Prepress, Composition and Image Preparation
This section covers Prepress, Composition and Image Preparation, the domain that tests copy handling, typesetting, layout, imposition, image resolution and plate or CTP preparation.
Core rules and concepts
- Copy preparation includes mark-up, proofreading and a style sheet; all corrections should be made before plate-making.
- Type is measured in points and picas: 12 points = 1 pica and 72 points = 1 inch. Leading is line spacing; kerning adjusts pairs; tracking adjusts a range.
- A layout needs trim size, bleed (commonly 3 mm), safe area, registration marks and slug information.
- Imposition arranges pages so that after folding and trimming they read in sequence; work-and-turn and work-and-tumble are common sheet layouts.
- Effective image resolution is calculated at final printed size; photographs usually need about 300 ppi, while line art needs more.
- Convert RGB images to CMYK for process colour, embed fonts, trap adjacent colours and supply a proofed PDF/X file.
- Conventional plates are exposed through film and developed; CTP exposes the plate directly, saving film and improving registration.
Worked example
A photograph is to print 10 cm wide at 300 ppi. Convert width to inches: 10 ÷ 2.54 = 3.937 in. Required pixels = 3.937 × 300 = 1,181 px. If the supplied file is only 600 px wide, effective ppi = 600 ÷ 3.937 = 152 ppi. That is below photo quality; rescanning is better than upsampling because interpolation cannot restore detail.
Traps
Do not mix ppi with dpi; do not omit bleed; do not enlarge a low-resolution logo; do not leave RGB images or missing fonts; do not impose pages without checking fold sequence and head direction.