SWE 423: Multimedia Systems

SWE 423: Multimedia Systems Chapter 4: Graphics and Images

Computer Graphics • Vector Graphics • Bitmap Images • Combining vectors and bitmaps

Models of Bitmapped Graphics • modeled by an array of pixel values • Bitmapped graphics does not use bitmaps (except for pure monochrome images) it use pixel maps

Models of Vector Graphics • stored as a mathematical description of a collection of individual lines, curves and shapes making up the image • e.g. line = two end points • Displaying a vector image • requires some computation to be performed in order to interpret the model and generate an array of pixel to be displayed • The process of interpreting the vector description known as rasterizing

Vector Graphics • Scalable • Resolution independent • No background • Cartoon-like • Inappropriate for photo-realistic images • Metafiles contain both raster and vector data

… Vector Graphics • Points can be identified by coordinates • Lines and shapes can be described by equations • Approximating abstract shapes on a grid of finite pixels leads to `jaggies‘ • Anti-aliasing can offset this effect • Bezier curves are drawn using four control points • Bezier curves can be made to join together smoothly into paths • Paths and shapes can be stroked and filled

… Vector Graphics • Geometrical transformations • translation, scaling, rotation, reflection and shearing - can be applied easily to vector shapes • Three approaches to 3-D modeling • constructive solid geometry • free-form modeling • procedural modeling • 3-D rendering models the effect of light and texture, as well as displaying the modeled objects in space • Ray tracing and radiosity are computationally expensive rendering algorithms that can produce photo-realistic results

Vector Graphics Tools • Adobe Illustrator • CorelDraw • Macromedia FreeHand • XARA X • Most can export your work to bitmap images

Metafile Format • Used for vector graphics • Can embed bitmap images • Collection of Graphics Device Interface (GDI) functions • “Rasterization” process • Default extension .WMF • Less space • Structure • header • size of the file • # of drawing objects • list of records

Other Common Vector File Formats • CGM • Computer Graphics Meta File • DRW • Micrographics Designer file format • CDR • CorelDRAW native format • HPGL • HP plotter print format • DXF • Drawing Exchange Files • created by Autodesk for AutoCAD

… Other Common Vector File Formats • EPS • Encapsulated PostScript Files • developed by Adobe Systems for compatibility with PostScript lang • device independent • may contain a TIFF or WMF image for screen presentation • AI • Illustrator native format

3-D Graphics • 2D: define shapes by paths • 3D: define objects by surfaces • Hierarchical modeling • A bicycle consists of a frame, two wheels, …

Rendering • In 3D, we have a mathematical model of objects in space, but we need a flat picture • Viewpoint • Position of camera • Scaling with distance • Lighting: position, intensity, type • Interaction of light: underwater, smoke-filled room • Texture

… Rendering • Wire frame • Hidden surface removal • Surface properties • Color and reflectivity • Lights • Shading • A color for each polygon • Interpolate color

SWE 423: Multimedia Systems