Transparency

Transparency is a property of Blocks with a wide range of effects on the game world. Its primary purpose is to tell the game engine if it needs to draw behind the block; an opaque block completely obscures the view behind it, while a transparent block does not. Thus, transparency applies not only to clear blocks like Ice and Glass, but also things like Cactus, which do not completely fill their cube.

By default, transparency also affects how Light propagates through the world. Opaque blocks completely prevent light from travelling through them, while transparent blocks diminish it by one intensity per block. This value can be overridden however, and there are several anomalies.

List of things affected by transparency
A lot of the effects of transparency are non-obvious. Minecraft does not have a "solid" property on blocks, so transparency is tested when a test for "solidity" would normally occur.


 * Opaque blocks suffocate you, while transparent blocks do not.
 * Monsters cannot spawn on transparent blocks.
 * You cannot attach Torches or other wall fixtures to transparent blocks, nor can you place Signs or other items on top of them.
 * Water which is completely surrounded by water or opaque blocks won't have a current. However, if the block is transparent the water will flow down like a waterfall.
 * Opaque blocks overhead can prevent Redstone Wires from connecting to each other. See the article for details.

This list is by no means complete.

Anomalies

 * Leaves are transparent when "Fancy" graphics are on, but opaque when "Fast" graphics are on. This means that you can toggle the graphics mode to achieve interesting effects, like placing torches on leaves or escaping suffocation when a tree grows too close to you. They propagate Light with a constant modifier of -1, regardless of the graphics mode.
 * A single Stone Slab is transparent, while a double Stone Slab is opaque. Both types completely block Light propagation.
 * Water is transparent, but it has a -2 modifier to Light propagation.
 * Similarly, Ice is transparent, but it has a -3 modifier to Light propagation.

Trivia

 * The property is actually stored as "Opacity" in the game; a transparent block has a value of "false."
 * Lava is transparent as you would expect, but it is set to completely block light propagation. This is unnoticeable since Lava has an intrisic Luminance of 15.
 * Farmland is explicitly set to block light propagation, despite the fact that it is already opaque.