Common way of animating 'motion' for walk cycle animations

I've just posted this at the Blender artists' forums before realising I would probably get a better response from a more game development-specific audience, so apologies for cross-posting! It's for the right reasons :)

I'm a programmer trying to animate a character walking for a game project, using Ogre. I've made a very simple walk cycle in Blender and exported it to Ogre, and it plays just fine. By fine, I mean it works, but there's terrible foot sliding. This is because I just animated the walk in-place (at the origin) in Blender, and of course I don't know what "speed of walk" that corresponds to, so when I move the character in-game the motion doesn't necessarily match up with the movement of the feet in the animation.

So my question is: what's the normal approach for this kind of thing? At work we use Maya, and the animators either animate a special 'moveTrans' node that represents the "position" of the character (or have the exporter generate it for them from the movement of the root node), then the game can read this to know how fast the animation moves the character. So in the Maya file, the character will walk forward for one cycle and this extra node will follow along with them by their feet. I've not seen anything like this in open-source land, and there's certainly no provision for that in the Ogre Exporter script. What do you chaps normally do for this?

Answer

There are two ways of doing it: root motion or "in code".

With root motion you have basically exactly what you're suggesting with the "moveTrans" node. You use the motion of that object to determine how quickly to play back the walk cycles. You might be able to do a reverse lookup of sorts to figure out that if you want to be at position X what frame you need to be at. But the real advantage of root motion is that to get truly realistic animations you need to account for the fact that movement isn't a true linear path. Real people move at a variable speed over time, or maybe they're shuffling left and right a bit, or whatever.

Mixamo did a good example with root motion control using Unity. There's a video of it here: http://vimeo.com/12052874

Now root motion does have its drawbacks. It can get tricky with things like trying to get the "walk" animation to work on stairs (ideally you'd have a separate stair/ladder climbing anim and do all the movement for that in the animation as well). I'm also not really sure how IK really plays into it, if at all.

Now doing it "in code" is a lot easier from the programming side of things. What I suggest doing is first making sure all your units are consistent. For example, 1 unit in Maya and your game is 1 meter. Then you get your artists to animate a character walking (complete with forward movement). Use that to figure out the speed of the animation, and export the animation without the forward velocity. Use that to figure out how fast to play back the animation. So if the walk animation at 1x speed is 5 m/s, and you want to have the player run at 7 m/s, you simply play back at 7/5 = 1.4x speed. If your model is scaled up or down multiply the animation playback rate by the inverse of the scale. A half height dude will have to walk twice as fast to move the same rate as a 1x scale guy.