Image and format terms, explained

The file4edit glossary explains sixteen image and file-format terms in plain language: lossy, lossless, alpha, aspect ratio, megapixel, DPI and PPI, compression artifact, chroma subsampling, quantization, dithering, raster versus vector, bit depth, sRGB, EXIF, unsharp mask and data URI. Each entry leads with a one-line definition and then says why it matters in practice, because a definition that only restates the term is worse than no entry at all. A filter box at the top of the page narrows the list as you type.

How it works

  1. Type the term you are looking for The filter box above the list matches against the term, its one-line definition and its full explanation, so you can search by a word inside an entry as well as by its name.
  2. Read the one-line definition Every entry opens with a single sentence — lossy is 'throws information away to get smaller', alpha is 'the transparency channel', DPI is 'only meaningful when printing'.
  3. Read what it means in practice Below the definition, each entry explains the consequence: why re-saving a JPEG degrades it every time, or why converting to a format without alpha needs a background colour.
  4. Clear the filter to browse With the box empty the whole list of sixteen terms shows again. If nothing matches what you typed, the page says so rather than leaving you with a blank list.

Questions

What is the difference between lossy and lossless compression?
Lossy compression throws information away to make a file smaller; lossless compression makes it smaller without discarding anything. JPEG, WebP and AVIF are lossy and discard the detail the eye is least likely to miss, which is why repeatedly editing and re-saving a JPEG degrades it every time. PNG and GIF are lossless: decoding gives back the exact original pixels, which is also why a lossless photograph is a large file.
Does DPI matter for images on the web?
No. DPI or PPI is a note about intended physical size and does nothing on a screen: a 1000-pixel image is 1000 pixels whether it is tagged 72 or 300 DPI. Changing the DPI value without resampling changes no pixels at all. It only becomes meaningful when the image is printed.
What is the alpha channel?
Alpha is a fourth value stored per pixel saying how opaque that pixel is, and it is what makes transparency possible. PNG, WebP and AVIF have it; JPEG and BMP do not. Converting an image to a format without alpha replaces the transparent areas with a solid background colour, which is why file4edit asks you to choose one.

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