Three-dimensional (3D) models represent a physical body using a collection of points in 3D space, connected by various geometric entities such as triangles, lines, curved surfaces, etc. Being a collection of data (points and other information), 3D models can be created manually, algorithmically (procedural modeling), or by scanning. Their surfaces may be further defined with texture mapping.
In the past, many computer games used pre-rendered images of 3D models as sprites before computers could render them in real-time. The designer can then see the model in various directions and views, this can help the designer see if the object is created as intended to compared to their original vision. Seeing the design this way can help the designer or company figure out changes or improvements needed to the product.
Shell models must be manifold (having no holes or cracks in the shell) to be meaningful as a real object. In a shell model of a cube, the bottom and top surface of the cube must have a uniform thickness with no holes or cracks in the first and last layer printed. Polygonal meshes (and to a lesser extent subdivision surfaces) are by far the most common representation. Level sets are a useful representation for deforming surfaces which undergo many topological changes such as fluids.
3D models can also be created using the technique of Photogrammetry with dedicated programs such as RealityCapture, Metashape and 3DF Zephyr. Cleanup and further processing can be performed with applications such as MeshLab, the GigaMesh Software Framework, netfabb or MeshMixer. Photogrammetry creates models using algorithms to interpret the shape and texture of real-world objects and environments based on photographs taken from many angles of the subject.
The first widely available commercial application of human virtual models appeared in 1998 on the Lands' End web site. The human virtual models were created by the company My Virtual Mode Inc. and enabled users to create a model of themselves and try on 3D clothing. There are several modern programs that allow for the creation of virtual human models (Poser being one example).
Over the last several years numerous marketplaces specializing in 3D rendering and printing models have emerged. Some of the 3D printing marketplaces are a combination of models sharing sites, with or without a built in e-com capability. Some of those platforms also offer 3D printing services on demand, software for model rendering and dynamic viewing of items. 3D printing file sharing and model rendering platforms include Shapeways, Sketchfab, Pinshape, Thingiverse, TurboSquid, CGTrader, Threeding, MyMiniFactory, and GrabCAD.
The medical industry uses detailed models of organs; these may be created with multiple 2-D image slices from an MRI or CT scan. The movie industry uses them as characters and objects for animated and real-life motion pictures. The video game industry uses them as assets for computer and video games.
A wide number of 3D software are also used in constructing digital representation of mechanical models or parts before they are actually manufactured. CAD- and CAM-related software is used in such fields, and with this software, not only can you construct the parts, but also assemble them, and observe their functionality.
The OWL 2 translation of the vocabulary of X3D can be used to provide semantic descriptions for 3D models, which is suitable for indexing and retrieval of 3D models by features such as geometry, dimensions, material, texture, diffuse reflection, transmission spectra, transparency, reflectivity, opalescence, glazes, varnishes, and enamels (as opposed to unstructured textual descriptions or 2.5D virtual museums and exhibitions using Google Street View on Google Arts & Culture, for example). The RDF representation of 3D models can be used in reasoning, which enables intelligent 3D applications which, for example, can automatically compare two 3D models by volume.
3D solid models can be tested in different ways depending on what is needed by using simulation, mechanism design, and analysis. If a motor is designed and assembled correctly (this can be done differently depending on what 3D modeling program is being used), using the mechanism tool the user should be able to tell if the motor or machine is assembled correctly by how it operates. Different design will need to be tested in different ways. For example; a pool pump would need a simulation ran of the water running through the pump to see how the water flows through the pump. These tests verify if a product is developed correctly or if it needs to be modified to meet its requirements.
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NUI are defined as interfaces in which a person interacts with the system with his/her body (hands, legs or any other parts of the body). Another characteristic of the NUI is that the learning process is fast, and the user can move from novice to expert in a quick transition. These user interfaces have previously been used in studies to create an interface that is adapted to children obtaining good results [3,25]. There are other studies that analyze the advantages and disadvantages of NUI and compare them with the standard interaction methods [26,27]. Rauterberg  carried out a study to compare four different types of interaction: (1) a command language, (2) a mouse, (3) a touch screen, and (4) a custom-made Digital Playing Desk. They used an implemented version of the computer game "Go-bang". The user had to play the game by moving a real chip on the virtual playing field using the four different types of interaction. A total of 304 visitors rated the usability of all four different interactions on a bipolar scale. The touch interaction was rated as being the easiest to use, followed by the mouse, the Digital Playing Desk, and the command language interface. Lacolina et al.  compared two interactive systems for natural exploration of 3D models. They compared two natural interfaces: multitouch vs. free-hand gestures. They concluded that both interfaces provide a natural dual-handed interaction and at the same time free the user from the need to use a separate device. The natural interfaces have proven to be useful for certain purposes such as learning [28,29] or rehabilitation . Most systems developed [10,18,19,23] for evaluating spatial short-term memory which are described in the virtual environments section have simple interaction methods (based on a screen, a mouse, and a keyboard).
The system is divided into three main components: the passive environment, the active environment and the user interaction. The passive environment was developed first. This passive environment consists of the objects that are static in the virtual world (e.g., buildings, the ground or the bench). To create all of these objects, we used a 3D model library called De Espona. These models were improved by using Blender and 3DS MAX to adapt the characteristics to the application requirements. The passive environment is composed by a city square surrounded by 16 visual cues including 8 buildings and 8 objects that are commonly found in a city (a streetlight, a bench, a trash can, a statue, a bin, a bus stop, a swing and a slide). The buildings are located distally, and the remaining visual cues are placed proximally to the city squared. Fig 6 shows the city square as seen from above. 2b1af7f3a8