I found some tutorials on makeing planer reflections but I need some helpl with this one. (The answer is almost definatly obvious, but i fail to see it…sigh)
I want to make a mirror that reflects the room that its in. The mirror is vertical, however, unlike those that are in the tutorials that i read. Where should i place my empty to get the desired effect? If need be, I will post a .blend. Or, if you could provide an example, that would also be VERY helpful. Thanks!
dante
Hi Dante, I’ve had some success in using mirored planes, but only when they are perfectly vertical. I had problems with angled mirrors.
But here we go, I hope I can remember this, if not I’m sure Theeth will set me straight.
Add Plane, select camera, Shift S to snap cursor to camera.
In top view, now add an Empty and place it under the Camera. Select the Empty and press N.
For X and Y coordinate use the same as for Camera. The Z coordinate value must be same as for the Camera but negative. The Plane should be in the middle (z=0). (Is not nessecary just makes the maths easier.)
The X axis of the Empty should point towards the plane and it’s Z axis should point up.
Make sure Plane is on a seperate Layer to the things it’s reflecting.
EDIT - But if this method still won’t produce the correct reflection, Add the Empty after Snapping the 3D cursor to the plane. You might have to experiment a bit with the X axis of the Empty. I can’t remember if it needs to be pointing away from the camera or towards it.
Maybe have to use Cube mapping for the Envmap material/texture.
Hope this helps,
Sonix.
Sonix, I think you may have mixed up horizontal with vertical. Vertical would be like a mirror on a wall, horizontal like the horizon or water or ground plane. Changing the z axis to negative moves the empty down. In the case of a mirror then you would change the y axis number to - or positive. For example I tested this to remind my self by adding a plane in front of the camera with a iconosphere in front of the plane then moving the camera in top view to the right a bit to get an angle view. Changing z moved the empty below the 0.0 line. But changing it to (I think positive y) moved it to the back of the plane. Then the enviroment map worked to reflect the iconosphere on the plane. Other than that your explanation was good.
Paradox
Here’s what I did for one of the WCs, using the way (of what I think) Sonix is describing:
http://www.aomega.dp.ua/anton/EscapeOfTheMonkey.jpg
The key is to place your empty so that your mirror plane is the “symmetry plane” between Camera and Empty.
Btw, is it possible to make this scene (Mirror on the wall in a room) without having to split one wall off and moving it to a different layer (non-rendered by the envmap). I thought I had to do it, but it is a pain 
May be some backface-culling could be implemented in envmapping code, so that polys that are between the empty and the center of the reflecting object are not rendered? Would be a good addition, me thinks.
Anton42
Thanks all for your replys. Anton, does the rotation of the empty matter?
dante
No, the rotation of the empty has no effect
Yes, the mirror has to be on a seperate layer, as does anything behind the mirror.
The empty has to be exactly specular to the camera - this means that if eg the camera is 5 units away from a certain point on the mirror, the empty should be 5 units away from the same point on the other side. The calculations get tricky if the plane isn’t horizontal or vertical, but I heard there’s a script somewhere that will automatically move the empty to match the camera and the mirror.
Yeah theres a script that does it, but I think its all in french…
I’ll look around
thanks
dante
here’s the thread about the script
https://blenderartists.org/forum/viewtopic.php?t=15685
and here the script itself -> fixed for 2.28
http://jmsoler.free.fr/util/blenderfile/py/Envmap_O_Matic228.py
Cheers for the corrections Paradox, you are quite correct. I wasn’t 100% sure at the time hence the edit. 
Sonix.