What the alpha channel stores
An image normally carries three channels: one for red, one for green, one for blue. An alpha channel adds a fourth, holding a number for each pixel that says how solid it is.
Typical files give alpha eight bits, allowing 256 levels between invisible and opaque. That range produces smooth gradients, which is why shadows and soft borders survive compositing. Eight bits per channel is the everyday standard for still images on the web.
How alpha combines with color
When an image is placed over another, the viewer blends them using the alpha value. Opaque pixels cover what is below, transparent pixels let it through, and partial values mix the two.
The same math handles anti-aliased text and feathered selections. Without alpha, every edge would be a hard step and every cutout would look like it was cut with scissors. Rendering engines read the values for each pixel and combine them with whatever sits underneath.
- RGB channels carry color
- Alpha carries opacity
- Full alpha range allows soft edges
- Rendering blends layers using those values
Formats with and without alpha
PNG, static WebP, GIF, and TIFF support transparency, while JPEG does not include an alpha channel at all. GIF restricts alpha to on or off, so its edges are harsh.
Filekind preserves alpha when converting between PNG and static WebP. Writing to JPEG fills transparent pixels with white, since the target format has nowhere to keep opacity. Keeping a PNG master makes it easy to repeat the conversion with the channel intact later.