Alpha Channel
A per-pixel transparency data channel. Enables semi-transparency and smooth cutouts in PNG and WebP.
An alpha channel stores per-pixel transparency as the fourth channel alongside RGB, corresponding to A in RGBA. Its theoretical foundation was established in Porter and Duff's 1984 alpha compositing paper.
Alpha values range 0-255 (8-bit):
- 0: Fully transparent - background shows through
- 255: Fully opaque - pixel color displays as-is
- 1-254: Semi-transparent - blended with background via compositing
Supporting formats:
- PNG: 8-bit alpha. De facto web standard for transparency
- WebP: Alpha in both lossy and lossless modes
- AVIF: Full alpha with high compression
- TIFF: Multiple alpha channels for print/DTP
JPEG lacks alpha support, so transparent images require PNG, WebP, or AVIF.
In web development, alpha channels are essential for transparent logos, UI overlays, compositing, and effects. Unlike CSS opacity which uniformly controls entire elements, alpha channels specify transparency per-pixel, enabling complex cutouts and smooth anti-aliasing at boundaries.