Video rendering is the process by which a computer processes information from a coded data source and uses that information to produce and display an image. The computer code may include instructions on creating images verbatim for playing back a movie or may provide a set of guidelines the computer uses to generate a custom image like a webpage. Video rendering can be one of the most hardware-demanding processes for a computer, especially when it's done in real time.
Real Time vs. Advanced Rendering
Whatever a computer displays on the screen is rendered in real time: The computer is computing all the coded data fast enough to display and update images with no perceivable lag. However, the computer can only render so much content complexity at once to continue the real-time rendering perception. The term rendering is used in video editing and processing to describe the computer taking extra time to render graphics and generate a full-motion-video playback version that works in real time. For example, a Pixar or Dreamworks computer animated movie features models that are too complicated for the computer to generate in real time, so the computer renders the content in advance so it can later be viewed in real time.
Motion Graphics vs. 3D Graphics
In addition to pre-recorded full motion video, computers can render motion graphics and 3D graphics. Motion graphics generally work with two-dimensional objects whereas 3D graphics work with polygons and other three-dimensional objects. Motion graphics use a combination of objects, images, footage and animation techniques to create video content. 3D graphics are different in that the computer renders video around three-dimensional virtual objects in three-dimensional space. For example, an older pixel/sprite video game from the 1980s uses motion graphics whereas a new three-dimensional game on a modern system uses 3-D graphics. The extra dimension doesn't equate to better image quality.
Adding Detail Layers
Elements like lighting, shading, reflections, shadows and other visual effects are added to the rendered video through additional layers. It would be very time consuming for a 3D artist to re-draw an object's shadow as it moves relative to a light source: Instead the computer uses calculations based on the virtual light source and virtual object to create a shadow. The virtual light source and corresponding shadows are different layer to the video. Both motion and 3D rendering are two-dimensional representations of space -- adding layers to both can give an illusion of depth.
GPU to the Rescue
The computer's processor doesn't go at video rendering alone. Graphics processing units, or GPUs, are a hardware counterpart to computer central processing units, or CPUs, that are much better suited for handling video rending complexities. CPUs are designed to handle large tasks very quickly one at a time, whereas GPUs are designed to handle dozens to thousands of small tasks simultaneously. Video rendering is a series of small tasks, making the GPU substantially better suited for the task.