Opacity

Opacity is a property of blocks. It determines ray tracing and collision result and controls several interaction behaviors.

Non-opaque blocks

 * Plants: sapling, yellow flower, red rose, brown mushroom, red mushroom, crops, reed.
 * Liquid: water, stationary water, lava, stationary lava.
 * Blocks: leaves, glass, step, mob spawner, wooden and cobblestone stairs, soil, snow, ice, cactus, fence.
 * Non-blocks: torch, fire, redstone (wire), sign post, wooden and iron door, ladder, minecart tracks, wall sign, lever, wooden and stone pressure plate, redstone torch off/on, button, portal.

Behaviors

 * Snow, doors, torches, buttons, ladders, levers, minecart tracks, pumpkins can only be placed on opaque blocks.
 * Mobs only spawn on opaque blocks and in non-opaque space.
 * Fireballs only start fire on opaque blocks.
 * Mushrooms only grow on opaque blocks.
 * Growing leaves will destroy any non-opaque blocks.
 * Opacity of adjacent blocks determines the direction of furnaces and chests.
 * Chests can't be opened if there are opaque blocks above.
 * Non-opaque blocks are not ignitable.
 * Non-opaque blocks change water behavior.
 * Block opacity affects the behavior of redstone electrics.