Nano Banana Image Gen

Nano Banana Image Gen

Generate editorial illustrations, marketing assets, and product mockups with Google's Gemini image-generation models, using character sheets and style templates as references.

When to Use

✅ Use for:

• Generating with gemini-2.5-flash-image, gemini-3.1-flash-image-preview, gemini-3-pro-image-preview
• Character-consistent illustrations across multiple scenes (same person, new setting)
• Style transfer using a reference image as the visual template
• Multi-image composition ("character from img 1 wearing outfit from img 2 in environment from img 3")
• Iterative image editing with conversation-style refinement
• Anything mentioning "Nano Banana", "Nano Banana 2", or "Nano Banana Pro"

❌ NOT for:

• Ideogram V3 generation — use existing next-app/scripts/gen-*.py scripts and the IDEOGRAM_API_KEY pattern
• Qwen image generation on M4 Max — see ~/.claude/QWEN_IMAGE_GUIDE.md (use v0.6.0)
• Generic prompt engineering tutorials
• Non-image Gemini API usage (text completion, embeddings, video)
• Image classification / object detection (use computer-vision-pipeline)

Core Process

flowchart TD
  S0[Request] --> S1{Need character<br/>or style consistency?}
  S1 -->|No, simple scene| M25[Use gemini-2.5-flash-image]
  S1 -->|Yes| MPRO[Use gemini-3-pro-image-preview]
  M25 --> CFG
  MPRO --> CFG[Set image_config.aspect_ratio<br/>response_modalities=IMAGE]
  CFG --> REFS{How many<br/>references?}
  REFS -->|0| P1[Prompt only]
  REFS -->|1| P1
  REFS -->|2+| P2[Label each ref's role<br/>character / style / lighting / etc]
  P1 --> SCENE[Write scenario:<br/>Refs + Relationship + Scene]
  P2 --> SCENE
  SCENE --> POS[Use POSITIVE framing<br/>describe what SHOULD be there<br/>negatives do not work]
  POS --> CALL[POST to generateContent]
  CALL --> OUT{Image OK?}
  OUT -->|Drift / wrong aspect| FIX[See troubleshooting.md]
  OUT -->|Yes| DONE[Save + convert to webp]
  FIX --> CALL

Step 1: Pick the right model

Full comparison: references/model-selection.md.

Step 2: Build the request correctly

The aspect ratio MUST be set in the generation config. Putting "16:9" in the prompt does not work.

config = {
    "responseModalities": ["IMAGE"],
    "imageConfig": {"aspectRatio": "16:9"},  # ← non-negotiable
}

Supported ratios: 1:1, 2:3, 3:2, 3:4, 4:3, 4:5, 5:4, 9:16, 16:9, 21:9. Nano Banana 2 adds 1:4, 4:1, 1:8, 8:1.

Endpoint: POST https://generativelanguage.googleapis.com/v1beta/models/{MODEL}:generateContent with header x-goog-api-key: $GEMINI_API_KEY.

Reference images attach as inline-data parts in contents[0].parts:

parts = [
    {"inlineData": {"mimeType": "image/png", "data": base64_of_char_sheet}},
    {"inlineData": {"mimeType": "image/png", "data": base64_of_style_ref}},
    {"text": prompt_with_role_labels},
]

A complete runnable example: scripts/generate.py.

Step 3: Use the official prompt formula

[Reference images] + [Relationship instruction] + [New scenario]

The relationship instruction is the load-bearing piece — explicitly label what role each reference plays. Without role labels, the model partly absorbs and partly ignores the reference.

Worked example for an editorial hero illustration:

Reference image 1 is the CHARACTER SHEET — preserve the man's face,
build, hair, and teal hoodie exactly.
Reference image 2 is the STYLE TEMPLATE — match its graphic-novel rendering,
warm leather-and-amber palette, atmospheric lighting, and cinematic composition.

Generate a new scene where: [scene description].

Wide cinematic 16:9 framing, character occupies the left third.
Laptop screens display abstract glowing UI shapes with no readable typography,
indistinct soft-blurred interface.

Deep dive on the formula and more variants: references/prompt-formula.md.

