why does mirring adds black rectangles to my model?

Support Modeling
1

hi…

i’m guessing it has something to do with normasl, but all i did was extrude a cube a few times, and add a mirror modifer, then right away i see it has some rectangle on it.

ok, i got it the mirroed half was INSIDE the original model, questions is, how do i make it mirror changes for a sepcific point, i have modelled the object changes to the right side should be applied to the left from the point on…

some thing like what happens during sculpt mode

2

How many axises are being mirrored? You might have both the X and Y mirrors turned on. Also, did you delete the half of the object that you were mirroring?

3

There are two or three things which will cause black face issues…

  • Normal mapping, usually fixed by recalculating normals.
  • Sandwiched faces (two faces in the same place), usually fixed by removing vertex doubles.
  • Smoothing on angles around a bend of over 90 degrees, or where there are edges too close together. Check for this by removing any subdiv modifiers and setting all edges as hard (non smooth).
  • Really really bad geometry (e.g. where faces twist in edgeflow so that they reconnect to areas but inside out) can make normals absolutely impossible to recalc.

Those ones are a few quickie possibilities. You got a screenshot or a blend file to give more clues?

4

lit was x axis, ike i said, i didn’t remove half of the object, how can i do that?

5

In Edit mode, select the faces on one side and hit Delete …

6

If you mirrored after the fact rather than interactively; not only will you have to recalculate normals, but also remove doubles.