Answer: Seedream 5.5 offers eleven aspect ratios — auto, 1:1, 4:3, 3:4, 16:9, 9:16, 2:3, 3:2, 21:9, 2:1 and 1:2 — with a background setting of auto, opaque or transparent, and PNG or JPEG output at 1K, 1.5K or 2K. Transparent PNG is a real asset; JPEG is not.
Most image-model write-ups treat framing as an afterthought, then people discover that a 21:9 cover cannot be cropped from a 1:1 render without losing the composition.
The ratio set, by job
| Ratio | Use |
|---|---|
| 16:9 | video stills, decks, web heroes |
| 9:16 | reels, story, mobile wallpaper |
| 1:1 | feed posts, avatars, product tiles |
| 4:3 / 3:4 | editorial spreads, print-ish layouts |
| 3:2 / 2:3 | photography convention, book covers |
| 21:9 | cinematic strips, site banners |
| 2:1 / 1:2 | extreme crops, skyscraper units |
| auto | let the model choose; use for exploration only |
auto is the one to be suspicious of. It is fine for a draft, but it makes every later step (an edit, a crop, a layer split) inherit a frame you did not choose.
Background: the switch that decides the file's future
| Setting | Output | Good for |
|---|---|---|
| auto | whatever the scene implies | full-frame imagery |
| opaque | solid, scene-consistent | photos, posters, anything composited later |
| transparent | alpha channel | cut-outs, stickers, product on white, overlays |
Two details matter more than the setting itself:
- Transparency requires PNG. A JPEG has no alpha channel, so asking for transparent + JPEG produces a matte that is not actually transparent.
- Transparency is not a mask. The edge follows the model's judgement; hair, glass and thin metal will need work in an editor.
Size and its two-step ladder
| Size | Credits | Typical use |
|---|---|---|
| 1K | 2 | drafts, thumbnails |
| 1.5K | 2 | web and editorial defaults |
| 2K | 4 | print-adjacent finals |
Same price for 1K and 1.5K, so the only reason to render at 1K is speed.
A workflow that avoids reframing
- Decide the delivery ratio before the prompt. A vertical brief written for a landscape frame produces a crop, not a composition.
- Write composition in the ratio's terms. 21:9 wants horizontal negative space; 9:16 wants headroom.
- Choose the background at generation time. Adding transparency afterwards is not a background setting — it is a cut-out job.
- Render finals at 2K, drafts at 1.5K. Both are one decision, not two renders.
Where this goes wrong
- Transparent + JPEG. No alpha. Use PNG.
- 21:9 for a thumbnail. The strip is unreadable at small sizes.
- `auto` in a chain. Every subsequent edit inherits the unplanned frame.
- Assuming 1:1 crops from 16:9. It crops, and the crop is a different picture.
FAQ
Is 4K available? No. Seedream 5.5 stops at 2K; Nano Banana 2.1 carries 4K.
Can I change the ratio on an edit? The edit re-renders, so yes — but the composition is judged against the new frame; expect to re-prompt.
Do extreme ratios cost more? No. Price follows resolution, not ratio.
Is transparent output available at every size? Yes, at 1K, 1.5K and 2K.
The ratio and background options are on Callirra's Seedream 5.5 page; pick the frame first, and the prompt gets shorter.