SRS

Super Rotation System, or SRS and sometimes called Standard Rotation System is the current Tetris Guideline standard for how tetrominoes behave, defining where and how the tetrominoes spawn, how they rotate, and what wall kicks they may perform. SRS traces its routes back to 1991 when BPS introduced its signature third and fourth rotation states for the S, Z, and I tetrominoes in the game Tetris 2+Bombliss. Two years later, in the game Tetris Battle Gaiden, BPS altered the spawn orientation of the T, L, and J tetrominoes so that they spawned flat-side first. It was not until the 2001 game, Tetris Worlds, that the wall kick system was introduced, and SRS took its final form. Henk Rogers, in his effort to unify all new Tetris games into the Tetris Guideline, required Arika to include a form of SRS in their 2005 game, Tetris The Grand Master 3 Terror-Instinct, where it is called "World" rule, in reference to Tetris Worlds.

Spawn Orientation and Location
The spawn orientations are included in the diagram on the right.
 * All tetrominoes spawn horizontally and wholly above the playfield.
 * The I and O tetrominoes spawn centrally, and the other, 3-cell wide tetrominoes spawn rounded to the left.
 * The J, L and T spawn flat-side first.
 * In Tetris Worlds, the tetrominoes spawn in rows 22 and 23 (or just row 22 in the case of the "I" tetromino), however, in later games the tetrominoes spawn 1 row lower.

Basic Rotation
The basic rotation states are shown in the diagram on the right. Some points to note:
 * When unobstructed, the tetrominoes all appear to rotate purely about a single point. These apparent rotation centers are shown as circles in the diagram.
 * It is a pure rotation in a mathematical sense, as opposed to the combination of rotation and translation found in other systems such as Sega Rotation and Atari Rotation.
 * As a direct consequence, the J, L, S, T and Z tetrominoes have 1 of their 4 states (the spawn state) in a "floating" position where they are not in contact with the bottom of their bounding box.
 * This allows the bounding box to descend below the surface of the stack (or the floor of the playing field) making it impossible for the tetrominoes to be rotated without the aid of floor kicks.
 * The S, Z and I tetrominoes have two horizontally oriented states and two vertically oriented states. It can be argued that having two vertical states leads to faster finesse.
 * For the "I" and "O" tetrominoes, the apparent rotation center is at the intersection of gridlines, whereas for the "J", "L", "S", "T" and "Z" tetrominoes, the rotation center coincides with the center of one of the four constituent minos.

Wall Kicks
When the player attempts to rotate a tetromino, but the position it would normally occupy after basic rotation is obstructed, (either by the wall or floor of the playfield, or by the stack), the game will attempt to "kick" the tetromino into an alternative position nearby. Some points to note:
 * When a rotation is attempted, 5 positions are sequentially tested (inclusive of basic rotation); if none are available, the rotation fails completely.
 * Which positions are tested is determined by the initial rotation state, and the desired final rotation state. Because it is possible to rotate both clockwise and counter-clockwise, for each of the 4 initial states there are 2 final states. Therefore there are a total of 8 possible rotations for each tetromino and 8 sets of wall kick data need to be described.
 * The positions are commonly described as a sequence of ( x, y) kick values representing translations relative to basic rotation; a convention of positive x rightwards, positive y upwards is used, e.g. (-1,+2) would indicate a kick of 1 cell left and 2 cells up.
 * The J, L, S, T and Z tetrominoes all share the same kick values, the I tetromino has its own set of kick values, and the O tetromino does not kick.
 * Several different conventions are commonly used for the naming of the rotation states. On this page, the following convention will be used:
 * 0 = spawn state
 * R = state resulting from a clockwise rotation ("right") from spawn
 * L = state resulting from a counter-clockwise ("left") rotation from spawn
 * 2 = state resulting from 2 successive rotations in either direction from spawn.

A wall kick example: The desired rotation is 0->L, and from the table above, the wall kick test order is ( 0, 0), (+1, 0), (+1,+1), ( 0,-2), (+1,-2).

