Colours and Computer Jimmy Lam The Hong Kong Polytechnic University

Agenda How colour is displayed on computer? How colour is displayed on printer or textile fabric? 8 colour modes Colour terms and definition Colour simplified Discussion

Colour Display on Computer Display of colour on computer monitor is based on R,G, B (additive colour) Colour output on colour printer or textile printer is based on C,Y,M, K (subtractive colour). The two colour modes (addition or subtraction), however, are not the same. Therefore, the display colour on the monitor is not the same as colour output on printer.

Colour modes on computer display (1) 1. Bitmap – pixels are 100% B/W 2. Grayscale – pixels are B/ W or up to 255 shades of gray 3. Duotone – printing method using 2 or more plates to add depth to grayscale 4. Indexed colour – colour table up to 256 colours

Color mode on computer display (2) 5. RGB – most widely used 6. CMYK – Cyan, magenta, yellow, black (K value) 7. Lab colour – 3-channel mode that can be know for true colour 8. Multichannel – 256 multi-level grayscale channels

Color display on computer The two most common modes used by computer display (monitor) and computer output (printer) are RGB mode and CYMK mode

Matching RGB with CYMK Colour gamut: refers to imaging devices, e.g. scanners, monitors, printers – device to display a subset of all visible colours Colour mapping: permits the best match in appearance to the source image What You See Is What You Get

Colour Definition(1) Typically defined as the presence or absence of light reflected/ not reflected from a surface Colour is defined by Hue, Value and Saturation in the Industry

Colour Definition (2) :Hue Primary hues – Red, Yellow, Blue Secondary hues – Green, Orange, Violet Tertiary hues - yellow-green, red- violet, etc.

Colour Definition (3) Value Value is concerned with the light and dark properties of color. Value – range of grays from white to black Closest to white – tints Closest to black - shades

Colour Definition (4) All hues can be made in all values. Adding white paint will make any pigment lighter. Adding black paint will make most pigments darker Value can exist without hue e.g. Black, white and gray

Web safe colour palette Colour can be displayed “correctly” in both computer monitor and colour printer

Questions Why colour cannot display correctly on both computer screen and colour printer? How these problems can be addressed today?