Computer Graphics An Introduction Jimmy Lam The Hong Kong Polytechnic University

Agenda  Introduction to Computer Graphics  Computer System  Image Specifications (resolution and colour depth)  Image mode (bit-mapped and vector)  Image Colour (RGB, CMYK and channel)  CAD/CAM system

The Computer  CPU – fetches, decodes and executes instructions through active RAM memory  Storage memory  I/O device – scanner, printer, monitor

Computer Image (1) Resolution –With larger screen you could see larger number of pixels –The most common display – 640 x 480 pixels or 1024 x 756 pixels –The greater the screen resolution – dpi (dot per inch), the clearer the image –Standard resolution is 72 dpi

Computer Image (2) Graphic board –It supports various numbers of bits per pixel –A bit is the smallest amount of binary information used by the computer and graphic boards operate on an 8-, 12-, 16- or 32-bit-per-pixel system

Computer Image (3)  The greater the bit depth the greater the number of colours that can be displayed  Standard board is 8-bit for 256 colours  16-bit board – 32,768  24-bit board – 16.7 million

Computer Image (4) Scanner  Available in B/W, greyscale and colour  Transfer artwork, images or founded objects into the computer  Requires good deal of memory space on the hard disk

 The higher the resolution of the scan, the more memory is required  If the resolution of scan is equal to the screen resolution, the scale of print out would be the same as seen on screen

Basic Image Modes

Bitmap (1)  Computer graphics - 2 main categories: bitmap/ vector.  Bitmap images--technically called raster images-- use a grid of colors known as pixels to represent images.  When working with bitmap images, pixels are edited rather than objects or shapes.

 Bitmap images are resolution- dependent, contain a fixed number of pixels  Details could be lost and appear jagged if scaled on-screen or if printed at a lower resolution Bitmap (2)

Vector  Vector graphics are made up of lines and curves defined by mathematical objects

 Vector graphics are resolution- independent  Can be scaled to any size and printed at any resolution without losing detail or clarity.  The best choice for representing bold graphics that must retain crisp lines when scaled to various sizes e.g. logos. Vector (2)

Bitmap Image Vector Image

Questions  What type of image file (bitmap or vector) is normally used to represent textile fabric (knitted or woven)?  For fashion design, what kind of image file will be used and why?

Image Colour  What colour images are displayed by computer monitor?  What colour images are printed out by computer printer and textile printer?  How do you match colour of your fabric samples to the computer monitor?

Colour: Grayscale  Grayscale uses tints of black to represent a design.  A brightness value ranging from 0% (white) to 100% (black).  Images produced using B/W or grayscale scanners are typically displayed in grayscale.  Convert artwork to high-quality black- and-white artwork.

Colour:RGB  Primary colours – Red, Green, Blue, most widely used  Called additive colors because white is created by adding R, G, and B together  Additive colors are used for lighting, television, and computer monitors

Colour: CMYK  Cyan, Magenta, Yellow, and Black  RGB model depends light source to create color, the CMYK model is based light- absorbing quality of ink printed on paper  Combining pure C, M, and Y pigments resulted in black. So they are called subtractive colors. Black (K) ink is added for better shadow density

Colour: Channels  Channels – image component contain pixel information for any given colour – e.g. RGB, CMYK, Grayscale  Resolution – measurement of fineness of detail, the higher the resolution, the finer the detail

CAD/CAM CAM – more process oriented, concept of time is important CAD defines a part, whereas CAM defines a part’s manufacturing processes, from raw material to finished product

Can CAD and CAM join as one?  Yes, but…  The differences between the two will impede their integration, because their workflows and final products are so different  Also because of the nature inherent in how they are used  It probably will for the foreseeable future

Questions  How can you match colour between two fabric samples?  Is it possible to use CAD system to eliminate colour different between two fabric samples?