Opacity



Opacity 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, Stairs, and single Stone Slabs, which do not completely fill their cube.

By default, Opacity 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 Opacity
A lot of the effects of Opacity are non-obvious. Minecraft does not have a "solid" property on blocks, so Opacity 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, nor can they spawn inside opaque blocks. However, they can spawn inside a non-solid transparent block, such as Flowers or Redstone Wires.
 * You cannot attach Torches or other wall fixtures to transparent blocks, nor can you place Signs or other items on top of them. Here's the full list of blocks that are restricted this way: Beds, Buttons, Redstone Repeaters, Ladders, Levers, Pressure Plates, Pumpkins, Rails, Snow (not Snow Block, which is OK), Torches (including Redstone Torches}, and Redstone Wire.
 * Opacity determines the direction Dispensers and Furnaces face when they are placed. It also dynamically changes the facing of Chests. These test a static version of Opacity that is not affected by the Leaf graphics toggle.
 * Water which is completely surrounded by water or opaque blocks won't have a current. However, if one of the surrounding blocks is transparent the water will flow down like a waterfall, making it much slower to swim up.
 * Opaque blocks overhead can prevent Redstone Wires from connecting to each other. See the article for details.
 * Tree leaves can overwrite transparent blocks when they grow, but they will not overwrite opaque blocks.
 * Chests cannot be opened if there is an opaque block on top of them, but transparent blocks are OK.
 * Opacity affects door orientation when placed. Doors open counter-clockwise by default, but if they have an opaque wall along their left side they open clockwise.
 * Ghast fireball explosions only start fires on opaque blocks.

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 don't have any extra effect on diffuse (block-generated) light, but they do diffuse sunlight.
 * A single Stone Slab is transparent, while a double Stone Slab is opaque. Both types completely block Light propagation.
 * Farmland is marked as transparent, but it completely blocks light propagation.
 * Water is transparent, but it has a -2 modifier to Light propagation, on top of the normal decrease of 1 per block.
 * Ice is transparent, and has the same -2 modifier to Light propagation.

Trivia

 * 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.
 * Only opaque blocks can be powered by Redstone power. This does not come up much in practice, because Redstone Wires and Redstone Torches can't be attached to transparent blocks. However, it can be observed by placing a torch underneath Glass, and then noting how adjacent wires are not powered. (If you replace the glass with something opaque they will get power.)
 * In the Nether, your spawn point will always be on top of an opaque block. This is hard to observe, since you warp back to the normal world when you die in the nether.