Liquid flowing tweak

[rdococ]

Minetest and Minecraft have very similar liquid flowing mechanics. But in Minecraft, if you dig a hole and then place a water source a few blocks away, it will flow straight towards the hole instead of spreading. If you place the water source first, it will spread, and digging the hole won't remove the spread. This doesn't happen in Minetest.

I think this is the reason for MTG's 'river water' block. If the water took more direct paths downhill instead of always spreading out, having a separate water type for rivers wouldn't be necessary, and while can't be removed now (and if it could, it would be an MTG issue, not engine), it may be worth adding support for this kind of liquid flow for future games.

Example of what I mean with a cliff edge. The water source doesn't bother spreading and heads straight for the edge of the cliff, and then downwards like river water is intended to do. (From Minecraft Wiki)

[sfan5]

see #13764