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Yes, I *still* have to look this up sometimes because I've used and made so many engines and tools that I don't know which hand is which and which way is up half the time ;) Gotta laugh at UE in the corner on its own there.
Ogre was right-handed (aka the canonical way if like me you learned from the Red Book) but had no concept of "up", technically. It could be whatever you wanted 
@sinbad oh no, I only knew a smaller version of that diagram. Crap. So if I learn Houdini I'll have to deal with yet another system...
@wildrikku @sinbad You can also get bigger versions that include projection matrices and stuff. It turns out that if you include enough parameters then nobody agrees.
@sinbad I queried the search engines for a while and never managed to find out why. Why would UE choose this coordinate system, breaking everyone's expectations about where the axes should be, all three in my case.
@kr4ft3r I can understand Z up, it makes 2D only calculations more sensible (truncate 3 items to 2, instead of having to swizzle XYZ to XZ like in Unity. Left hand coordinates are usually because people want positive coords to go into the screen not behind you
@runevision @kr4ft3r yeah UE was never designed for 2D games
@sinbad @runevision @kr4ft3r I think Magic Carpet, Dungeon Keeper and Fable were all the same coordinate system as UE. The thinking was indeed “it’s just top-down 2D, but you can also zoom in and out.”
@bbbscarter @sinbad @runevision @dev_ric If, on top of the top-down-first approach we also imagine that the game designer likes their top-down figures to face to the right, this may hold some excuse for why X+ is forward. I guess.
@kr4ft3r @bbbscarter @runevision yeah exactly - imagine drawing your top-down dungeon maps on graph paper, as so many of us old folks did - that’s canonically an XY coordinate system. Need to add height, use the next letter along 
@sinbad @kr4ft3r @runevision yup, exactly that. I’ll be honest, it’s still the most intuitive coordinate system for me, even though I know better - for many 3D games you do the initial map editing top down, like it’s a board game, and then raise/lower things.
@sinbad @kr4ft3r @bbbscarter @runevision yeah ok - if I think about drawing maps for a raycasting engine I can't argue with the Z up logic there 
I wonder if it's a case of what games you grew up playing. For me, it was the side on platformers like Dizzy and such. But if you're coming from Bomberman or early football games, I guess you're looking down through the screen at those worlds.
@dev_ric @sinbad @kr4ft3r @runevision yup, agreed. In my case I was trained in The Bullfrog Way, where the mindset was very board-game centric.
@bbbscarter @sinbad @kr4ft3r @runevision I loved Bullfrog games (massive thanks for those!), but it actually never twigged with me how board-game like they were. And now it's so obvious! Theme Hospital even had an actual board depicting level progression... How did I not realise? 
@sinbad That's all fine but I can't grasp the reason behind X forward, which also implies that X would be vertical in 2D, idk about real mathematicians but this is the opposite from what most coders expect.
@kr4ft3r when +Z is up and left handed, +X being forward makes +Y the right vector. People like positive bases
@sinbad Well thanks, that makes sense. Still doesn't explan why UE chose to be sitting alone over there, could it be some programmer's decades old arbitrary decision or was there a proper reason, I guess former and can leave it at that.
@kr4ft3r bear in mind this was decided by Tim Sweeny in 1998 before most of the others were even made, so you could say they all just decided to do something else
@kr4ft3r also all the choices are arbitrary, none of them are right or wrong - most math texts tend to be right-handed but even MS threw that out the window because they wanted increasing depth to be +Z
@sinbad @kr4ft3r in a WYSIWYG editor, I'll agree. But in code-only editors, Y up is the only way 3D makes sense to me. You're looking at a screen of X,Y units and adding a depth to it. You even have Z buffering to log the depth of a thing the further into the screen it goes. I can't fathom why anybody would then think it best to flip that organic axis over and have Z pointing upwards. 
@sinbad So I build and UV models in Lightwave, export them into Blender to rig, animate, and export the .fbx, and then import into Unreal.
Maybe I should add a stage involving Maya or something so I can get all four corners crossed off....