Explosion

An explosion is a physical event in the Minecraft world, caused by a primed TNT, a (charged) creeper, or a fireball launched by Ghasts. An explosion can destroy nearby blocks, propel and damage nearby entities, and start fire.

Properties

 * Position . For example, A TNT explosion happens at the center of a primed TNT, which is a 0.98 × 0.98 × 0.98 cube.
 * Power . For the power of each type of explosion,
 * TNT explosion has 4;
 * creeper explosion has 3, or 6 when charged by lightning;
 * fireball explosion has 1.
 * Ability to generate fire . Currently only fireball explosion can start fire.



Destroying blocks
An explosion can destroy nearby blocks. Its effect is evaluated independently on many explosion rays originating from the explosion center, as shown in the right figure.


 * Each ray has a blast force randomized in [0.7 × power, 1.3 × power ].
 * The effect of the blast force is examined at checkpoints on the ray with step length of 0.3.
 * The blast force is absorbed (block resistance / 5 + 0.3) × step length by the block (no matter whether destroyed) at the each checkpoint, and attenuated by step length × 0.75 between checkpoints, until completely absorbed or attenuated.
 * A block is considered destroyed if it can't completely absorb the blast force at any checkpoint in it (air blocks can be destroyed too).

From the above process, the following results can be deduced:


 * The maximum travel distance in the air of an explosion (i.e. only attenuated, not absorbed by blocks) = ⌊1.3 × power / (step length × 0.75)⌋ × step length = 10.2 (charged creepers), 6.9 (TNT), 5.1 (creepers), 1.5 (fireballs). For example, a TNT explosion can destroy a torch 7 blocks away.
 * The minimum block resistance required to absorb maximum blast force of an explosion happening in nearby air = ((1.3 × power - attenuation steps × step length × 0.75) / step length - 0.3) × 5 . To not be destroyed, a block has to absorb all blast force at the first checkpoint in it.
 * The attenuation steps is related to the collision model of the exploding entity. For explosion in air, there is at least one attenuation step. TNT and creeper explosion are always 0.49 and 0.5 meter away from nearest block (2 att. steps), but fireball explosion can happen anywhere (1 att. step).
 * Thus, the block resistances are 121.00 (charged creepers), 77.67 (TNT), 56.00 (creepers), 16.42 (fireballs).
 * So water, stationary lava, obsidian, and bedrock are always indestructible, and fences and less resistant blocks can be destroyed by fireballs. These are theoretical values, and in reality less resistant blocks are not always destroyed.

Note that the effect of multiple explosions, no matter how simultaneous, on one block is evaluated independently and serially per explosion, and blocks don't have "temporary health" and such properties across explosion history. That means explosions have no cumulative effect on blocks.

Destroyed blocks have 30% chance of being dropped as items later.

Interaction with entities
An explosion has different effects on entities (the player, mobs, floating items, etc.) than blocks. Entities are damaged and propelled by an explosion if within its blast radius of 2 × power.


 * For each entity within the radius, define impact = (1 - distance from the explosion / radius) × exposure.
 * Apply (impact × impact + impact) × 4 × radius + 1 point (half-heart) of damage to the entity.
 * Propel the entity so that its velocity increases by impact in the direction from explosion to the entity.

From the above process, the following results can be deduced:


 * Entities will always get at least 1 point of damage if they are within the radius, regardless of their explosion exposure.
 * The maximum velocity gain that an entity can obtain from a TNT explosion is 1, at the explosion center with 100% exposure.

Different damage effects will ensue. For example, existing items will be destroyed, and the armor on the player will absorb part of the damage. Items dropped in the process of, or actually after, the explosion are not affected because they have no interaction between the explosion.

The propulsion effect is often used for TNT cannons.



Calculation of explosion exposure
Explosion exposure is simply how much an entity is visible from the explosion center, and is approximated with the ratio of visible sample points on the entity. The approximation algorithm has sampling error that results in directional asymmetry of propulsion. For example, a typical TNT cannon has maximum range in the west direction partly because the primed TNT has largest sampled exposure in that direction.

Causing fire
If the explosion has the ability, it will randomly start fire in 1/3 of all destroyed air blocks that are above opaque blocks. All destroyed non-air blocks will be set to air after the explosion is animated, but strangely these just generated fire blocks are also reverted to air. This bug makes fireballs actually unable to start fire (still in Beta 1.5_01).

Blast Resistance
The data below is pulled from a running instance of Minecraft (Beta 1.5) and kept in a template. [] 

Typical blast radii
The player will get certain damage if within these radii of a 100% exposure ground 1-, 2-, or 4-TNT explosion, as shown in the following figures with the amount of damage shown on each circle.

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