1. Week One Raster & Vector Formats

2. 2 Computer Graphics Vector Graphics Pictures are produced via a series of ‘draw commands’ These commands will draw short straight line segments and other primitives such as a circle Short straight line segments - strokes

3. 3 Some Sample Commands PenUp / PenDown SetPenColour (SomeColour) SetPenWidth (Width) SetPenStyle (Style) LineBy (Dx, Dy) / LineTo (x, y) Commands stored in a Display File which is executed to reproduce a display, e.g. WMF

4. 4 Raster Graphics (1) More common and the main thrust of this module Think of a Display (say, a Screen) as being described in terms of ‘a series of plottable points’ Each point is referred to as a Pixel The dimensions of the screen are referred to as its ‘resolution’ e.g. 1024 x 768

5. 5 Raster Graphics (2) The greater the resolution the larger the number of pixels If we have a black & white display then each pixel is either set (on) or not set (off) A Raster is a simply a portion of memory which contains a ‘map’ of the display contents Raster Graphics

6. 6 Raster Graphics (3) For Black & White Displays each pixel may be represented in memory as 1 bit For Colour Displays we require more than 1 bit for each pixel to be held, e.g. –4 colours requires 2 bits/pixel –16 colours require 4 bits/pixel –2 N colours require N bits/pixel Conceptually, the Raster has depth (3D)

7. 7 Raster Graphics (4) Shapes - straight lines; polygons etc. - are thus depicted in terms of a series of points (set pixels) Problem of selecting the appropriate sequence of points to best represent a straight line Many algorithms exist for drawing lines, circles, in-fill, etc.

8. 8 Image Size Example: 1024 x 768 resolution in 256 colours 256 colours => 8 bits per pixel so, 1024 x 768 x 8 = 6,291,456 bits for image or, more commonly, 6,291,456 / 8 = 786,432 bytes 786,432 / 1024 = 768 kB

9. 9 Video Just as with cine film, video is really a sequence of still images (frames) viewed in rapid succession. Important concept: frame rate –number of images displayed per second –often expressed in Hertz (Hz)

10. 10 Video Size What size is 2 minutes of video, each frame of resolution 640 x 480, shot in 24-bit colour at a frame rate of 25Hz ? One frame: 640 x 480 x 24 / 8 / 1024 = 900kB One second of footage: 900 x 25 / 1024 = 21.97MB Two minutes of footage: 21.97 x 120 / 1024 = 2.57GB