Object-Apply-Rotation What's the point?
HI 忠鑫 崔 KKIDS ,
In this Lesson from ~3:55, Jonathan gives a great example for when you would Apply the Rotation. There are a few more situations, but not many and it is beyond the scope of a Basics Course to go much deeper into this subject.
In general, when modelling, you would try to avoid Applying the Rotation and only Apply the Scale.
Think of applying the rotation like you are re-defining or re-setting what you already did, like you are starting all over with no history. So if you rotate a plane 45 degrees, the object will remember it's rotation and it its mind it has a 45 degree rotation. You can see that in the N panel in the transforsm, the rotation will mark 45 degrees. But when you apply the rotation, it's like the plane is thinking oh never mind, I have always been this way and he no longer remembers that 45 degree and goes back to no history, in other words, the rotation goes back to zero, and you can see that in the value in the N panel.
You have the global axis. Transforms(location, rotation, and scale) transform the local axis(data) in reference to the world/global axis. When you apply any transform this makes changes the data to represent the visual transform. To get a grasp of local verses global try this:
1. Open blender and select the default cube. Rotate on x axis -20 degrees. Now imagen that the cube is the upper half of a human and they are blend over at the waist.
2. Hit R then hit z(this is global z) and move mouse. This would be the same rotation as if a person rotated at their hips. Hit z a second time(local z). Now the person would be rotating at the shoulders. Right click to exit.
3. Now apply rotation.(Ctrl+a->rotation). Repeat step 2. You will notice that now when you hit the second z(local) it now rotates the same as the global.
So the answer to your question is you want to apply the rotation when you need the local axis to match the global axis. This is something that comes with experience and a lot of trial and error. It always depends on the model and what your end goal is for that model.