Mirror Door

A mirror door is a type of underworld transition glitch performed by using the mirror on the same frame as a subtile transition. These only have useful effects on subtile doors. They can be performed on all normal doors and both west and north auto doors.

Execution

To execute a mirror door:

1. Lift a pot within the supertile you plan to do the glitch in.
2. Position Link 1 frame's worth of movement from a subtile transition. You likely want to snap even if it's not an auto door because of the next requirement.
3. Most Y-items are unusable while Link has "door state", and the mirror is no exception. To get rid of door state, you can take damage (like from a bomb), or you can use the door-state cancel you get from snapping.
4. Press Y and the d-pad towards the door on the same frame.

Mechanics

Mirror doors do two things:

1. Behave as a door juke, bringing you into the next room without a transition.
2. Execute the "Open Triforce Room" routine.

When the Open Triforce Room routine executes, it begins a countdown (decrements by 1 each frame) with whatever the current value of $C8[0x2] is. During this countdown, most other routines are stopped, including sprites and a full loss of player control. When the value reaches 0x00FF, it jumps to 0 and the game continues as normal. In the northeast quadrant of the supertile, a graphically busted hole will appear in the middle of the north wall. This is the Triforce door, and it can be used as a normal transition.

The value stored at $C8 fluctuates throughout gameplay. It changes on every supertile transition as it's used in decompression routines that occur on supertile load. It's generally a very high value, which would mean waiting a long time for the countdown to resolve. $C8 is also used to temporarily cache Link's Y-position when lifting a pot, and this is why players should lift a pot within the supertile they're performing the trick in. Link's Y-position grows downwards, meaning it'll store a lower value when he's towards the top of the underworld map compared to when he's lower in the map. This ultimately means that the farther south Link is on the underworld map, the longer this glitch will take to complete. However, even from the lowest row, picking up a pot allows the mirror doors to resolve in a time reasonable enough to be useful in a speedrun.

Caveats

If a pot is not picked up or the value of $C8[0x2] changes from another action (such as using the map), then the countdown will take incredibly long. Long enough that a run may as well be dead, as the timer can take upwards of 65,535 frames (the maximum unsigned 16-bit integer, and translating to about 18.2 minutes). Also, as the timer counts down to 255 (0xFF) rather than 0, a value that is too small is functionally worse than other arbitrarily high values. However, the only pots that are affected by this are those on the upper quadrants of the highest row on the underworld map. In terms of functionality, no mirror door with a practical use is lost to this caveat.