Introduction

In computer graphics rendering means creating a digital image with the use of a computer - to be more exact the generation of a view of a virtual scene under an arbitrary viewing direction. Conventional methods use a scene description based on geometrical primitives to create a view. McMillan uses the term geometry-based rendering for these methods [MB95]. With those methods regular shaped 'artificial' objects can easily be described. The modeling of curved surfaces, however, is more complex. These surfaces have to be approximated with a polygonal mesh. Natural objects as trees or items of daily life as clothing must be laboriously constructed with thousands of single faces.

Recently there have been great advances in three-dimensional scanning methods, which promise to simplify the process of model building. However, this automated model creation also verifies our suspicion: the geometry of the real world is extremely complex [MB95]. Therefore the modeling process is a laborious and time consuming task. But not only the modeling, also displaying the scene is algorithmicaly complex and needs high computational power. To rotate or to move an object at interactive frame rates, it is necessary to generate at least 5 images/sec. For complex scenes this can only be done with dedicated hardware.

In contrast, image-based rendering uses pictures for describing the scene. These images can be photographs, video takes or synthetic images. The modeling process is replaced by image recording. New views are produced through modification (interpolation, reprojection etc.) of existing input data. A great advantage of image-based rendering is that the effort to display an object is independent of the complexity of its internal representation. A tree is represented by just as much pixels as a cube (assuming both are the same virtual size). Thus, in an image-based rendering system the computational costs of displaying new images are constant and independent of scene complexity. In Table 1 the differences of the two methods are summarized [Kan97].

 

Table: Comparison between geometry-based rendering and image-based rendering

geometry-based rendering	image-based rendering
use of 3-D models	uses a collection of images
effort depends on scene complexity	effort independend of scene complexity
sophisticated software for realism	realism depends on input images
dedicated hardware	only main processor necessary
conventional rendering pipeline	pixel projection and pixel interpolation
	only static scenes
	not metric