What causes lengthdir using the wrong angle sign in GameMaker?

GameMaker measures angles counterclockwise but the room y axis points down, so mixing trig from another engine or negating the angle sends lengthdir results in the mirrored vertical direction.

How do I catch this when it only happens for some players?

Capture it automatically from the player's device with the full stack trace, the device and OS, the build, and a breadcrumb trail of what they did. That turns a vague complaint into a specific, reproducible issue you can fix without owning the hardware it happens on.

How to Fix lengthdir Using the Wrong Angle Sign in GameMaker

Q: How do you fix lengthdir using the wrong angle sign in GameMaker?

Use lengthdir_x(len, dir) and lengthdir_y(len, dir) directly with GameMaker's angle convention rather than hand-rolling sin and cos with the wrong sign.

Quick answer: Use lengthdir_x(len, dir) and lengthdir_y(len, dir) directly with GameMaker's angle convention rather than hand-rolling sin and cos with the wrong sign.

A projectile aimed with manual trig that flies down instead of up in GameMaker has the angle sign wrong. Using the built-in lengthdir functions with GameMaker's convention fixes it. Here is how.

How to fix it

1. Use lengthdir functions

Compute movement with x += lengthdir_x(spd, direction) and y += lengthdir_y(spd, direction). These already account for GameMaker's angle convention and downward y.

2. Remember angles are counterclockwise

In GameMaker 0 is right and angles increase counterclockwise, so lengthdir_y internally uses -sin. Do not add your own negative sign on top of it.

3. Set direction and speed directly

For simple motion, set the built-in direction and speed variables; GameMaker derives hspeed and vspeed with the correct signs automatically.

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.

Reproduce it once with full context and the fix writes itself. The hunt is the expensive part.