Understanding Digital Images
Pixels, vectors, and resolution
Raster images are grids of pixels; vector art is defined by mathematical paths and scales without loss. Know the relationship between pixel dimensions, physical size, and resolution (ppi): pixels = inches x ppi. Print work typically targets 300 ppi at final size; screen work is measured in pixels and CSS pixels, with higher-density displays using 2x or 3x assets.
Colour models and bit depth
Understand RGB (additive, screen), CMYK (subtractive, print), grayscale, and spot colours. Learn why RGB has a wider gamut than CMYK and why bright RGB colours can shift when converted. Know 8-bit vs. 16-bit channels and that bit depth affects tonal smoothness, not physical dimensions.
File formats and compression
Know when to use native editable formats versus delivery formats: lossless compression preserves all data, lossy compression discards data permanently and degrades with repeated saves. Understand transparency support, alpha channels, and why a flattened format cannot be re-edited easily.
Traps to watch
- Resampling vs. resizing: increasing ppi while keeping pixel dimensions shrinks physical size; it does not add detail.
- Upscaling: enlarging a raster image beyond its native pixels invents data and softens detail.
- Colour mode mismatch: designing in RGB and converting late can produce dull or shifted colours.
- Effective resolution: an image placed small and scaled up in layout loses effective ppi.
Rule of thumb
Decide the final output medium first, then set colour mode, pixel dimensions, and resolution to match it. Converting later is always more expensive than starting correctly.