/sketch

Style Transfer Reference

Purpose: Match an existing brand or illustration style when generating new images — either by referencing a source image, extracting reusable style tokens, or pinning style weight against content. Invoke when cross-asset cohesion must follow an already-shipped look (brand illustrations, a licensed style guide, a sibling product's art direction) rather than a freshly invented one.

Scope Boundary

• sketch style: reference-image prompts for Gemini, style-token extraction, weight / negative-prompt controls, comparison notes for SDXL / Flux when Gemini is not the right tool.
• sketch batch (sibling): consumes the locked style anchor to fan out N assets. Run style first.
• Vision (elsewhere): creative direction and mood boards. If the ask is "what style?" → Vision, not style.
• Muse (elsewhere): converts final palette / typography / spacing into design tokens after style is locked.

If the user hands over a PNG and says "match this" → style. If they say "decide on a style" → Vision first.

Workflow

INTAKE     →  collect style sources: reference images, brand guide PDF,
              existing asset URLs, palette hex list
           →  confirm licensing: can we use this style? (see Anti-Patterns)
           →  decide: Gemini-native vs external (SDXL / Flux) — see "Model Choice"

EXTRACT    →  derive style tokens: palette, line weight, shading mode,
              lens / lighting / mood, grain / noise signature
           →  write a reusable STYLE_TOKEN string (≤ 40 words)

APPLY      →  build prompt: Subject + STYLE_TOKEN + Composition + Technical
           →  attach 2–4 reference images via inlineData (never fileData)
           →  add negative prompt against known leakage patterns

VERIFY     →  render 3 A/B pairs against the reference, eyeball cohesion,
              pHash reference vs output (expect distance 20–35 — too close
              = plagiarism risk, too far = style drift)

Reference-Image Prompt Patterns

Gemini accepts up to 14 reference images per request; keep each under 4 MB.

from google import genai
from google.genai import types
import base64, pathlib

def ref(path: str) -> types.Part:
    data = pathlib.Path(path).read_bytes()
    return types.Part.from_bytes(
        data=data, mime_type="image/png"
    )  # inlineData, never fileData for style refs

parts = [
    types.Part.from_text(text=(
        "Generate a new illustration that matches the visual style of "
        "the attached references. Subject: a fox reading a letter. "
        "STYLE: flat vector, 4-color limited palette (#0B3954 #087E8B #BFD7EA #FF5A5F), "
        "1.5px outline, slight paper grain, soft ambient shadow. "
        "Do not copy the references' subjects."
    )),
    ref("refs/brand_hero.png"),
    ref("refs/brand_secondary.png"),
]

Rule: always tell the model to match style and not subject, otherwise it reconstructs the reference content.

Style Token Extraction

A good STYLE_TOKEN is a short, dense, reusable string that composes with any subject.

Target length: 20–40 words. Pin the token in metadata.json so batch can reuse it verbatim.

Style Strength Controls

Gemini does not expose a numeric style weight. Simulate it with prompt + references:

For SDXL / Flux workflows (out of Sketch scope), you would use IP-Adapter or LoRA weight (0.4–0.9). If the user needs numeric control, that is the hand-off signal — see "Model Choice" below.

Negative Prompts for Style Leakage

Common leakage modes and their counter-prompts:

--- leakage: reference subject bleeds into output ---
negative: "copying reference subject, same composition as reference"

--- leakage: photorealism sneaks into flat illustration ---
negative: "photographic, realistic skin, bokeh, depth of field"

--- leakage: extra colors outside palette ---
negative: "colors outside the stated palette, gradient, neon"

--- leakage: wrong outline ---
negative: "sketchy lines, variable line weight, crosshatch"

Gemini supports negative phrasing inside the main prompt — no dedicated negative_prompt field. Wrap them with "Avoid: …".

Model Choice: Gemini vs Dedicated Style Models

If the user brings LoRA weights or IP-Adapter config, Sketch stops — route to an external pipeline (not an in-repo agent). Sketch still delivers the Gemini-native version if the brief tolerates it.

Anti-Patterns

• Upload a copyrighted reference ("make it like Ghibli") — policy risk and likely content-filter rejection. Ask the user to confirm licensing.
• Use fileData URL for reference images — model silently returns text-only, no image. Always inlineData Base64.
• Pack 10+ references of the same shot — model averages to mud. 2–4 diverse angles of the same style outperform.
• Include the STYLE_TOKEN once then vary it across the batch — cohesion collapses. Pin it verbatim.
• Ask Gemini for numeric style weight (style_weight=0.7) — no such parameter exists; that signals "wrong model".
• Skip the A/B against reference — style drift is invisible without side-by-side.
• Match style so closely the output is a near-duplicate of the reference (pHash hamming < 10) — that is plagiarism, not style transfer.

Handoff

• To batch: STYLE_TOKEN string + reference image paths + recommended seed stride. batch fans out.
• To Muse: finalized palette hex list and typography hints if the style will become design tokens.
• To Vision: if the reference style is unclear or inconsistent, bounce back — creative direction must land before style is useful.
• To Growth: one hero render plus STYLE_TOKEN so marketing asset requests stay on-brand.