Belt Counter

Factorio gives you production graphs, but they cover the whole factory. If you want to know how much iron is actually moving down one specific belt, over the last minute, the game won't tell you. You count items by eye, or you build a contraption out of combinators and nixie tubes, or you guess.

I wanted a number. So I built a small mod called Belt Counter. You place a building that looks like a combinator, wire it to a belt, and it shows you that belt's throughput over time, broken down by item and by quality.

The counting is almost free, because Factorio already does the hard part. A belt can be wired to the circuit network in "pulse" mode, where it emits one signal every time an item moves onto it. You don't scan the belt or track positions. You wire it up, set it to pulse, and add up the signals. In Space Age those signals carry quality, so a rare iron plate and a normal iron plate show up as different signals, and the per-quality breakdown falls out for free.

There is one subtlety. The counter can also push its own measured rate back onto the circuit network, so you can wire it to a lamp or a display. But it reads from the same network it writes to, and if you aren't careful it counts its own output as input and the number runs off to infinity. The fix is to subtract last tick's output from this tick's reading before you accumulate anything. Once I had a test that fed the counter its own output and checked the rate held steady, I stopped worrying about it.

The interesting problem was the graph. I wanted a line chart over time, with the timescales Factorio's own production screen uses: five seconds, a minute, ten minutes, an hour. The production screen does exactly this. But it is hard-coded engine UI. There is no way to open it scoped to a single entity, and no way to feed it your own data.

Then I hit the part that surprised me. Factorio's GUI toolkit has no chart element. At all. You get frames, labels, tables, progress bars, sprites, around two dozen widget types, and not one of them draws a line. If you want a graph, you draw it yourself.

You can't draw into a GUI either. The only drawing API, LuaRendering, draws into the game world, not the interface. So the workaround is a bit of a hack. You create a hidden surface, an off-map dimension with a flat dark floor. You draw your line on that floor with LuaRendering. Then you point an in-game camera at that patch of the hidden surface and drop the camera into your GUI. The player thinks they are looking at a chart. They are looking at a security-camera feed of a line I painted on the floor of a void dimension that exists only to hold graphs.

It works. It is faintly ridiculous, and that is most of why I like it.

I think the reason these projects are fun is the same reason they are worth doing. The constraints. A platform that handed you a chart widget would have produced a more boring mod and a less interesting afternoon. The camera trick is in no manual. You find it by staring at what the API does give you, a world you can draw on and a camera that can point at any surface, and noticing that those two things compose into something nobody designed them to do.

Small tools are a good place to find these. The whole thing is a few hundred lines of Lua. It does one job. And buried inside it there is a private dimension whose only purpose is to be filmed.

Belt Counter is on GitHub. It needs Factorio 2.0, and Space Age for the quality breakdown.