{| cellpadding="2" style="border:1px solid darkgray;" 1. Initial position. Attempt to rotate 0->L.
 * - valign=top
 * width="200pt"|
 * width="200pt"|

2. Test 1, ( 0, 0) fails. (Basic rotation fails.) 3. Test 2, (+1, 0)</tt> fails.
 * width="200pt"|
 * }

{| cellpadding="2" style="border:1px solid darkgray;"
 * - valign=top
 * width="200pt"|

4. Test 3, (+1,+1)</tt> fails. 5. Test 4, ( 0,-2)</tt> fails.
 * width="200pt"|
 * width="200pt"|

6. Final position. Test 5, (+1,-2)</tt> succeeds.
 * }

Arika SRS
In their games Tetris The Grand Master 3 Terror-Instinct and Tetris The Grand Master Ace, Arika were required to include a form of SRS as the default rotation system, in order to conform more closely to Henk Rogers' Tetris Guideline. Arika's implementation of SRS uses the exact same wall kick data for the J, L, S, T and Z tetrominoes as the Guideline's standard; however, the I tetromino uses the wall kick data shown below:

The logic behind Arika's modifications is that the I wall kicks are now symmetric about the y-axis when rotating from or to a horizontal orientation. One noticeable consequence of this is illustrated in the following example:

{| cellpadding="2" style="border:1px solid darkgray;"
 * - valign=top
 * width="150pt"|


 * width="350pt"|In the symmetric position, only Arika SRS allows the clearing of 4 lines by rotating counter-clockwise.
 * }

How Guideline SRS Really Works


Instead of directly assigning a set of ( x, y)</tt> kick translations to each of the 8 possible rotations, TTC actually employed a different approach, and instead assigned a set of ( x, y)</tt> "offset" values to the 4 rotation states. The kick translations are then derived by taking the difference between pairs of offset data. When rotating from A to B, subtracting B's values from A's will give the kick translation for the rotation one way; and subtracting A's values from B's will give the kick translation for rotating back the other way.

There is another complexity to TTC's implementation: the derived translations are relative to a different datum. So far on this page, kick translations have been defined relative to "basic rotation", but TTC uses what forum user nightmareci has named "true rotation". "True rotation" is still a mathematical pure rotation with no translation involved; however, the rotation center always coincides with the center of one of the four constituent minos. (Recall that the apparent rotation center of the I and O tetrominoes in basic rotation coincided with the intersection of gridlines). This means that for "true rotation", the rotation center for the O piece is not at the geometric center, so the piece will have a "wobble" when rotated. The first kick translation has to be used to correct for this wobble.

An example of deriving kick translations from the offsets:

The offsets for J, rotation state 0 are: ( 0, 0)</tt>, ( 0, 0)</tt>, ( 0, 0)</tt>, ( 0, 0)</tt>, ( 0, 0)</tt>. The offsets for J, rotation state R are: ( 0, 0)</tt>, (+1, 0)</tt>, (+1,-1)</tt>, ( 0,+2)</tt>, (+1,+2)</tt>.

<tt>( 0, 0)</tt> - <tt>( 0, 0)</tt> = <tt>( 0, 0)</tt>, <tt>( 0, 0)</tt> - <tt>(+1, 0)</tt> = <tt>(-1, 0)</tt>, <tt>( 0, 0)</tt> - <tt>(+1,-1)</tt> = <tt>(-1,+1)</tt>, <tt>( 0, 0)</tt> - <tt>( 0,+2)</tt> = <tt>( 0,-2)</tt>, <tt>( 0, 0)</tt> - <tt>(+1,+2)</tt> = <tt>(-1,-2)</tt>.

Therefore, the kick translations for the J rotation 0->R, relative to "true rotation" (which is conveniently the same as "basic rotation" for the J tetromino), are: <tt>( 0, 0)</tt>, <tt>(-1, 0)</tt>, <tt>(-1,+1)</tt>, <tt>( 0,-2)</tt>, <tt>(-1,-2)</tt>.