The integration of 3D models into physics simulation applications has transformed fields like gaming, virtual reality, and scientific research. Whether we're developing a video game with realistic physics or simulating physical phenomena for academic purposes, loading 3D models correctly is crucial for ensuring accurate and engaging simulations. In this part we will go through the process of loading 3D models in our physics simulation application, covering key concepts, steps, and best practices.

3D Models: 3D models are digital representations of objects or scenes created using specialized software. They consist of vertices, edges, and faces that define the shape and structure of the object. Common file formats for 3D models include OBJ, FBX, and STL.

Physics Simulations: Physics simulations involve the use of mathematical models and algorithms to replicate real-world physical behaviors. These simulations can include gravity, collisions, fluid dynamics, and more, providing a realistic interaction between objects within a virtual environment.

In this Part we will focus on loading and manipulating 3D models, leaving Physics Simulation for later.

To start we need to import our 3D models into our application. The exact process will depend on the software we are going to use, but the general steps will be as follows:

1. Select a Compatible Format: Ensure our 3D model is in the format required by our application (e.g., OBJ, STL). This means we must select a file format that best covers our needs. For example, if we want to perform simple physics calculations, geometric data is all we need, but if we are designing a game we need a lot of data about the object’s representation. For most of the file formats we can find plenty of information or software libraries on the internet.
2. Prepare the Model: Clean up the model by removing unnecessary elements and ensuring it is optimized for our application. This includes checking for non-manifold geometry, applying transformations, and reducing polygon count if necessary. This step is usually done in third party applications like Blender. In these applications we can create our models ar modify existing ones, to adapt them to our specific needs.
3. Import the Model: Create an import function within our application to load the 3D model. This may involve specifying import settings such as scale, orientation, and material properties. Once our model has been brought to our specifications we can develop the necessary code to do this task. After loading the model we create the meshes required for our calculations and drawing.

The gl_model class is designed to handle model loading in a straightforward and clear manner.

model1 = new gl_model();
model1->load(model_name);
model1->set_scale(vec3(1,1,1));

In the model_loader sample application we load the model and simply rotate and scale it using the mouse. The gl_model class implements the necessary interfaces to perform these actions and finally draw the model as we did with all the simple objects so far.

Our library supports three model formats. They each have different characteristics and are very easy to implement. These formats are:

1. OBJ: this format supports material attributes and textures for model representations, making better for use in games and visualization applications.
2. STL: This format is very simple supporting only the geometry. This makes ideal for physics simulation and fast drawing.
3. OFF: This format enables per vertex color in the model making ideal for scientific representations of 3D models

Sometimes the 3D modeling applications save the meshes in a clockwise order and we need to invert them to match the counterclockwise drawing we have. For this reason, we can ‘tell’ the loading mechanism to do the inversion by passing another argument either to the constructor or the load function.

In this part we were introduced to the loading of 3D models in our application.