Transparency is a fourth channel
Porter and Duff's 1984 compositing paper introduced the alpha channel — the A in RGBA — so each pixel carries its own opacity alongside red, green and blue [1]. A transparent background is simply a layer of pixels whose alpha is 0.
Formats that can store it
Only some formats can hold transparency:
- PNG — optional full alpha channel; the W3C published the Third Edition in 2025 [2].
- WebP — lossy or lossless with alpha, standardised as RFC 9649 in 2024 [3].
- AVIF — stores alpha as a separate image item at the same bit depth [4].
- GIF — only one fully transparent palette colour, no soft edges [5].
- JPEG — no alpha at all; JFIF allows only greyscale or colour channels [6].
Why some cut-outs get a halo
Graphics software stores alpha two ways: straight (colour and opacity separate) or premultiplied (colour already multiplied by opacity). Microsoft notes premultiplied alpha gives better results when images are filtered or layered [7]; when the two are mixed up, edge pixels come out too dark or tinted — the thin fringe you sometimes see around a cut-out on a new background. Downloading a clean PNG and placing it on the final backdrop in one step avoids most of it.
