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I was trying to create a symmetric wall by doing:

  1. Shift + D to duplicate the wall on the right side and move it to the left. enter image description here

  2. Change the Scale of Y of the duplicated object to -1. enter image description here

  3. Ctrl + A to apply scale. enter image description here

And the color of the object changed. enter image description here How can I change it back?

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    $\begingroup$ go in Edit mode, select all the vertices and ctrl N? $\endgroup$ – moonboots Sep 17 '18 at 9:55
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    $\begingroup$ Welcome to bse. Like wrapping paper one side has decoration the other doesn't. For a mesh face the "decoration" is the normal pointing out, what you want to see. Inverting the scale (negative scales) can have the effect of flipping the side we see. The ctrl N suggested above flips the normal back. There are options to make all normals consistent, or flip them on a face by face basis. Normals $\endgroup$ – batFINGER Sep 17 '18 at 12:08
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Blender does not fix surface normals(vectors defining surface direction) automatically when you invert the geometry so they are pointing the wrong way - to the inside of the object. The best thing to do would be to flip the normals:

enter image description here

Note that you need to be in edit mode and have the geometry selected. You can also reach this function from Faces menu that can be called with ctrl+f, you can just hit f once the menu is called - that's a very quick way to reach the function.

You could also use Recalculate Outside(ctrl+n) or Recalculate Inside(ctrl+shift+n) functions, however this might lead to undesired result if you have a mesh that has normals already set up in some particular way or if you are working with geometry that the automatic algorithm may not be suitable for (for example simple planes might confuse it since it cannot determine where the outside should be and if you have a few disconnected planes in one object that might be a problem). Most people will advise to recalculate normals in this situation, however it is a lot more logical to just flip them instead if you know that the problem is caused by inverting the geometry(scaling with a negative value).

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