when they are inside rock, because the mineral and the rock/gravel/dirt are both opaque, so it is all treated as opaque (same "type"), and you only have to draw the front-most opaque surface.

The Blue_Crystal is getting drawn because you have a transparent material next to an opaque material: two different types.

The air-filled blocks inside rock get drawn because there you have a surface where rock meets air: two different types. It is drawn "black" because there is no illumination inside the empty space. If you were to place a minilight inside an underground empty space, then close up the space around it, then dig somewhere else and use the transparency bug to view it from a distance, I believe you would see a lit-up cube because the minilight provides light.