Materials on Back / Reversed Faces and UV Tools Fix

[Fletch]

Twilight is ok with you painting on the back face.
What did you say?
Yes, that's right... it's ok to paint on the back face in SketchUp. Twilight will render the material just fine... but with some caveats. So just be prepared for unexpected results.

1. If you are planning on turning the material into an emitter: be prepared for black. Emitter materials must decide which way to shine the light. They opt for the front face.
2. If you are planning on turning the material into a metal: be prepared for possible unexpected results... reflections may be determined by face normal.
3. If you paint a material on a back face, but this face is inside of a group, and then you paint the group with a completely different material: THAT group-wide material will be the material that is seen as being now on the FRONT face and will take precedent over the back face material.
4. If you are planning on applying a specular or bump map be prepared for unexpected results. The bump map may now be coming out of the face instead of going into it, as the face's normal is used in calculating many things.
5. If you are planning on applying glass to the face, be prepared for unexpected results... realistic glass measures distance and refractions utilizing information gained from which direction the face is oriented. Reversed faces can make a big difference between "realistic" and "wrong".

So what do I do now?
Quickly check your SU scene by using the "Face Style" toolbar and clicking the "monochrome" button. Then step through all your scenes to make sure you are only seeing white faces. Blue faces should give you a warning something may render in an unexpected way.

So, you've seen the light now, and realize you have painted several things on the back faces in your scene, and want to put the materials onto the front face? This is going to be terribly tedious, especially with curved or wavy surfaces. But take heart... there's a free tool that makes the fix a joy.
UV Tools for SketchUp by ThomThom the

After following the installation instructions carefully, you will now have this amazing plugin in your "Plugins" pull-down menu.

1. Select any and all faces that are reversed containing a material painted on the back face
2. choose "UV Toolkit"> "Backface To Frontface" ... it will automatically find all materials on all backfaces and paint them onto the front face for you... this will work for an entire scene, except components or groups (you must open each of them and it will work inside them.)
3. With the reversed faces still selected now right-click on one of them and choose "Reverse Face" in your SketchUp context menu.

Done. All better now.

Happy rendering.

[unclebim]

Thank you for the explanation, this is quite useful!

[Anwar]

Thanks alot , I under stood the problem and I solve it although I dont know how and why become this tow deffirent faces

[Fletch]

A computer thinks in 1 and 0.
A face in 3D has a Front (1) and a Back (0). This is so that the render engine can calculate the angles of bouncing light and reflections. If it does not know if the face is a 1 or a 0, front or back, then it can not calculate accurate reflection, refraction, angles, etc.

I am not sure that front face = 1 and back face = 0, this is only for a simple example.