weird behavior with grid faces

I'm trying to make a low poly hills/terrain and ran into some weird behavior with the corner faces

I make a 10x10 grid of size 10

I select a square of 4 faces and I press G + Z + mouse move up to move these faces up 1m

the NE and SW corners look correct

But for some reason the NW and SE corners are different and don't slope downwards on the corners

I tried using edge slide but the same thing happens.

Does anyone know what is going on or how I can fix this and have all corners be the same?

@thibsert's explanation is completely correct, but to also answer the question of how to fix this:

Use the modifier Triangulate.

Here is an example with Shortest Diagonal:

And here an example with Longest Diagonal:

...Of course, you can also edit the mesh manually and add appropriate edges so that it is built exactly as you want it.

• Thx, I didn't know that :) Commented Aug 26, 2022 at 21:33
• That did the trick. Thanks! Commented Aug 26, 2022 at 22:06

I don't have any solution, but here is at least an explanation.

A graphic card can only display Triangles. When Blender seems to display a Quad, it is in fact divided into two Tris.

The thing is, you can divide an 'ABCD' Quad into two pairs of Tris : ABC + ADC, or ABD + BCD.

That's not an issue if A,B,C and D are co-planar : both pair will look like a flat quad face. But if they aren't, they will look like two distinct triangles, and that's what append to your corners.

If you Triangulate (Ctrl+T) your grid before moving the hill up, the result will be exactly the same, but the reason should look more obvious !

Please note that both are, geometrically speaking, perfectly valid. That's why I can't give you a solution : that is not a "problem"...

• It all makes sense now. Thanks for the explanation Commented Aug 26, 2022 at 21:12
• @PeterHearn A thank you is expressed here with an upvote ;-) What should I do when someone answers my question? Commented Aug 26, 2022 at 21:28