Module 1: Vector Foundations

Vector vs Raster Explained

Before you draw anything in Illustrator, you need to understand what kind of image it creates and why that matters. Choosing the wrong type of file is one of the most common reasons logos look blurry on banners, print badly on T-shirts or cannot be edited by a printer. Once you understand vector and raster images, you will know which program to open, which file to ask clients for and which format to send.

Raster images: a grid of pixels

A raster image, also called a bitmap, is made of a grid of tiny coloured squares called pixels. Every photograph from a phone camera is a raster image. Programs like Adobe Photoshop are designed mainly for raster editing.

Raster images have a fixed number of pixels. A photo that is 1200 by 800 pixels contains exactly that many pixels. If you enlarge it beyond its size, the software has to invent pixels, and the image becomes soft or blocky, which designers call pixelated. Raster images are excellent for photographs and complex textures, where smooth, subtle colour changes matter.

Common raster formats are JPG (photos, small file size, no transparency), PNG (supports transparency, good for screen graphics) and WebP (efficient format for websites). Photoshop's PSD files are mostly raster too.

Vector images: shapes described by mathematics

A vector image is made of paths: points, lines and curves described mathematically, with fills and strokes applied to them. Instead of storing millions of pixels, the file stores instructions such as "draw a circle at this position with this radius and fill it with this blue".

Because the shapes are calculated each time they are displayed or printed, vector artwork can be scaled to any size without losing quality. The same logo file can be printed on a business card or on a building wrap and remain perfectly sharp. Vector files are also easy to edit: you can change a colour or move a curve without damaging anything.

Vectors are ideal for logos, icons, typography, illustrations with flat or gradient colours, diagrams, maps and anything that must be cut by machines, such as vinyl signs or laser-cut materials. Common vector formats are AI (Illustrator's native format), SVG (for the web), PDF (when saved with vector content) and EPS (an older format still requested by some printers).

Comparing them side by side

Illustrator can contain raster images, for example a photo placed inside a poster, and Photoshop can contain some vector elements like shapes and type. But each program is strongest at its own type of graphic.

A worked example: the blurry banner

Imagine a hypothetical school in Machakos that wants a 3-metre banner for its sports day. The administrator sends the printer the school logo copied from the school's Facebook page: a JPG of 400 by 400 pixels. Printed at banner size, each pixel becomes several millimetres wide, and the logo looks jagged and blurry.

A designer recreates the logo in Illustrator by tracing its shapes with vector tools, then saves it as AI, PDF and SVG. The printer can now print it at any size, the sign maker can cut it from vinyl, and the web developer can use the SVG on the school website. One afternoon of vector work solves the problem permanently.

How to tell what you have

• Zoom in very far. If you see square pixels, it is raster. If edges stay crisp, it may be vector.
• In Illustrator, open the file and switch to Outline mode with View > Outline (Ctrl+Y on Windows, Cmd+Y on Mac). Vector shapes show as thin outlines; a placed raster image shows as a box with a cross.
• Be careful: a PDF or EPS can contain a raster image inside it. The file extension alone does not guarantee vector content.

Common mistakes to avoid

• Enlarging a small JPG and expecting it to look sharp.
• Renaming a JPG to .ai or .svg. Changing the extension does not change the content.
• Sending only a PNG logo to a printer or sign maker who needs vector artwork.
• Using vector for a photograph; tracing a detailed photo produces huge, unattractive files.