What is an alpha channel?
The alpha channel is the part of an image that records transparency. Every pixel in a PNG or WebP image carries four numbers: red, green, blue, and alpha. The first three decide the color; the fourth decides how see-through the pixel is.
RGB tells you the color. Alpha tells you the opacity.
A pixel with (255, 0, 0, 255) is a fully opaque red dot — a wall of red. A pixel with (255, 0, 0, 128) is the same red, painted at 50% opacity, so whatever is behind it shows through. A pixel with (255, 0, 0, 0) is invisible.
When you see a checkerboard pattern in Photoshop, Figma, or your file browser, that checkerboard is not part of the image — it is a UI convention to show you the pixels whose alpha is below full opacity.
Which formats have an alpha channel?
- PNG: yes, full 8-bit alpha. The default choice for logos and UI assets.
- WebP: yes, full 8-bit alpha in both lossy and lossless modes.
- GIF: only 1-bit alpha — each pixel is either fully visible or fully transparent. No soft edges.
- JPEG: no alpha channel at all. JPEG cannot store transparency.
- AVIF: yes, full alpha. Newer, increasingly supported.
- SVG: yes — transparency is part of the vector model rather than a per-pixel channel.
Why can't JPEG store transparency?
JPEG was designed for photographs in the 1990s, before transparency was a common need on the web. Its compression algorithm works on three color components only. There is no fourth slot for alpha, which is why exporting a transparent design as JPEG forces a background fill (usually white).
When you convert a transparent PNG to JPEG with the Triim converter, you can pick that fill color (this is sometimes called the "matte"). White is the conventional default, but black or a brand color is just as valid.
How alpha affects trimming and background removal
When a tool says it "trims transparent pixels," it is reading the alpha channel and finding the smallest rectangle that still contains every pixel above a tiny opacity threshold. The result is a tighter crop with the same image content. The Triim trim tool does exactly this in your browser, with a default threshold of alpha > 10 so faint anti-aliased edges aren't mistakenly clipped.
Background removal is a different operation. The image probably has no alpha to start with — it's a product shot on a white backdrop. The job is to create the alpha channel by deciding which pixels are background. Triim's color-key remove-bg tool does this by measuring how close each pixel's color is to a reference (the top-left pixel or a hex you supply), zeroing out the alpha of pixels within a tolerance, and softly fading the alpha of pixels just outside the tolerance band.
Quick mental model
The alpha channel is a grayscale mask the same size as the image:
- White (255) means the pixel is fully visible.
- Black (0) means the pixel is fully invisible.
- Gray means the pixel blends with whatever is underneath.
Most image-editing operations can be described in those terms: trimming shrinks the rectangle so the alpha mask stops including invisible rows and columns; background removal writes a new alpha mask; matting a JPEG flattens the mask onto a single solid color.
Do all of this in your browser
Every tool on Triim runs entirely in your browser — no upload, no server, no storage. Try the trim tool, remove-bg, or convert and you'll see the alpha channel in action without any file ever leaving your device.