Saving an RGBA image as JPEG and expecting transparency to survive, when the editor quietly composites it onto a solid colour first.
Editing and retouching
Alpha channel
Also called alpha transparency, transparency channel or rgba. Here is what it means, when it changes what you export, and what people get wrong.
Editing and retouchingRuns on your deviceUpdated 10 August 2026
The short answer
An alpha channel is an extra layer of information in an image file that records how see-through each pixel is. It sits alongside the red, green and blue channels, giving RGBA. With 8 bits per pixel it stores 256 levels of opacity, from fully transparent to fully opaque.
Updated
The short version
Alpha channel at a glance
| Quick fact | Detail |
|---|---|
| Also written | RGBA (red, green, blue, alpha) |
| Typical depth | 8 bits, giving 256 opacity levels |
| Value range | 0 is fully transparent, 255 is fully opaque |
| Two flavours | Straight (unassociated) and premultiplied (associated) |
| PNG uses | Straight, unassociated alpha |
| No alpha support | JPEG |
Slide the table sideways to see every column.
How does an alpha channel actually work?
Every pixel in a colour image already carries three numbers: how much red, how much green, how much blue. An alpha channel adds a fourth number that says how much of that colour should be used at all. At 8 bits the scale runs from 0 to 255, where 0 means the pixel is invisible, 255 means it is solid, and 128 means roughly half strength.
When the image is drawn, the renderer mixes each pixel with whatever is behind it in proportion to the alpha value. Alpha never changes the colours themselves. It only controls how much of them shows, which is why a badly cut edge keeps its leftover background colour even after the alpha says it should be mostly transparent.
What is the difference between straight and premultiplied alpha?
There are two ways to store the same transparency. With straight alpha (also called unassociated alpha) the red, green and blue values are kept at full strength and the alpha value sits separately. With premultiplied alpha (also called associated alpha) the colour values have already been multiplied by the alpha, so a pixel that is half transparent stores half-strength colour.
PNG stores straight, unassociated alpha. TIFF can store either and flags which one it is using. Premultiplied data is convenient for compositing and for scaling or blurring, because the maths comes out right without an extra step. Straight data keeps the true colour of a pixel even where it is fully transparent, which matters if you plan to re-edit the edge.
The practical point is that software has to know which kind it is handling. Reading premultiplied data as if it were straight (or the reverse) puts a dark or unnaturally bright rim around every soft edge, which is one of the quieter causes of halos on cut-outs.
Why does the bit depth of the alpha channel matter?
A 1-bit alpha channel has only two states: a pixel is either fully there or fully gone. GIF works this way, designating one palette entry as transparent. The result is a hard, stair-stepped edge on any curve, and no way to represent a soft shadow or a strand of hair.
An 8-bit alpha channel has 256 steps, so the edge pixels can be partly opaque. That is what makes anti-aliased outlines, blurred edges, glass, smoke and fine hair look believable on a new background. It is also what lets you keep a real drop shadow that darkens whatever it lands on rather than sitting on a grey patch.
Which image formats carry an alpha channel?
| Format | Alpha | Notes |
|---|---|---|
| PNG | Yes, 8 or 16 bits per pixel | Also supports simple 1-bit palette transparency |
| WebP | Yes, 8-bit | Available in both lossy and lossless modes |
| AVIF | Yes, full alpha | Alongside HDR and 8, 10 or 12-bit depth |
| TIFF | Yes | Flagged as associated or unassociated alpha |
| HEIC / HEIF | Yes | The alpha plane is stored as an auxiliary image |
| GIF | 1-bit only | One palette entry is treated as transparent |
| JPEG | No | Transparency is composited away on save |
If a file needs to keep transparency and be opened anywhere, PNG remains the safe answer. WebP and AVIF carry the same transparency at smaller sizes and are a good choice for the web.
What people get wrong about alpha channel
Each one is a real failure mode, not a style preference.
Treating premultiplied data as straight alpha, which draws a dark or bright rim along every soft edge.
Assuming alpha cleans up the colours at the edge, when it only hides them, so spilled background colour stays in the RGB data.
Relying on GIF for transparency on a curved subject and getting a jagged outline because GIF alpha is on or off only.
Questions people ask
Alpha channel, answered
The follow-up questions people search for once they have the definition.
Is an alpha channel the same thing as a mask?
They are closely related but not the same. A mask is an editable control inside a working document that decides what shows. An alpha channel is opacity data saved into the image file itself. Most editors let you convert one into the other, but only the alpha channel travels with an exported PNG.
What does 32-bit RGBA mean?
It means 8 bits for each of red, green, blue and alpha, so 32 bits per pixel in total. It does not mean the image has more colours than a standard 24-bit photo. The extra 8 bits are purely the transparency information.
Does adding an alpha channel make the file bigger?
Usually yes, because there is a fourth channel of data to store. The increase is often modest, since a typical cut-out has large areas of pure transparent and pure opaque that compress well. Detailed semi-transparent edges, such as hair, cost more.
Can a JPEG have a transparent background?
No. JPEG has no alpha channel at all. If you export a transparent image to JPEG, the transparent areas are filled with a background colour, normally white, and that fill is permanent. Use PNG, WebP or AVIF if the transparency has to survive.
Same cluster
More from the glossary
Neighbouring entries, so every term in the set is one hop from every other.
Aspect ratio
Aspect ratio is the relationship between an image's width and its height, written as two numbers such as 1:1, 4:5 or 16:9.
White balance
White balance is the adjustment that makes neutral things look neutral under whatever light you shot in.
AVIF
AVIF stores AV1-coded pictures inside a HEIF container.
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Sources
Where the facts on this page come from. Every link opens the specification, standard or documentation the claim was read at.
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