[Mini Howto] Mirror tool on Blender

[jda]

Archmage asked me about the Mirror tool in Blender.
So I thought I'd post a little tutorial about it here. Specially 'cause I link to a couple very nice videos too!

Note: You can use the Mirror tool in either Object or Edit mode - people say it works differently whether you do it in one or the other mode but (though I haven't actually bothered to test this myself) I think that's just because (I think) you can set the pivot Point independently for each mode.
1. Select what you want to mirror: whole object (object mode) or part of a mesh (edit mode, selection by either vertices, edges or faces).
2.Make a copy of your selection (SHIFT+D).
3. Set the said pivot point: It's the icon immediately to the right of the Viewport one. Click it and you'll get a menu-list of some five different options. Probably the most used ones are the Median point and the 3D cursor:
3.a. The Median point option will simply mirror your selection around the selection's median point (it's "center"). (@Archmage: This is probably what you'll want to use for the axe. It's what I used to mirror the half skyship I worked on for most of the time and make it whole again (joining the two halves)). Note: Sometimes this will put the mirrored selection just at the same position it was before (only flipped/mirrored), sometime it will displace it.
3.b. The 3D cursor option will mirror your selection around the 3D cursor's position. This is what I used e.g. to rotate (the pivot point can be used to rotate to) the outrider's arms: I placed the 3D cursor at about the shoulder's center of the arm and then rotated it around that point (if I had rotated it around the median point, the shoulder would have gone "into" the torso...).
Very nice getting started with blender video which includes an excellent explanation of the Pivot point: http://www.rab3d.com/tutorial_started.html#video (see from about time: 01:45).
(This video and the one before it on that page were actually what I followed to start modelling in blender! )
4. Press CTRL+M, select what axis you want to mirror it on (X, Y or Z) and press ENTER.
5. If needed, move the mirrored copy to the apropriate position. Hint: in Edit mode, use the Snap-tool, then select everything (A twice) and Remove doubles (W then 6). You're done!
Excellent video on the snap tool: http://blenderb.blogspot.com/2009/10/transform-snap-tool.html

EDIT: Thought I'd mention:
The above does not work in the UV map editor. However there is a mirror funcion on this window. And it's actually simpler:
1. Select what you want to mirror.
2. Press M.
3. A menu pops up asking whether to mirror on the X or the Y axis. Make your selection. Done.

EDIT #2:
John.d.h suggested bellow that using the Mirror modifier would be a better option. He also kindly provided an excelent link to learn about this modifier. Here it is: http://wiki.blender.org/index.php/Doc:Manual/Modifiers/Mesh/Mirror  
I think that tool actually is a better option than what I described above in most cases. But I can think of at least one exception. Look at my second reply (post #5) on this thread for that.

ElimiNator also noted to flip the normals. This is a very important subject! Though the orientation of your normals will make no difference to the 3D model itself, it will completely scr** up your UV mapping/texturing later on if they're not correct!
Mirroring (with either method) may set your normals facing the wrong way! I won't go much into this here but here's a few tips:
- You can see how the normals will affect your texture by looking at your model in "Textured" mode (instead of Object or Edit), you can switch to and from this viewport by pressing ALT+Z. If you see a transparent face where it should be solid, chances are (100% unless you're actually working with Blender transparency) the normal to that face is flipped the wrong way.
- You can press CTRL+F then 1 to flip the selected face(s) normals.
- CTRL+N in Edit mode recalculates your "outside normals" for the entire mesh. Sometimes fixes it, sometimes messes it up further, but no harm and trying and undoing it afterwards if it didn't work.
- Pressing CTRL+A then 2 in Object mode sometimes also affects normals (usually NOT if they're correct already )

[John.d.h]

Isn't it a heck of a lot easier to use the Mirror modifier?

[-Archmage-]

No I don't think so John...

Jda, thanks, I learned a lot, about snapping and mirroring.

[jda]

Isn't it a heck of a lot easier to use the Mirror modifier?

I know it's used and it's somewhat more automatic. But I never used it before myself (and I'm usually not very keen on automatisation myself ).
Still you may have a very good point. Care to give a link to an easy overlook? (yeah I could find it myself but I'm sure you know of a really simple one that just lets you know in which situations it's good for and how it generally looks ). And you're good at providing links!

No I don't think so John...

Jda, thanks, I learned a lot, about snapping and mirroring.

You're very welcome. I've learned a lot of tricks from you too!

[John.d.h]

http://wiki.blender.org/index.php/Doc:Manual/Modifiers/Mesh/Mirror

I think the biggest difference between the two approaches is that while you have mirror modifier, you can edit symmetrically, meaning if you move a vertex on the right, the corresponding vertex on the left will move as well.

[jda]

I knew you'd provide the perfect link, John! Thanks! 

That (the Mirror modifier) would actually not have been apropriate for my skyship because it has a cabin you can look into (hence, I needed to edit the outside of the cabin and the inside as well (for various reasons). An earlier version of the skyship actually had two openings, one on the front of the ship (that it keeps) and one at about the middle of the ship, where you could see the pilot's feet hanging out of (I had problems with the overal dimensions of the ship and, though I did have another option, I opted for simpling "closing" that opening); this mid-section of the ship allowed some looking (specially with camea zoom) into the inside of the cabin from a side-rear standpoint). Hence it was crucial that I could edit the inside of the ship as well.
Using the method I described above meant I could look at the inside planes of the cabin seamlessly. The mirror modifier would just have drawn the outside on top of it - not good. ;(

Still, I think for Archmage's double bladed axe, the mirror modifier would be best! Do use it instead, Arch! It will not only save you some extra steps to join the halves later, it will allow you to edit it at any given time!
I mean... suppose you'd finished one blade, had duplicated, mirrored and joined it with the other half... and then thought to make an extra edit on it... you'd have to repeat that edit on the other blade too... Even more work!
Furthermore, if you're using the mirror modifier you can see, at every step, exactly how it will look with the two blades. I guess that helps.

The link John gave is pretty nice, it moves on to further options and complications of the modifier... Just read up to the point you figure out what you need, nevermind the rest for now, and just start using it.
That's how I do most of my learning.

[-Archmage-]

I already made two axes and posted them in the dwarves topic, what are you missing?

[jda]

I already posted on the Dwarves topic you were fast and i'd just posted here.

[-Archmage-]

Yah, I noticed that, right as I returned to the main page from posting.

[ElimiNator]

Isn't it a heck of a lot easier to use the Mirror modifier?

Yes I think so, and just flip the normals too.

[Loronal]

I have had problems when I mirror al the uvs get screwed on my humans. This even happens in omegas tutorial (I think omega made the tutorial on lest.110mb) download the model and the right side will be totally weird

[jda]

Yes, Pheonix, I did mention that on my EDIT #2 on the first post.
If you need more information on the sollutions I mentioned, you'd better search for it on the Net. I surelly won't have time (and also don't really have inside knowledge about it) to go into that.