COS3712 MAY JUNE EXAM

COS3712 MAY JUNE EXAM May /June 2011 COMPUTER SCIENCE COMPUTER GRAPHICS Duration: 2 hours Total: 70 marks Examiners: ........................................................................................................... ........................................................................................................... MEMORANDUM 2 COS3712 May/June 2011 QUESTION 1 [10] a) Differentiate between the object oriented and image oriented pipeline implementation strategies and discuss the advantages of each approach? What strategy does OpenGL use? [6] b) Give brief definitions of the following terms in the context of computer graphics: i) Antialiasing [2] ii) Interpolation [2] Interpolation is a way of determining value (of some parameter) for any point between two endpoints of which the parameter values are known (e.g. the colour of any points between two points, or the normal of any point between two points) In the object oriented strategy vertices are defined by a program and flow through a sequence of modules that tranforms them, colors them, and determines where they are visible etc. The output is the pixels in a frame buffer. Advantage – hardware to build an object based system is fast and relatively inexpensive. Image oriented approaches loop over pixels, or rows of pixels called scanlines. For each pixel we work backwards determining which geometric primitives can contribute to its colour. Advantages – we need only limited display memory at any time – we can hope to generate pixels at the rate and in the order required to refresh the display – handles global effects well. OpenGL uses the object oriented strategy. In the conversion between analog values of object or world coordinates and colours to the discrete values of screen coordinates and colours, rasterised (rendered) line segments and edges of polygons often become jagged or pixels that contrast too strongly with there neighbourhood are displayed.. This can be prevented by using antialiasing. Antialiasing blends and smooths points, lines, or polygons to get rid of sharp contrasts or other unwanted patterns.