How to prevent Blender randomly flipping textures assigned to a face of a cube

Question

I'm encountering a problem that Blender sometimes flips a texture assigned to a face of a cube. The screenshot below illustrates the issue:

The texture "castlewallside1a" has been assigned to both of the visible faces of the cube. This texture includes a grass strip, which is supposed to be at the bottom (as seen in the preview). The screenshot shows that it's been correctly assigned to the face on the left, but is inverted when assigned to the face on the right. Both assignments were done the same way: select the face; unwrap it; select and assign the texture. The cube itself is aligned normally (the barely visible face at the top is the top of the cube).

I've tried inverting the cube before assigning the texture, but this does not help. The only workaround I've found is to create a second texture that's already flipped and assign it to the second face. While this does work, it doubles the number of textures and should not be necessary.

Is there a way to prevent this from happening? Also, is there a way to manually rotate the texture after application? Rotating the face is not a solution, as this twists the cube into a knot.

Answer 1

Susu's comment was dead on: the UV editor is the way to address this issue. I'm new to Blender and had never used the UV editor, but read up on it after seeing the comment. For others in the same position, the UV editor allows you to accurately control the mapping of textures to the faces of an object, including mapping different textures to different faces from a single texture map. Here's the approach that worked for me:

1. Prepare a single texture file containing all the textures you want to apply (the texture atlas). For example, you could build a cross-shaped graphic with six textures for an unwrapped cube.
2. In Blender, press the UV Editing button at the top to split the screen between the 3D Viewport and the UV Editor.
3. Put the 3D Viewport in Edit Mode and unwrap all faces of the cube.
4. Select each face in turn and use the UV Editor window to select the portion of the atlas that should be used to texture the face, rotating or scaling as required.

This doesn't address the question of why Blender randomly flipped the texture in the first place, but it does make it irrelevant.