Introduction

In this arti­cle we're going to see how to rotate objects in Uni­ty. There are sev­er­al ways to achieve this, in this case we will use the Rotate method of the Trans­form class, this will direct­ly mod­i­fy the rota­tion para­me­ters that are observed at the top of the inspec­tor with the GameOb­ject selected.

Go to the project’s main page

Before we start I invite you to watch the video I made to sum­ma­rize this arti­cle. Eng­lish Sub­ti­tles available.

How are rotations expressed mathematically?

In order to describe math­e­mat­i­cal­ly we have to know direct­ly or indi­rect­ly the fol­low­ing parameters:

-Rota­tion axis

-Amount of angles to rotate

-Sense of rotation

Let's imag­ine that the object we want to rotate is a sphere. We take it in our hands and stick a rod through the exact cen­ter of the sphere. Then we take the rod and ori­ent it from north to south par­al­lel to the ground. Final­ly we make it turn counterclockwise.

Were you able to visualize it in your mind?

My descrip­tion of the rota­tion was accu­rate although per­haps it was not draft­ed in the best way to be under­stood. How­ev­er, let us assume that we could all have imag­ined exact­ly the same thing.

In the first place we can say that the axis of rota­tion is the rod. If we become strict the axis of rota­tion is an imag­i­nary line that pass­es through the two ends of the rod.

The num­ber of angles to rotate is half a turn, this math­e­mat­i­cal­ly is 180° or π radians.

The direc­tion of rota­tion is coun­ter­clock­wise or counterclockwise.

These three para­me­ters can be ful­ly described using a vec­tor, which is what we are going to do in this article.

Workstation

We are going to use the rota­tion sta­tion which con­sists of three gears that are ori­ent­ed in the direc­tion of the world's three x,y,z axes.

The three gears are posi­tioned per­pen­dic­u­lar to each other.

In the hier­ar­chy we can find them inside the GameOb­ject "#9 Rota­teOb­jects". They have the names GearX, GearY and GearZ indi­cat­ing the axis of rota­tion of each gear.

The script to com­plete is called "Rota­teOb­jects" and is assigned to GameOb­ject with the same name in the hierarchy.

In fig­ure 4 we see that the three GameOb­jects of the hier­ar­chy are already assigned in the fields of the inspector.

When open­ing the Script we find the FixedUp­date method defined and inside some com­ments that say that the gears must be rotat­ed there.

If it is not clear what a method is in my chan­nel there is a video in which I talk about meth­ods in pro­gram­ming (eng­lish sub­ti­tles avail­able) and there is also an arti­cle in the pro­gram­ming sec­tion of this page.

Why should the rotation code go in the FixedUpdate method?

In the Arti­cle 1 about read­ing key­board and mouse inputs we dis­cussed why read­ing instruc­tions were in the Update method.

The FixedUp­date method as its name indi­cates is an update of the object state but exe­cut­ed at a cer­tain fixed time. 

This means that no mat­ter how much the frame rate of the game changes, we will have the same num­ber of exe­cu­tions of this method per second.

Colo­care­mos en el méto­do FixedUp­date una instruc­ción que hará que los engrana­jes giren una deter­mi­na­da can­ti­dad de ángu­los en algún sen­ti­do respec­to de un eje. Esto sig­nifi­ca que en cada lla­ma­da de FixedUp­date se apli­cará la rotación, lo que gener­ará un movimien­to con­tin­uo de rotación.

Solution

The Rotate method of the Trans­form class has dif­fer­ent ver­sions, we are going to use one in which the first para­me­ter is the vec­tor that com­plete­ly express­es the rota­tion and in the sec­ond para­me­ter we indi­cate the type of coor­di­nates to use, glob­al coor­di­nates that are unique in the whole world of the game or local coor­di­nates that are spe­cif­ic to each par­tic­u­lar object, which would be as if each object had its own north.

By math­e­mat­i­cal prop­er­ties of the vec­tors I rewrote the vec­tor as the prod­uct of its mod­ule or norm mul­ti­plied by a unit vec­tor (norm 1) describ­ing the direction.

This allows us to sep­a­rate on the one hand the direc­tion of rota­tion and on the oth­er hand the num­ber of angles to rotate.

Anoth­er advan­tage of express­ing the vec­tor that way is that we can change the direc­tion of rota­tion sim­ply by using neg­a­tive val­ues in the inspector.

Conclusion

You can sim­ply rotate objects in Uni­ty using the Rotate method of the Trans­form class.

If we don't need math­e­mat­i­cal pre­ci­sion, we can play with num­bers and get dif­fer­ent results.

With this type of rota­tion we direct­ly affect the rota­tion vec­tor of the Trans­form com­po­nent, this is fine if we seek to show only the ani­ma­tion rota­tion. If the rota­tion is need­ed to pro­duce phys­i­cal inter­ac­tions, Rigid­Body should be used.