Nano Banana transparent background: no alpha option, use RMBG

Nano Banana has no documented transparent-background setting. Google's Gemini image docs list JPEG or PNG output and do not mention alpha, and on Sume the background parameter is documented for GPT Image 2.5 only. To get a cutout from a Nano Banana picture, send it to RMBG 1.0, which returns a PNG with alpha.

Google's facts are from its Gemini API page; Sume's from the Image API docs and the OpenAPI document. Read 2026-10-01.

What does Google's page say about output?

It lists output as JPEG or PNG, ratios from 1:1 to 21:9, and notes that all generated images carry a SynthID watermark. Transparency is not mentioned there, so this post does not claim Nano Banana can or cannot do it natively.

Where does Sume offer transparent output?

Two routes are documented: background: "transparent" on GPT Image 2.5, and the dedicated RMBG 1.0 tool. Setting background on a model that does not list it should return 400 unsupported_parameter.

Which route gives transparency?

Summary of the options named in this post.

How do I cut out a Nano Banana image?

Generate the image, take the output URL, then call RMBG 1.0 with it as a public HTTPS image_url. RMBG is billed at $0.0225 per image.

curl -X POST https://api.sume.com/v1/rmbg-1.0/remove \
  -H "Authorization: Bearer $SUME_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "image_url": "https://media.sume.com/img/EXAMPLE/0.png"
  }'

Which should I pick?

If you need the subject with soft edges from the first render, ask GPT Image 2.5 for a transparent background. If you already like a Nano Banana image, cut it out with RMBG. Test your own subjects; neither doc promises edge quality.