OpenGL® 3D models are collections of connected points in 3D space that form polygons to represent objects in virtual space. They are usually created with 3D modeling software and can include textures, surface normals, and bump maps. Libraries are used to load models into in-memory data structures for use by OpenGL®, but features like animation require code to be written. Procedurally generated models can also be created. Problems can arise with reading files, proprietary storage methods, and surface splitting.
OpenGL® 3D models, in the most general sense, are collections of connected points in three-dimensional (3D) space, called vertices, that combine to form a sequence of polygons that can be rendered to represent an object in virtual space. Almost all 3D objects that are not shape primitives, such as triangles, can be considered OpenGL® 3D models. In a more practical sense, references to an OpenGL® 3D model usually refer to a complex 3D object that has been built in a 3D modeling program. A complete model most often includes textures, surface normals, and bump maps, and, for animated objects, skeletal information and vertex weights.
OpenGL® is designed to be just a library of graphics functions used for quick visualization routines, so it doesn’t provide any real high-level support for 3D models. Instead, intermediate programming libraries are usually employed to load models from popular file formats and put them into in-memory data structures that can be used by OpenGL®. Fully realized OpenGL® 3D models are very complex to create, so they are almost always created with full modeling software. While a library can be used to load a complete model in OpenGL®, implementing features such as animation or motion requires that the code be written by the programmer, because OpenGL® has no innate capabilities beyond displaying the model.
Not all OpenGL® 3D models need to be loaded from a file. They can be built into a program using a sequence of algorithms or procedures. These procedurally generated models can be large primitives, such as a sphere or torus, or they can be more complex shapes, such as a helix. Through the use of fractal algorithms, 3D models that approximate mountains, trees, or rippling water can be created and manipulated as needed.
There are some complexities that can arise when using OpenGL® 3D models. The most common is the problem of reading a file, because not all 3D modelers provide the specifics on how to save a file. Another problem can be proprietary methods of storing vertex, texture, or material information. This would mean that some parts of the model could be loaded upside down or texture mapping could be completely useless.
One problem you may encounter with OpenGL® 3D models, regardless of file format, is the splitting of surfaces from modeler to OpenGL®. Surfaces such as quadrilaterals are sometimes divided into triangles using one formula, while OpenGL® might use another. This can cause patterns with holes to appear on the surface and can be a difficult problem to fix. Poor subdivisions can also lead to models that have been oversubdivided, causing a simple model to become very memory intensive as the number of polygons increases.
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