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