Arrow traps seeing farther left

Verified against the spelunky.ai port v0.1.10 on 28 October 2026.

An Arrow Trap fires at the first thing that moves in a line in front of it, which runs to the first block in its row. With nothing in the way, that line is a block longer for a trap facing left than for one facing right, so a player can stand safely 6 blocks from one kind, and not the other.

The player stands midway between two Arrow Traps that face each other across an open room, as far from one as from the other. He turns round, and only the left-facing trap fires; its arrow stops short of him. He takes two steps toward the right-facing one before it fires too ^[1].

A step after it's made, each trap makes its sensor, an invisible strip (oArrowTrapTest) along its row that sets it off when anything moving touches it ^[2]. To size it, the trap scans its row for a block, up to 97 pixels away. The left-facing trap's code measures the strip from where the scan stopped back to the trap:

    xAct = x-1;
    while (not collision_point(xAct, y+8, oSolid, 0, 0))
    {
        if (x - xAct > 96) break;
        xAct -= 1;
    }
    if (xAct > x-16) { xAct = x-16; }
    obj = instance_create(xAct, y, oArrowTrapTest);
    obj.image_xscale = ceil(((x-1)-xAct)/16);

In the open the scan stops 97 pixels out, and the strip is ceil(96/16), 6 blocks. The right-facing trap's code scans as far, but then takes 8 more pixels off and starts the strip at the block in front of it:

    xAct -= (x+8);
    if (xAct < 32) { xAct = 32; }
    obj = instance_create(x+16, y, oArrowTrapTest);
    obj.image_xscale = ceil((xAct-16)/16);

That's ceil(73/16), 5 blocks. With a block within 5 blocks of either trap, both strips reach it, and they agree; only in the open does the left-facing one see a block farther ^[3].

Stand 6 blocks in front of an Arrow Trap with open floor between: a left-facing trap fires when the player moves, a right-facing one doesn't. Play reaches it at every trap with more than 5 open blocks in front of it. The replay of the clip's staged scene, on a custom level, puts two traps 11 open blocks apart: the player midway turns round and sets off only the left-facing one, then walks left until the right-facing one fires ^[4].

The spelunky.ai port reproduces it exactly: it's in docs/quirks.md's list of the 1.1 source's bugs ^[3], and the oracle's fingerprint of the replay pins both sensors. No option turns it off.