Rendering is the process of generating an image from a model, by means of a software program. The model is a description of three dimensional objects in a strictly defined language or data structure. It would contain geometry, viewpoint, texture and lighting information. The image is a digital image or raster graphics image. The term may be by analogy with an "artist's rendering" of a scene. 'Rendering' is also used to describe the process of calculating effects in a video editing file to produce final video output.
It is one of the major sub-topics of 3D computer graphics, and in practice always connected to the others. In the 'graphics pipeline' it's the last major step, giving the final appearance to the models and animation. With the increasing sophistication of computer graphics since the 1970s onward, it has become a more distinct subject.
It has uses in: computer and video games, simulators, movies or TV special effects, and design visualisation, each employing a different balance of features and techniques. As a product, a wide variety of renderers are available. some are integrated into larger modelling and animation packages, some are stand-alone, some are free open-source projects. On the inside, a renderer is a carefully engineered program, based on a selective mixture of disciplines related to: light physics, visual perception, mathematics, and software development.
In the case of 3D graphics, rendering may be done slowly, as in pre-rendering, or in real time. Pre-rendering is a computationally intensive process that is typically used for movie creation, while real-time rendering is often done for 3D video games which rely on the use of graphics cards with 3D hardware accelerators.