Imaging Concepts
Raster versus vector
Digital images come in two families. Raster (bitmap) images are grids of pixels, each storing colour values. Enlarging a raster image beyond its pixel dimensions forces software to invent pixels, so edges soften or block. Vector images describe shapes with mathematical paths, fills, and strokes; they scale without loss and suit logos, icons, diagrams, and text, but not continuous-tone photographs.
Resolution, dimensions and colour
Image dimensions are measured in pixels (width × height). Resolution is expressed as pixels per inch (ppi) for screens or dots per inch (dpi) for printers. These describe how densely pixels are packed, not how many exist. Colour depth is the number of bits per pixel: 8 bits per channel across red, green, and blue gives 24-bit colour, about 16.7 million colours. Colour models matter: RGB is additive and used by displays; CMYK is subtractive and used for ink printing. Their gamuts differ, so RGB colours may shift when converted to CMYK.
Traps: confusing ppi with dpi; assuming a high dpi value adds detail to a low-pixel image; thinking file size depends only on dimensions (colour depth and compression also matter); treating RGB and CMYK as interchangeable.
Worked example: a 3000 × 2000 pixel photo printed at 300 ppi is 10 × 6.67 inches. Printed at 150 ppi the same file is 20 × 13.33 inches but looks less sharp. Rule of thumb: pixels are fixed; resolution only changes physical size and apparent sharpness.