Geometries are used to describe the physical and logical dependencies of a fixture type. For example, dependencies between Pan and Tilt rotation as well as dependencies between a Master Dimmer and a Dimmer of a subfixture. A valid GDTF fixture type needs to have at least one geometry.

This chapter describes how to create geometries in an easy way.

To make things easier, look at your luminaire and think about the different construction elements. "Disassemble" the luminaire in your mind and think of the individual parts and what they do exactly.

By identifying the parts and their functions, you can decide what type of geometry to use:

**Static elements**(for visualisation only, e.g. a housing) ► Geometry Type**Normal Geometry****Rotating elements**(e.g. the yoke and head of a moving light) ► Geometry Type**Axis****Illuminated elements**(e.g. a lens) ► Geometry Type**Beam**

For a detailed description of all available geometry types, please see the GDTF Spec.

If you would like to use your own 3D models, you now know which models (parts) you need. To get these models, there are basically two ways:

- You already have a 3D model of the fixture. Then you can use a 3D modelling software of your choice to export the needed parts as separate files.
- You don't have a 3D model of the fixture, but maybe a drawing or picture. Then you can use a 3D modelling software to create the models from scratch and export them as separate files.

When using high-resolution CAD models used for design and manufacturing of the real device, you will likely exceed the recommended vertices count of 1200 for the whole fixture. Such a high level of detail is not needed for visualising the fixture. You should reduce the vertices count or re-draw the models, keeping only the most relevant details.

Learn more about the requirements for 3D models in the GDTF Spec.

Learn more about the requirements for 3D models in the GDTF Spec.

Next, think about the correct order of the geometries. Usually you start with the part of the fixture which has the mounting points to attach it to a truss or to put it on the floor. It can be a bracket or the base of a moving light, for example. This geometry will be your **Top Level Geometry**.

Any other geometries which are physically or logically attached to the Top Level Geometry will be **Child Geometries**. A Child Geometry itself can have children as well. This means you create a **tree structure**.

One of the easiest luminaires in terms of geometries is a PAR can:

It consists of two geometries:

- The
**"Body"**, which represents the housing of the PAR can. It has no special functions and is therefore a**Normal Geometry**type. Also, it is the part where you will attach a clamp to mount the fixture somewhere, so it is the**Top Level Geometry**. - The "
**Beam"**, which represents the front lens of the light source. It appears bright if you look at the fixture and it emits the beam, so it is a**Beam**geometry type. It is physically attached to the body, therefore it is a**Child Geometry**of the body. You can tell that by the indentation in the list.

If you move around the Body geometry, you will see that the Beam moves along - this is how the top-down principle in the tree structure works. You will make use of this concept in many other places when creating a GDTF fixture.

This simple moving head, on the other hand, consists of four geometries:

- The
**"Base"**(**Normal Geometry**type), which is the**Top Level Geometry** - The
**"Yoke"**, which can rotate around the Pan axis and is therefore an**Axis**geometry type. It is physically connected to the Base and therefore a**Child Geometry**of the Base. - The
**"Head"**, which can rotate around the Tilt axis and is therefore an**Axis**geometry type as well. It is phyiscally connected to the Yoke and therefore a**Child Geometry**of the Yoke. - The
**"Beam"**, which represents again the front lens (not the light source itself!) and is therefore a**Beam**geometry type. It is physically connected to the Head and therefore a**Child Geometry**of the Head.

You will define whether an Axis geometry is a Pan or Tilt axis when creating the corresponding DMX channels.

We will now create a basic moving light, using only default models.

In a new GDTF file, the Geometry page will look like this:

- Click on Add Top Level Geometry.

The Add Geometry pop-up opens.

- Fill in the
**Name**,**Length**,**Width**,**Height**,**Geometry Type**and**Select Model**as in the screenshot above. Then click on OK.

The Geometry page will now look like this:

- With the Base geometry selected in the list, click on Add Child Geometry.

The Add Geometry pop-up opens again.

- Fill in the properties as in the screenshot above. Then click on OK.

Back on the Geometries page, use the blue arrow handle to adjust the position of the Yoke on the Z axis.

- With the Yoke geometry selected in the list, click on Add Child Geometry.

The Add Geometry pop-up opens again.

- Fill in the properties as in the screenshot above. Then click on OK.

Back on the Geometries page, use the blue arrow handle to adjust the position of the Head on the Z axis.

- With the Head geometry selected in the list, click on Add Child Geometry.

The Add Geometry pop-up opens again.

- Fill in the properties as in the screenshot above. Then click on OK.

Back on the Geometries page, use the blue arrow handle to adjust the position of the Beam on the Z axis.

Learn more about the different Lamp Properties in the GDTF Spec.

The Beam geometry must be positioned outside of the head.

Otherwise there will be no light output in the visualisation!

Now all geometries are created and positioned correctly.

If you would like to know how to deal with more complex geometry structures, have a look at the chapter How to Deal With Complex Structures.