Flipped polys, double sidded and, recalculate...

Just not sure what is happening.

There are times when I start a model no matter how I have “double sided” set in the mesh menu, it is only one sided.

And sometimes just the opposite. Sometimes - most of the time actually - I have the model stuck in double-sided mode. It will not display one side only.

And then the model that was single sided, then decides to display as double sided.

I have an ATI graphics card and I am using 2.56, usually the latest build. But this has been happening consistently in 2.5x

So obviously I am missing something key here. Is there a known bug with this or am I missing something?

Attached file has double sided on with a box that has some of the polys flipped. It should show as solid. And then when you switch to unchecked it should be as it is.

However this may work just fine on your machine. Please let me know what you find.


Box.blend (878 KB)


Had a look at your file and the reason it is sometimes not showing the faces correctly is because your normals on the faces that are invisible are facing the wrong way.

To correct, select all the faces that are displaying incorrectly and press CTRL+N whilst in edit mode. That will recalculate normals to the outside and hopefully will fix your issue.

normal and also any criss crossing edges
also for normal you avhe to llok at it in viewport to make certain all normal are in right direction

the normal algo does not always work nciely


Please read what I said and look again. I faced them the wrong way intentionally. If the face is double sided it should be solid regardless of which way it is facing.

If you load the file up and it is not double sided and you are using 2.5x and the check box is checked for double sided, the question is, why?

And also why on some of my files - most actually - that the opposite is true? Double sided is unchecked and the faces remain double sided.

This is the default - double sided. And when you model this way it is problematic because Blender always flips polys around and you are always having to recalculate.

The problem arises when you have no way to get feed back on this and it is guesswork until you render or export the file to another application.

Could be I am missing something very basic here.

well you were also in glsl mode
and i think this changes the behavior in viewport for texture!

i tried to save under 2.49 and lost the mat !

so did you test this in texture only and not glsl
which i cannot really test got bug with video card!

i did see a difference by changing to texture mode and re calculate the normal
then i can see all faces on cube !

append in 2.49

and changes with double or single face
faces becomes dark in texture mode
and by changing don’t see anything all greyish!

to get it as in 2.49
go to n transform panel at bottom

click on solid texture

Very good observation Ricky.

Seems to be Textured Solid is the thing the makes the most difference here and that may be what I was missing.

It is still not correct in my opinion. The polys should not be black but see-through. At least I have more to work with now.


Can anyone else confirm that this is odd behavior? Or that it is undesirable?

Because in LightWave for instance if the poly is flipped you can see through the back side if it is not there. If it is double sided it is solid.

This Blender display has really had me for a loop since I have been using it. It just seemed to get stranger in 2.5. For me this is not at all desirable behavior when modeling but at least I understand a little better what is going on now.

Hey Richard. Same thing happening here (2.58). No matter whether I check double sided or not the wall of my room - a single polygon, renders double sided. This seems like a serious bug, or a real hideous limitation.

can you upload a sample file so we can look at it!

happy 2.5

Yeah, something screwy has been happening here for quite a while. With texture solid off the mesh will show as double sided pretty much all of the time and with the normals pointed in the “wrong direction” the polys will appear darker. With texture solid on, regardless of whether or not the double sided is checked, the polys will behave as single sided in OpenGL. In my opinion, I think the behavior is mapped to the wrong control, such that the texture solid check box is behaving like the double sided check box, and the double sided check box isn’t working the way it should or not at all. Texture solid is doing double duty on display options, while double sided is doing a double dose of nothing.

Well, I could upload a blend file with an cube with its polys flipped and double sided turned off :slight_smile: But I think everyone can do that :slight_smile: When you render, because the polys are only single sided, you would expect the poly nearest the camera to be invisible, but alas, that doesn’t happen. The non-normal side of the poly still renders. Like the previous poster said, it works in the viewport in texture mode just fine (not solid mode though), but rendering still renders both sides (I assume).

Yes, the render has always been wrong as far as I remember, as it is always double sided.