Step 4: Layer character consistency

Three techniques, in order of impact:

1. Words + reference together — describe identifying details in text and attach the reference. "Young Latino man, teal hoodie, dark curly hair" + image. Image alone drifts.
2. Iterative re-feeding — for multi-scene consistency, attach the previous generation in the next request alongside the original sheet. Identity tightens with each pass.
3. Annotated references — Nano Banana reads labels and captions inside reference images. A labeled character sheet outperforms a clean illustration.

Details + edge cases: references/character-consistency.md.

Step 5: Style references are separate from character refs

For matching an existing visual style, attach two images with explicit roles:

• Ref 1: the character (face/build/clothing to preserve)
• Ref 2: a successful piece in the target style (rendering, palette, lighting, composition to match)

Single-reference style transfer drifts. Dual-reference with role labels is the documented pattern. See references/style-references.md for the full pattern and when to use 3+ refs.

Step 6: Suppress unwanted text/labels with positive framing only

There is no negative prompt. Two things that work:

• Positive framing: describe what should occupy the space. Not "no text on the laptop screens" → "the laptop screens display abstract glowing UI shapes, indistinct soft-blurred interface, no readable typography."
• Use Nano Banana Pro: its text-rendering architecture is also good at not rendering text when told to. The 2.5-flash model is unreliable here.

Anti-Patterns

Anti-Pattern: Aspect ratio in the prompt

Novice: "I asked for a 16:9 wide editorial illustration in the prompt — Gemini gave me a square." Expert: Aspect ratio belongs in generationConfig.imageConfig.aspectRatio, never in prompt text. The model ignores prompt-level dimensions. The default is square. Timeline: This config field landed with the 2.5-flash-image GA release in late 2025. Earlier docs and tutorials often omit it; verify against the current docs.

Anti-Pattern: Single reference, expecting both character AND style fidelity

Novice: "I attached the character sheet and asked for the same style — Gemini gave me 16-bit pixel art." Expert: A single illustrated reference is ambiguous about role. The model treats it as a loose mood board and may hallucinate aesthetic interpretations that aren't in the source. Attach two references with explicit role labels (Reference 1 is the CHARACTER, Reference 2 is the STYLE TEMPLATE). Character fidelity and style fidelity are different jobs. Timeline: This became necessary with the move from Imagen-style "one ref + style flag" pipelines (Ideogram V3) to Gemini's reference-as-context model. Pre-2026 patterns don't transfer.

Anti-Pattern: Negative prompts ("no text", "no watermark")

Novice: "I added 'no text, no labels, no watermarks' but Gemini still wrote 'CONCERT TICKETS' across the screen." Expert: Gemini does not honor negative prompts the way Ideogram or Stable Diffusion do. Describe what should be there instead — abstract shapes, blurred interface, plain surface. Or upgrade to Nano Banana Pro, whose text architecture is precise enough to reliably not generate text on command. Timeline: This is a structural property of Gemini's image head, not a temporary limitation. It applies to all current models in the family.

Anti-Pattern: Using gemini-2.5-flash-image for character work

Novice: "I'll save money by using 2.5-flash for the editorial hero illustration." Expert: 2.5-flash is the weakest character-consistency model in the family and unreliable for strict style adherence. Pro is the right tool for character/style work; 2.5-flash is for cheap simple scenes. The cost difference is rarely worth the regeneration loop. Timeline: Pro became available (preview) in early 2026. Before that, the choice was between 2.5-flash and external tools like Ideogram.

References

Consult these for deep dives — they are NOT loaded by default:

Quick Reference: Repo Conventions (jbuds4life)

When this skill is invoked inside the jbuds4life repo:

• Character sheets live in experiments/blog-image-styles/character-sheets/ (alex, jordan, marcus, mei, sofia — all *-v3-sheet.png)
• Existing successful heroes (use as STYLE TEMPLATE refs) live in next-app/public/images/blog/*.webp
• API key in next-app/.env.local as GEMINI_API_KEY
• Output convention: save timestamped PNG first, then convert to webp via cwebp -q 85, then place at next-app/public/images/blog/{slug}-hero.webp
• The blog frontmatter og_image_url: /images/blog/{slug}-hero.webp is what's served as the OG image

Sources

• Gemini API: image generation
• Gemini models reference
• Google Cloud — Ultimate prompting guide for Nano Banana
• Google Cloud — Gemini 2.5 Flash Image on Vertex AI
• Google Developers — Introducing Gemini 2.5 Flash Image