What is meant exactly by 'Deformable Mesh Area'? (context: Was Reading when/when not to use triangles)

Question

I have been learning basic modelling and texturing using Maya. I read many times to avoid using Tris and I can see why sometimes I get pinches or the edge flow breaks down.

But I was reading this thread: https://blenderartists.org/t/why-tris-are-bad/547399

The top answer says:

"Tris have their purpose and place. Those what blindly repeat never ever use tris probably can’t even tell you why. They don’t deform well, can cause pinching artifacts, don’t play nice with subdivision surfaces, and kill the holy edge loop. You can use tris. Just not on deformable areas and are best avoided on large planar areas as well. They work very well tucked away behind the ears though…"

It seems this guy knows what he is talking about and I understand that I should only use them in hidden areas. But what exactly does he mean by 'They don't deform well' and 'not in deformable areas'?

I googled it, it seems like any curved area is 'deformed' (https://catlikecoding.com/unity/tutorials/mesh-deformation/#:~:text=The%20mesh%20is%20deformed%20because%20a%20force%20is,entire%20object%20would%20move%20without%20changing%20its%20shape.). But is the answerer meaning deforming as in animation or live movement?

Answer 1

Think of an ocean surface, various sizes of waves. where all verticles move (try it in blender with ocean modifier). You don't get strange artifacts with simulating that with quads Not even if you later to decide to subdivide it. Clean topology makes life easier, but there can be exceptions to the quads rule. In some cases a tris might bend better, depending on the shape you might need a quad to have some bending angle divinding it into 2 tris.

Answer 2

"Deformed," in the context of 3D modelling and animation, means that it has a different shape than it originally did. What's "original"? That depends on the context. Most often, it means that it has a different shape than the final shape the modeller gave it-- that is, yes, it's about animation (which is live/dynamic movement)-- but a modeller might say, during the process of creating that final shape, that he or she is deforming the model from a shape it had yesterday, an hour ago, or a second ago. What's a different shape? It means that one or more edges have changed their length (but not all of them uniformly, that would be transformation) and/or that one or more face angles, the angle between two adjacent faces, has changed.

It is most often used in the context of "skeletal deformation" or "armature deformation", where you use an armature to deform the shape, but there are a lot of different tools with which you can deform a mesh. The important distinction is between deformation and "transformation" or "static geometry", and more about those in a second. It is frequently confused with "malformation", or ugly deformation, by those new to animation, but those are not the same-- deformation says nothing about how appealing the change in shape is, only that it is changed. It's totally appropriate to say, "That's great deformation!", which is of course not something you would ever say about a human baby.

Let's give an example. You have a robot model. In real life terms, this robot has some bits that are rigid metal. Without a massive amount of force, those individual bits are never going to change their individual shapes. They may change their relationship to each other, but only rigidly. They will never stretch or bend. We can say that those bits are static geometry. They do not deform. They only ever transform. (And in the context of the original question, we can safely triangulate this, because once we give it custom normals or a normal map to fix anything we dislike, those normals are never really going to change.)

The robot also has some rubber hoses connecting its limbs to its body. Those hoses are going to smoothly bend and stretch to reach their targets. In contrast to the metal bits, these hoses do deform. (And here, their normals are going to be dynamically adjusted with that deformation, ultimately from all faces neighboring that sample's face, meaning tris and quads are going to give us different results.)