question about mirror modifier

Hi Folks

newbie here
if I take the default cube in the default top view and subdivide it twice
then select the half of the vertices on the left side of the screen and delete them
then I apply a mirror modifier
it mirror the remaining vertices and edges over the Z axis
I see this in different tutorials and folks then just proceed from there
but the default in the mirror modifier is to mirror across the X axis
if I deselect that, and select the Z axis, the effect goes away
what am I missing?
I can work with it like it is but it seem to me like it doesn’t work right
I’m sure I’m just thinking about it wrong:spin:
please straighten me out


The modifier is working, but both sides are already symetrical so you don’t see anything. Earlier, you subdivided the cube and deleted the one side, this let you see what was happening when you mirrored the x axis. If you remove the bottom half, you’ll be able to see the results of mirroring along the z axis.

if I remove the bottom half of the cube, what remains is symmetrical about the Y axis so mirroring about the X or the Z should give be back the full cube.
this is what I get

that looks like mirroring about X or Z would work but in fact mirroring about Y (what’s shown in the pic) produces a whole cube
I’m lost:(


I think you are confusing yourself thinking about what “axis” it is mirroring around.
When you select mirror on Y, every vertex is duplicated around the center point in the y “direction” not on the axis. It is helpful to work in the front view 1. Select the left side, delete vertices and add your mirror. This is the “default” way the mirror is intended to work. If you also delete the bottom half in front view, then turn the mirror on Z as well you will again have the full cube. Hope that helps.

around the center point in the y "direction" not on the axis.

bingo, that explains it:)
seems like an odd way to think about it but if that’s how it works I can deal with it


If you look at the grid Blender displays in 3D view, the green line is the Y axis, the red line is the X axis, and if you imagine another line going straight up and down, that’s the Z axis (also the blue arrow on the transform manipulator). The little pink dot that you see while in edit mode or when the object is selected is the objects origin. The mirror modifier mirrors geometry around the origin. If you delete half of the cube on the Y axis like you did in your attached photo, since what’s left is already symmetrical on the X and Z axis, the only operation with a visible result is to mirror along the Y axis, and since you deleted half the cube and the origin was right in the center, it creates a full cube. If that’s hard to grasp, let me know and I can make a .blend file that will explain it all.

EDIT I was writing this before anyone posted, so ignore it.

seems like an odd way to think about it but if that’s how it works I can deal with it

took me a while to get used to as well…i guess saying “y” axis is easier than “xz” plane

I get it
I just thought I was mirroring something across a line
blender mirrors across the origin in the direction of a line
this seems odd to me but it’s not a problem, just an adjustment in my thinking

thanks again