Alpha and transparency
Which export formats can keep transparency, the difference between Straight and Premultiplied, what Matte is for, and how to check the result before you commit to it.
A canvas in Celas Studio always has an alpha channel while you work. What survives on export depends entirely on the format you pick: some keep it perfectly, some cannot represent it at all, and one keeps only a hard, all-or-nothing version of it.
Which formats can hold it
| Format | Alpha |
|---|---|
| PNG (still, sequence, sheet) | Yes, lossless |
| WebP (still, sequence, sheet) | Yes |
| JPEG, BMP | No: always flattened |
| MOV (QuickTime) | Yes, straight only |
| WebM (VP9) | Yes |
| Animated WebP | Yes |
| MP4 (H.264 / HEVC) | No: the Channels control is locked to RGB |
| Animated GIF | A hard cutout only, see below, not soft transparency |
For a still, sequence or sprite sheet, Background offers Canvas colour, Black, White and Transparent. Transparent only appears for PNG and WebP, because JPEG and BMP have nowhere to put it. Pick either of those and the dialog says so directly: "JPEG cannot store transparency, exporting on the canvas colour", with BMP shown the same way. The image is flattened onto whichever colour Background is set to, so if you need a transparent still, switch Format to PNG or WebP first.
For video, transparency lives in the Channels control instead of Background: set it to RGB + Alpha, which is only available for MOV, WebM and animated WebP. MP4 greys the control out and forces RGB, because neither H.264 nor HEVC can carry an alpha channel at all. For a transparent moving image, MP4 is not an option regardless of any other setting.
GIF is a special case
A GIF has one palette entry set aside as fully transparent, and a pixel is either that entry or it is not. There is no partial alpha. A soft edge or a drop shadow turns into a hard, slightly ragged cutout. For a clean edge, export MOV, WebM or animated WebP instead and keep GIF for quick previews where the cutout does not matter.
Straight vs Premultiplied
Where Channels is set to RGB + Alpha, a Color control appears with two options, Straight and Premultiplied. They store the same transparency, but the colour underneath it is kept differently, and the choice matters once the clip is composited over something else.
- Straight keeps the RGB values exactly as painted, independent of the alpha channel, so a half-transparent red pixel is still stored as full-strength red with 50% alpha alongside it.
- Premultiplied blends the colour toward the Matte colour as alpha drops, so a half-transparent red pixel over a black matte is stored as a darker red. It is the format some compositing and playback software expects, and it avoids a pale fringe around soft edges in software that does not handle alpha correctly.
Straight is the default. Switch to Premultiplied only if the software you are handing the file to asks for it.
Matte
The Background control becomes Matte the moment you choose Premultiplied, and offers Canvas colour, Black or White, never Transparent, because Premultiplied needs an actual colour to blend into the edges. Pick whatever background you will actually be compositing the clip over; the closer the match, the cleaner the edge. Choosing Straight instead removes the control altogether, since straight alpha keeps colour and transparency apart and has nothing to matte against.
Checking the result
The Video and Animated GIF dialogs both play a live preview with an eight-pixel checkerboard behind anything transparent, so you can see exactly what will and will not survive before you click Export. The GIF preview runs through the same colour-reduction pass the final file gets, so what you see there is what you get.