University of Babylon
College of Computer Technology
Department of Information Network
Day: Thursday
Date: 12-12-2013
Stage: Second
Topic: Introduction to Computer Graphics
Lecture8: Rendering and Projections
By
Suad Abdullelah Alasadi
1. Vector and Raster Data
Computer images are classified into two general types: those defined as a pixel map and
those defined as one ormore vector commands. In the first case we refer to raster
graphics and in the second case to vector graphics. Figure 2-1, on the following page,
shows two images of a cross, first defined as a bitmap, and then as a set of vector
commands.
The left-side image of Figure 2-1 shows the attribute of each pixel encoded in a bitmap.
The simplest scheme consists of using a 0-bit to represent a white pixel and a 1-bit to
represent a black pixel. Vector commands, on the other hand, refer to the geometrical
elements in the image. The vector commands in Figure 2-1 define the image in terms of
two intersecting straight lines. Each command contains the start and end points of the
corresponding line in a Cartesian coordinate plane that represents the system’s video
display.
An image composed exclusively of geometrical elements, such as a line drawing of a
building, or a machine part, can usually be defined by vector commands. On the other
hand, a naturalistic representation of a landscape may best be done with a © 2003 by
CRC Press LLC bitmap. Each method of image encoding, raster- or vector-based, has its
advantages and drawbacks. One fact often claimed in favor of vector representation is
the resulting memory savings. For example, in a video surface of 600-by-400 screen
dots, the bitmap for representing two intersecting straight lines encodes the individual
states of 240,000 pixels. If the encoding is in a two-color form, as in Figure 2-1, then 1
memory byte is required for each 8 screen pixels, requiring a 30,000-byte memory area
for the entire image. This same image can be encoded in two vector commands that
define the start and end points of each line. By the same token, to describe in vector
commands a screen image of Leonardo’s Mona Lisa would be more complicated and
memory consuming than a bitmap.
Figure 2-1 Raster and Vector Representation of a Graphics Object In the 3D graphics
rasterized images are mostly used as textures and backgrounds. 3D rendering is based on
transformations that require graphics objects defined by their coordinate points.
Software operates mathematically on these points to transform the encoded images. For
example, a geometrically defined object can be moved to another screen location by
adding a constant to each of its coordinate points. In Figure 2-2 the rectangle with its
lower left-most vertex at coordinates x = 1, y = 2, is translated to the position x = 12, y =
8, by adding 11 units to its x coordinate and 6 units to its y coordinate.
2. The Rendering Pipeline
A common interpretation of the rendering process is to consider it as a series of trans
formations that take the object from the coordinate system in which it is encoded, into
the coordinate system of the display surface. This process, sometimes referred to as the
rendering pipeline, is described as a series of spaces through which the object mi- grates
in its route from database to screen. Waterfall model of the rendering pipeline is
shown in
The rendering can be defined as transform the image from 3D world coordinate
to 2D image coordinate , the following figures shows the steps of rendering
3D Primitives
3D Modeling Coordinates
Modeling Transformation
Lighting
Viewing Transformation
Projection Transformation
Clipping
View Port Transformation
Scan Conversation
2 D Image Coordinates
Image Display
3. PROJECTIONS
Transform 3D objects on to a 2D plane using projections.
3.1 types of projections
*Perspective projection
*Parallel projection
In parallel projection, coordinate positions are transformed to the view
plane along parallel lines.
In perspective projection, object position is transformed to the view plane
along lines that converge to a point called projection reference point (center
of projection).
Perspective Projection
Parallel Projection