How to Fix GameMaker Procedural Room Instances Snapping Off the Grid

Misaligned walls in a GameMaker generated dungeon usually mean instances are placed at unsnapped pixel positions. Snapping to the cell size aligns everything.

How to fix it

1. Place by cell, then convert

Work in grid cell coordinates during generation and convert to pixels only at placement time with x = cell_x * CELL_SIZE. This keeps every instance exactly on the grid.

2. Snap any pixel-space math

If a position is computed in pixels, snap it with floor(x / CELL_SIZE) * CELL_SIZE before instance_create_layer so rounding errors never leave a one-pixel seam between tiles.

3. Use a ds_grid as the source of truth

Generate the layout into a ds_grid of cell types, then iterate the grid and create one instance per filled cell at the snapped pixel position, so visuals always match the logical map exactly.

Catching the ones you can't reproduce

The hardest version of this to fix is the one you can't reproduce — it only happens on a player's hardware, OS, driver, or save state, under conditions that simply aren't present on your machine. A report that says “it crashed” or “it froze” gives you nothing to act on, so the bug survives release after release while quietly costing you players.

Automatic error capture closes that gap. Each failure arrives with its full stack trace, the device and OS, the build number, and a breadcrumb trail of what the player did right before it broke, so even a failure you have never seen becomes a specific, reproducible issue. Fold identical failures into one signature ranked by how many players each hits, and your worklist sorts itself worst-first instead of arriving as a stream of vague complaints.

This is where a tool like Bugnet earns its place. Its SDK captures every GameMaker error automatically with the full stack trace plus device, OS, memory, build, and game-state context, folds duplicates into one grouped issue with an occurrence count, and ties each to the build it first appeared on — so you fix the problem that hurts the most players first and confirm it is gone when its signature disappears from the next release.