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Definition
An alpha channel is an additional data channel in an image that controls transparency. Each pixel has an alpha value from 0 (fully transparent) to 255 (fully opaque). Formats that support alpha channels include PNG, WebP, AVIF, and SVG. JPEG does not support transparency.
In a standard RGB image, each pixel stores three values: red, green, and blue intensity. An alpha channel adds a fourth value that specifies the pixel's opacity. This creates RGBA images where pixels can be partially transparent, enabling smooth blending with backgrounds.
Alpha transparency is essential for logos, icons, and overlays that need to appear on different colored backgrounds. Unlike GIF's binary transparency (pixels are either fully transparent or fully opaque), alpha channels support 256 levels of transparency, allowing smooth anti-aliased edges and gradient fading effects.
PNG-24 with alpha channel is the most widely used format for transparent web graphics. WebP and AVIF also support alpha channels with better compression. When converting transparent images to JPEG (which has no alpha support), transparent areas are typically filled with white or black. Maintaining transparency during format conversion requires choosing a format that supports alpha channels.